diff --git a/.github/actions/install-binaryen/action.yml b/.github/actions/install-binaryen/action.yml
new file mode 100644
index 0000000000..b3c3615521
--- /dev/null
+++ b/.github/actions/install-binaryen/action.yml
@@ -0,0 +1,90 @@
+name: Install Binaryen
+
+inputs:
+  repository:
+    description: 'Repository name with owner. For example, actions/checkout'
+    default: WebAssembly/binaryen
+  ref:
+    description: >
+      The branch, tag or SHA to checkout. When checking out the repository that
+      triggered a workflow, this defaults to the reference or SHA for that
+      event.  Otherwise, uses the default branch.
+    default: latest
+  build:
+    description: Whether we should build from source
+    default: false
+runs:
+  using: composite
+  steps:
+    - name: Restore cached binaryen
+      if: ${{ inputs.build && inputs.build != 'false' }}
+      id: cache-binaryen
+      uses: actions/cache/restore@v4
+      with:
+        path: binaryen
+        key: ${{ runner.os }}-binaryen-${{ inputs.ref }}
+
+    - name: Checkout binaryen
+      if: ${{ inputs.build  && inputs.build != 'false' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      uses: actions/checkout@v4
+      with:
+        repository: ${{ inputs.repository }}
+        path: binaryen
+        submodules: true
+        ref: ${{ inputs.ref == 'latest' && 'main' || inputs.ref }}
+
+    - name: Install ninja (Linux)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Linux' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      shell: bash
+      run: sudo apt-get install ninja-build
+
+    - name: Install ninja (MacOS)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'macOS' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      shell: bash
+      run: brew install ninja
+
+    - name: Build binaryen
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os != 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: |
+        cmake -G Ninja .
+        ninja
+
+    - name: Install binaryen build dependencies (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: opam install conf-cmake conf-c++
+
+    - name: Build binaryen (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: |
+        opam exec -- cmake . -DBUILD_STATIC_LIB=ON -DBUILD_TESTS=off -DINSTALL_LIBS=off -DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc
+        make -j 4
+
+    - name: Cache binaryen
+      if: ${{ inputs.build && inputs.build != 'false' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      uses: actions/cache/save@v4
+      with:
+        path: binaryen
+        key: ${{ runner.os }}-binaryen-${{ inputs.ref }}
+
+    - name: Set binaryen's path
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os != 'Windows' }}
+      shell: bash
+      run: echo "$GITHUB_WORKSPACE/binaryen/bin" >> $GITHUB_PATH
+
+    - name: Copy binaryen's tools (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' }}
+      shell: bash
+      run: cp $GITHUB_WORKSPACE/binaryen/bin/wasm-{merge,opt}.exe _opam/bin
+
+    - name: Download Binaryen
+      if: ${{ ! inputs.build || inputs.build == 'false' }}
+      uses: Aandreba/setup-binaryen@v1.0.0
+      with:
+          token: ${{ github.token }}
+          version: ${{ inputs.ref }}
diff --git a/.github/workflows/build-wasm_of_ocaml.yml b/.github/workflows/build-wasm_of_ocaml.yml
index 70f4c5ab10..db3892fe2e 100644
--- a/.github/workflows/build-wasm_of_ocaml.yml
+++ b/.github/workflows/build-wasm_of_ocaml.yml
@@ -16,6 +16,8 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
+        os-name:
+          - Ubuntu
         ocaml-compiler:
           - "4.14"
           - "5.0"
@@ -27,30 +29,50 @@ jobs:
           - false
         all_jane_street_tests:
           - false
+        wasi:
+          - false
         include:
           - os: macos-latest
+            os-name: MacOS
             ocaml-compiler: "5.3"
             separate_compilation: true
             jane_street_tests: false
             all_jane_street_tests: false
+            wasi: false
           - os: windows-latest
+            os-name: Windows
             ocaml-compiler: "5.2"
             separate_compilation: true
             jane_street_tests: true
             all_jane_street_tests: true
+            wasi: false
           - os: ubuntu-latest
+            os-name: Ubuntu
             ocaml-compiler: "5.2"
             separate_compilation: true
             jane_street_tests: true
             all_jane_street_tests: true
+            wasi: false
           - os: ubuntu-latest
+            os-name: Ubuntu
             ocaml-compiler: "5.2"
             separate_compilation: false
             jane_street_tests: true
             all_jane_street_tests: false
+            wasi: false
+          - os: ubuntu-latest
+            os-name: Ubuntu
+            ocaml-compiler: "5.3"
+            separate_compilation: true
+            jane_street_tests: false
+            all_jane_street_tests: false
+            wasi: true
 
     runs-on: ${{ matrix.os }}
 
+    name:
+       ${{ matrix.wasi && 'WASI / ' || '' }}${{ (! matrix.separate_compilation) && 'Whole program / ' || ''}}${{ matrix.ocaml-compiler }} / ${{ matrix.os-name }}${{ matrix.all_jane_street_tests && ' / Jane Street tests' || ''}}
+
     steps:
       - name: Set git to use LF
         if: ${{ matrix.os == 'windows-latest' && matrix.ocaml-compiler < 5.2 }}
@@ -77,15 +99,63 @@ jobs:
         with:
           node-version: latest
 
+      - name: Set-up Rust toolchain
+        if: matrix.wasi
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+
+      - name: Checkout Wasmtime
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: bytecodealliance/wasmtime
+          path: wasmtime
+          submodules: true
+
+      - name: Build Wasmtime
+        if: matrix.wasi
+        working-directory: ./wasmtime
+        run: |
+          cargo build
+          echo `pwd`/target/debug >> "$GITHUB_PATH"
+
+      - name: Checkout Virgil
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: titzer/virgil
+          path: virgil
+
+      - name: Build Virgil
+        if: matrix.wasi
+        working-directory: ./virgil
+        run: |
+          export PATH=$PATH:`pwd`/bin
+          echo `pwd`/bin >> "$GITHUB_PATH"
+          make
+
+      - name: Checkout Wizard engine
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: titzer/wizard-engine
+          path: wizard-engine
+
+      - name: Build Wizard engine
+        if: matrix.wasi
+        working-directory: ./wizard-engine
+        run: |
+          make -j 4
+          echo `pwd`/bin >> "$GITHUB_PATH"
+
       - name: Set-up OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v3
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
 
-      - name: Set-up Binaryen
-        uses: Aandreba/setup-binaryen@v1.0.0
+      - name: Install Binaryen
+        uses: ./wasm_of_ocaml/.github/actions/install-binaryen
         with:
-          token: ${{ github.token }}
+          build: true
 
       - name: Pin faked binaryen-bin package
         # It's faster to use a cached version
@@ -129,7 +199,7 @@ jobs:
           opam install . -t
 
       - name: Run tests
-        if: ${{ matrix.separate_compilation }}
+        if: ${{ matrix.separate_compilation && ! matrix.wasi }}
         working-directory: ./wasm_of_ocaml
         run: opam exec -- dune build @runtest-wasm
 
@@ -138,11 +208,33 @@ jobs:
       # See https://github.com/libuv/libuv/issues/3622
 
       - name: Run tests with CPS effects
-        if: ${{ matrix.ocaml-compiler >= '5.' && matrix.separate_compilation }}
+        if: ${{ matrix.ocaml-compiler >= '5.' && matrix.separate_compilation && ! matrix.wasi }}
         continue-on-error: ${{ matrix.os == 'windows-latest' }}
         working-directory: ./wasm_of_ocaml
         run: opam exec -- dune build @runtest-wasm --profile with-effects
 
+      - name: Run tests (WASI runtime - node)
+        if: ${{ false }}
+        working-directory: ./wasm_of_ocaml
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
+      - name: Run tests (WASI runtime - Wizard engine)
+        if: ${{ matrix.wasi }}
+        working-directory: ./wasm_of_ocaml
+        env:
+          WASM_ENGINE: wizard-fast
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
+      - name: Run tests (WASI runtime - wasmtime)
+        if: ${{ false }}
+        working-directory: ./wasm_of_ocaml
+        env:
+          WASM_ENGINE: wasmtime
+          WASI_FLAGS: --enable trap-on-exception
+          RUST_BACKTRACE: 0
+        continue-on-error: true
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
       - name: Run Base tests
         if: matrix.all_jane_street_tests
         continue-on-error: ${{ matrix.os == 'windows-latest' }}
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 758928f661..7900183f6f 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -122,10 +122,10 @@ jobs:
       - run: opam install conf-pkg-config conf-mingw-w64-g++-i686 conf-mingw-w64-g++-x86_64
         if: runner.os == 'Windows'
 
-      - name: Set-up Binaryen
-        uses: Aandreba/setup-binaryen@v1.0.0
+      - name: Install Binaryen
+        uses: ./.github/actions/install-binaryen
         with:
-          token: ${{ github.token }}
+          build: true
 
       - name: Install faked binaryen-bin package
         # It's faster to use a cached version
diff --git a/compiler/bin-wasm_of_ocaml/cmd_arg.ml b/compiler/bin-wasm_of_ocaml/cmd_arg.ml
index f079595690..d449f5a23b 100644
--- a/compiler/bin-wasm_of_ocaml/cmd_arg.ml
+++ b/compiler/bin-wasm_of_ocaml/cmd_arg.ml
@@ -44,7 +44,11 @@ let normalize_effects (effects : [ `Cps | `Jspi ] option) common : Config.effect
   | None ->
       (* For backward compatibility, consider that [--enable effects] alone means
         [--effects cps] *)
-      if List.mem "effects" ~set:common.Jsoo_cmdline.Arg.optim.enable then `Cps else `Jspi
+      if List.mem "effects" ~set:common.Jsoo_cmdline.Arg.optim.enable
+      then `Cps
+      else if List.mem "wasi" ~set:common.Jsoo_cmdline.Arg.optim.enable
+      then `Disabled
+      else `Jspi
   | Some ((`Cps | `Jspi) as e) -> e
 
 type t =
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index efc1be643b..def4ad4d5a 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -89,9 +89,12 @@ let build_runtime ~runtime_file =
     [ ( "effects"
       , Wat_preprocess.String
           (match Config.effects () with
+          | `Disabled -> "disabled"
           | `Jspi -> "jspi"
           | `Cps -> "cps"
-          | `Disabled | `Double_translation -> assert false) )
+          | `Double_translation -> assert false) )
+    ; "wasi", Wat_preprocess.Bool (Config.Flag.wasi ())
+    ; "trap-on-exception", Wat_preprocess.Bool (Config.Flag.trap_on_exception ())
     ]
   in
   match
@@ -113,7 +116,9 @@ let build_runtime ~runtime_file =
             ; file = module_name ^ ".wat"
             ; source = Contents contents
             })
-          Runtime_files.wat_files
+          (if Config.Flag.wasi ()
+           then ("libc", Runtime_files.wasi_libc) :: Runtime_files.wat_files
+           else Runtime_files.wat_files)
       in
       Runtime.build
         ~link_options:[ "-g" ]
@@ -175,7 +180,10 @@ let link_and_optimize
   @@ fun opt_temp_sourcemap' ->
   let primitives =
     Binaryen.dead_code_elimination
-      ~dependencies:Runtime_files.dependencies
+      ~dependencies:
+        (if Config.Flag.wasi ()
+         then Runtime_files.wasi_dependencies
+         else Runtime_files.dependencies)
       ~opt_input_sourcemap:opt_temp_sourcemap
       ~opt_output_sourcemap:opt_temp_sourcemap'
       ~input_file:temp_file
@@ -293,7 +301,13 @@ let build_js_runtime ~primitives ?runtime_arguments () =
     | _ -> assert false
   in
   let init_fun =
-    match Parse_js.parse (Parse_js.Lexer.of_string Runtime_files.js_runtime) with
+    match
+      Parse_js.parse
+        (Parse_js.Lexer.of_string
+           (if Config.Flag.wasi ()
+            then Runtime_files.js_wasi_launcher
+            else Runtime_files.js_launcher))
+    with
     | [ (Expression_statement f, _) ] -> f
     | _ -> assert false
   in
@@ -530,9 +544,12 @@ let run
              tmp_wasm_file
          in
          let wasm_name =
-           Printf.sprintf
-             "code-%s"
-             (String.sub (Digest.to_hex (Digest.file tmp_wasm_file)) ~pos:0 ~len:20)
+           if Config.Flag.wasi ()
+           then "code"
+           else
+             Printf.sprintf
+               "code-%s"
+               (String.sub (Digest.to_hex (Digest.file tmp_wasm_file)) ~pos:0 ~len:20)
          in
          let tmp_wasm_file' = Filename.concat tmp_dir (wasm_name ^ ".wasm") in
          Sys.rename tmp_wasm_file tmp_wasm_file';
diff --git a/compiler/bin-wasm_of_ocaml/dune b/compiler/bin-wasm_of_ocaml/dune
index 1870a60f7c..024b987d36 100644
--- a/compiler/bin-wasm_of_ocaml/dune
+++ b/compiler/bin-wasm_of_ocaml/dune
@@ -27,6 +27,9 @@
   gen/gen.exe
   ../../runtime/wasm/runtime.js
   ../../runtime/wasm/deps.json
+  ../../runtime/wasm/runtime-wasi.js
+  ../../runtime/wasm/deps-wasi.json
+  ../../runtime/wasm/libc.wasm
   (glob_files ../../runtime/wasm/*.wat)
   (glob_files ../../runtime/wasm/runtime-*.wasm))
  (action
diff --git a/compiler/bin-wasm_of_ocaml/gen/gen.ml b/compiler/bin-wasm_of_ocaml/gen/gen.ml
index a9f3c0e1b2..8da7f7258e 100644
--- a/compiler/bin-wasm_of_ocaml/gen/gen.ml
+++ b/compiler/bin-wasm_of_ocaml/gen/gen.ml
@@ -32,15 +32,22 @@ let check_js_file fname =
 (* Keep the two variables below in sync with function build_runtime in
    ../compile.ml *)
 
-let default_flags = []
+let default_flags = [ "trap-on-exception", `B false ]
 
-let interesting_runtimes = [ [ "effects", `S "jspi" ]; [ "effects", `S "cps" ] ]
+let interesting_runtimes =
+  [ [ "effects", `S "jspi"; "wasi", `B false ]
+  ; [ "effects", `S "cps"; "wasi", `B false ]
+  ; [ "effects", `S "disabled"; "wasi", `B true ]
+  ; [ "effects", `S "cps"; "wasi", `B true ]
+  ]
+
+let defaults = [ "effects", "disabled" ]
 
 let name_runtime standard l =
   let flags =
     List.filter_map l ~f:(fun (k, v) ->
         match v with
-        | `S s -> Some s
+        | `S s -> if List.mem (k, s) ~set:defaults then None else Some s
         | `B b -> if b then Some k else None)
   in
   String.concat ~sep:"-" ("runtime" :: (if standard then [ "standard" ] else flags))
@@ -67,11 +74,13 @@ let print_flags f flags =
 
 let () =
   let () = set_binary_mode_out stdout true in
-  let js_runtime, deps, wat_files, runtimes =
+  let js_launcher, deps, js_wasi_launcher, wasi_deps, wasi_libc, wat_files, runtimes =
     match Array.to_list Sys.argv with
-    | _ :: js_runtime :: deps :: rest ->
-        assert (Filename.check_suffix js_runtime ".js");
+    | _ :: js_launcher :: deps :: js_wasi_launcher :: wasi_deps :: wasi_libc :: rest ->
+        assert (Filename.check_suffix js_launcher ".js");
+        assert (Filename.check_suffix js_wasi_launcher ".js");
         assert (Filename.check_suffix deps ".json");
+        assert (Filename.check_suffix wasi_deps ".json");
         let wat_files, rest =
           List.partition rest ~f:(fun f -> Filename.check_suffix f ".wat")
         in
@@ -79,13 +88,17 @@ let () =
           List.partition rest ~f:(fun f -> Filename.check_suffix f ".wasm")
         in
         assert (List.is_empty rest);
-        js_runtime, deps, wat_files, wasm_files
+        js_launcher, deps, js_wasi_launcher, wasi_deps, wasi_libc, wat_files, wasm_files
     | _ -> assert false
   in
-  check_js_file js_runtime;
+  check_js_file js_launcher;
+  check_js_file js_wasi_launcher;
   Format.printf "open Wasm_of_ocaml_compiler@.";
-  Format.printf "let js_runtime = {|\n%s\n|}@." (Fs.read_file js_runtime);
+  Format.printf "let js_launcher = {|\n%s\n|}@." (Fs.read_file js_launcher);
   Format.printf "let dependencies = {|\n%s\n|}@." (Fs.read_file deps);
+  Format.printf "let js_wasi_launcher = {|\n%s\n|}@." (Fs.read_file js_wasi_launcher);
+  Format.printf "let wasi_dependencies = {|\n%s\n|}@." (Fs.read_file wasi_deps);
+  Format.printf "let wasi_libc = %S@." (String.escaped (Fs.read_file wasi_libc));
   Format.printf
     "let wat_files = [%a]@."
     (Format.pp_print_list (fun f file ->
diff --git a/compiler/lib-wasm/binaryen.ml b/compiler/lib-wasm/binaryen.ml
index 5886e28c6b..190aeab387 100644
--- a/compiler/lib-wasm/binaryen.ml
+++ b/compiler/lib-wasm/binaryen.ml
@@ -36,6 +36,8 @@ let common_options () =
     ; "--enable-bulk-memory"
     ; "--enable-nontrapping-float-to-int"
     ; "--enable-strings"
+    ; "--enable-multimemory" (* To keep wasm-merge happy *)
+    ; "--enable-stack-switching"
     ]
   in
   if Config.Flag.pretty () then "-g" :: l else l
@@ -111,9 +113,9 @@ let dead_code_elimination
   filter_unused_primitives primitives usage_file
 
 let optimization_options =
-  [| [ "-O2"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
-   ; [ "-O2"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
-   ; [ "-O3"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
+  [| [ "-O2"; "--skip-pass=inlining-optimizing" ]
+   ; [ "-O2"; "--skip-pass=inlining-optimizing" ]
+   ; [ "-O3"; "--skip-pass=inlining-optimizing" ]
   |]
 
 let optimize
@@ -133,6 +135,7 @@ let optimize
   command
     ("wasm-opt"
      :: (common_options ()
+        @ (if Config.Flag.trap_on_exception () then [] else [ "--traps-never-happen" ])
         @ Option.value ~default:optimization_options.(level - 1) options
         @ [ Filename.quote input_file; "-o"; Filename.quote output_file ])
     @ opt_flag "--input-source-map" opt_input_sourcemap
diff --git a/compiler/lib-wasm/gc_target.ml b/compiler/lib-wasm/gc_target.ml
index 4557751d77..6947379306 100644
--- a/compiler/lib-wasm/gc_target.ml
+++ b/compiler/lib-wasm/gc_target.ml
@@ -574,9 +574,13 @@ module Value = struct
        return ())
       (val_int (if negate then Arith.eqz n else n))
 
-  let eq x y = eq_gen ~negate:false x y
+  let eq x y =
+    if Config.Flag.wasi () then val_int (ref_eq x y) else eq_gen ~negate:false x y
 
-  let neq x y = eq_gen ~negate:true x y
+  let neq x y =
+    if Config.Flag.wasi ()
+    then val_int (Arith.eqz (ref_eq x y))
+    else eq_gen ~negate:true x y
 
   let ult = binop Arith.(ult)
 
@@ -1302,7 +1306,12 @@ module Math = struct
     { W.params = List.init ~len:n ~f:(fun _ : W.value_type -> F64); result = [ F64 ] }
 
   let unary name x =
-    let* f = register_import ~import_module:"Math" ~name (Fun (float_func_type 1)) in
+    let* f =
+      register_import
+        ~import_module:(if Config.Flag.wasi () then "env" else "Math")
+        ~name
+        (Fun (float_func_type 1))
+    in
     let* x = x in
     return (W.Call (f, [ x ]))
 
@@ -1345,7 +1354,12 @@ module Math = struct
   let log10 f = unary "log10" f
 
   let binary name x y =
-    let* f = register_import ~import_module:"Math" ~name (Fun (float_func_type 2)) in
+    let* f =
+      register_import
+        ~import_module:(if Config.Flag.wasi () then "env" else "Math")
+        ~name
+        (Fun (float_func_type 2))
+    in
     let* x = x in
     let* y = y in
     return (W.Call (f, [ x; y ]))
@@ -1688,21 +1702,34 @@ let handle_exceptions ~result_typ ~fall_through ~context body x exn_handler =
          x
          (block_expr
             { params = []; result = [ Value.value ] }
-            (let* exn =
-               block_expr
-                 { params = []; result = [ externref ] }
-                 (let* e =
-                    try_expr
-                      { params = []; result = [ externref ] }
-                      (body
-                         ~result_typ:[ externref ]
-                         ~fall_through:`Skip
-                         ~context:(`Skip :: `Skip :: `Catch :: context))
-                      [ ocaml_tag, 1, Value.value; js_tag, 0, externref ]
-                  in
-                  instr (W.Push e))
-             in
-             instr (W.CallInstr (f, [ exn ]))))
+            (if Config.Flag.wasi ()
+             then
+               let* e =
+                 try_expr
+                   { params = []; result = [ Value.value ] }
+                   (body
+                      ~result_typ:[ Value.value ]
+                      ~fall_through:`Skip
+                      ~context:(`Skip :: `Catch :: context))
+                   [ ocaml_tag, 0, Value.value ]
+               in
+               instr (W.Push e)
+             else
+               let* exn =
+                 block_expr
+                   { params = []; result = [ externref ] }
+                   (let* e =
+                      try_expr
+                        { params = []; result = [ externref ] }
+                        (body
+                           ~result_typ:[ externref ]
+                           ~fall_through:`Skip
+                           ~context:(`Skip :: `Skip :: `Catch :: context))
+                        [ ocaml_tag, 1, Value.value; js_tag, 0, externref ]
+                    in
+                    instr (W.Push e))
+               in
+               instr (W.CallInstr (f, [ exn ]))))
      in
      let* () = no_event in
      exn_handler ~result_typ ~fall_through ~context)
diff --git a/compiler/lib-wasm/generate.ml b/compiler/lib-wasm/generate.ml
index f3028fcdb7..6773e61d6d 100644
--- a/compiler/lib-wasm/generate.ml
+++ b/compiler/lib-wasm/generate.ml
@@ -26,8 +26,7 @@ let times = Debug.find "times"
 let effects_cps () =
   match Config.effects () with
   | `Cps | `Double_translation -> true
-  | `Jspi -> false
-  | `Disabled -> assert false
+  | `Disabled | `Jspi -> false
 
 module Generate (Target : Target_sig.S) = struct
   open Target
@@ -1144,6 +1143,36 @@ module Generate (Target : Target_sig.S) = struct
       :: context.other_fields;
     name
 
+  let add_missing_primitives ~context l =
+    let failwith_desc = W.Fun { params = [ Value.value ]; result = [] } in
+    List.iter l ~f:(fun (exported_name, arity) ->
+        let name = Code.Var.fresh_n exported_name in
+        let locals, body =
+          function_body
+            ~context
+            ~param_names:[]
+            ~body:
+              (let* failwith =
+                 register_import ~import_module:"env" ~name:"caml_failwith" failwith_desc
+               in
+               let* msg =
+                 Constant.translate (String (exported_name ^ " not implemented"))
+               in
+               let* () = instr (CallInstr (failwith, [ msg ])) in
+               push Value.unit)
+        in
+        context.other_fields <-
+          W.Function
+            { name
+            ; exported_name = Some exported_name
+            ; typ = None
+            ; signature = func_type arity
+            ; param_names = []
+            ; locals
+            ; body
+            }
+          :: context.other_fields)
+
   let entry_point context toplevel_fun entry_name =
     let signature, param_names, body = entry_point ~toplevel_fun in
     let locals, body = function_body ~context ~param_names ~body in
@@ -1301,6 +1330,10 @@ let add_start_function = G.add_start_function
 
 let add_init_function = G.add_init_function
 
+let add_missing_primitives =
+  let module G = Generate (Gc_target) in
+  G.add_missing_primitives
+
 let output ch ~context =
   let t = Timer.make () in
   let fields = G.output ~context in
diff --git a/compiler/lib-wasm/generate.mli b/compiler/lib-wasm/generate.mli
index 79408d8cb6..4ada928d41 100644
--- a/compiler/lib-wasm/generate.mli
+++ b/compiler/lib-wasm/generate.mli
@@ -33,6 +33,9 @@ val add_start_function : context:Code_generation.context -> Wasm_ast.var -> unit
 
 val add_init_function : context:Code_generation.context -> to_link:string list -> unit
 
+val add_missing_primitives :
+  context:Code_generation.context -> (string * int) list -> unit
+
 val output : out_channel -> context:Code_generation.context -> unit
 
 val wasm_output : out_channel -> context:Code_generation.context -> unit
diff --git a/compiler/lib-wasm/link.ml b/compiler/lib-wasm/link.ml
index f56bd9e500..d4a33f3481 100644
--- a/compiler/lib-wasm/link.ml
+++ b/compiler/lib-wasm/link.ml
@@ -173,7 +173,20 @@ module Wasm_binary = struct
 
   let reftype' i ch =
     match i with
-    | 0x6a | 0x6b | 0x6c | 0x6d | 0x6e | 0x6f | 0x70 | 0x71 | 0x72 | 0x73 -> ()
+    | 0x68
+    | 0x69
+    | 0x6a
+    | 0x6b
+    | 0x6c
+    | 0x6d
+    | 0x6e
+    | 0x6f
+    | 0x70
+    | 0x71
+    | 0x72
+    | 0x73
+    | 0x74
+    | 0x75 -> ()
     | 0x63 | 0x64 -> heaptype ch
     | _ ->
         Format.eprintf "Unknown reftype %x@." i;
@@ -181,12 +194,13 @@ module Wasm_binary = struct
 
   let reftype ch = reftype' (input_byte ch) ch
 
-  let valtype ch =
-    let i = read_uint ch in
+  let valtype' i ch =
     match i with
-    | 0x7b | 0x7c | 0x7d | 0x7e | 0x7f -> ()
+    | 0x7B | 0x7C | 0x7D | 0x7E | 0x7F -> ()
     | _ -> reftype' i ch
 
+  let valtype ch = valtype' (read_uint ch) ch
+
   let limits ch =
     match input_byte ch with
     | 0 -> ignore (read_uint ch)
@@ -201,32 +215,99 @@ module Wasm_binary = struct
     reftype ch;
     limits ch
 
+  type comptype =
+    | Func of { arity : int }
+    | Struct
+    | Array
+    | Cont
+
+  let supertype ch =
+    match input_byte ch with
+    | 0 -> ()
+    | 1 -> ignore (read_uint ch)
+    | _ -> assert false
+
+  let storagetype ch =
+    let i = read_uint ch in
+    match i with
+    | 0x78 | 0x77 -> ()
+    | _ -> valtype' i ch
+
+  let fieldtype ch =
+    storagetype ch;
+    ignore (input_byte ch)
+
+  let comptype i ch =
+    match i with
+    | 0x5D ->
+        ignore (read_sint ch);
+        Cont
+    | 0x5E ->
+        fieldtype ch;
+        Array
+    | 0x5F ->
+        ignore (vec fieldtype ch);
+        Struct
+    | 0x60 ->
+        let params = vec valtype ch in
+        let _ = vec valtype ch in
+        Func { arity = List.length params }
+    | c -> failwith (Printf.sprintf "Unknown comptype %d" c)
+
+  let subtype i ch =
+    match i with
+    | 0x50 ->
+        supertype ch;
+        comptype (input_byte ch) ch
+    | 0x4F ->
+        supertype ch;
+        comptype (input_byte ch) ch
+    | _ -> comptype i ch
+
+  let rectype ch =
+    match input_byte ch with
+    | 0x4E -> vec (fun ch -> subtype (input_byte ch) ch) ch
+    | i -> [ subtype i ch ]
+
+  type importdesc =
+    | Func of int
+    | Table
+    | Mem
+    | Global
+    | Tag
+
   type import =
     { module_ : string
     ; name : string
+    ; desc : importdesc
     }
 
   let import ch =
     let module_ = name ch in
     let name = name ch in
     let d = read_uint ch in
-    let _ =
+    let desc =
       match d with
-      | 0 -> ignore (read_uint ch)
-      | 1 -> tabletype ch
-      | 2 -> memtype ch
+      | 0 -> Func (read_uint ch)
+      | 1 ->
+          tabletype ch;
+          Table
+      | 2 ->
+          memtype ch;
+          Mem
       | 3 ->
           let _typ = valtype ch in
           let _mut = input_byte ch in
-          ()
+          Global
       | 4 ->
           assert (read_uint ch = 0);
-          ignore (read_uint ch)
+          ignore (read_uint ch);
+          Tag
       | _ ->
           Format.eprintf "Unknown import %x@." d;
           assert false
     in
-    { module_; name }
+    { module_; name; desc }
 
   let export ch =
     let name = name ch in
@@ -256,6 +337,7 @@ module Wasm_binary = struct
   type interface =
     { imports : import list
     ; exports : string list
+    ; types : comptype array
     }
 
   let read_interface ch =
@@ -263,7 +345,11 @@ module Wasm_binary = struct
       match next_section ch with
       | None -> i
       | Some s ->
-          if s.id = 2
+          if s.id = 1
+          then
+            find_sections
+              { i with types = Array.of_list (List.flatten (vec rectype ch.ch)) }
+          else if s.id = 2
           then find_sections { i with imports = vec import ch.ch }
           else if s.id = 7
           then { i with exports = vec export ch.ch }
@@ -271,7 +357,7 @@ module Wasm_binary = struct
             skip_section ch s;
             find_sections i)
     in
-    find_sections { imports = []; exports = [] }
+    find_sections { imports = []; exports = []; types = [||] }
 
   let append_source_map_section ~file ~url =
     let ch = open_out_gen [ Open_wronly; Open_append; Open_binary ] 0o666 file in
@@ -405,6 +491,13 @@ let generate_start_function ~to_link ~out_file =
   Generate.wasm_output ch ~context;
   if times () then Format.eprintf "    generate start: %a@." Timer.print t1
 
+let generate_missing_primitives ~missing_primitives ~out_file =
+  Filename.gen_file out_file
+  @@ fun ch ->
+  let context = Generate.start () in
+  Generate.add_missing_primitives ~context missing_primitives;
+  Generate.wasm_output ch ~context
+
 let output_js js =
   let js = Driver.simplify_js js in
   let js = Driver.name_variables js in
@@ -642,17 +735,20 @@ let compute_dependencies ~files_to_link ~files =
 
 let compute_missing_primitives (runtime_intf, intfs) =
   let provided_primitives = StringSet.of_list runtime_intf.Wasm_binary.exports in
-  StringSet.elements
+  StringMap.bindings
   @@ List.fold_left
-       ~f:(fun s { Wasm_binary.imports; _ } ->
+       ~f:(fun s { Wasm_binary.imports; types; _ } ->
          List.fold_left
-           ~f:(fun s { Wasm_binary.module_; name; _ } ->
-             if String.equal module_ "env" && not (StringSet.mem name provided_primitives)
-             then StringSet.add name s
-             else s)
+           ~f:(fun s { Wasm_binary.module_; name; desc } ->
+             match module_, desc with
+             | "env", Func idx when not (StringSet.mem name provided_primitives) -> (
+                 match types.(idx) with
+                 | Func { arity } -> StringMap.add name arity s
+                 | _ -> s)
+             | _ -> s)
            ~init:s
            imports)
-       ~init:StringSet.empty
+       ~init:StringMap.empty
        intfs
 
 let load_information files =
@@ -688,6 +784,72 @@ let gen_dir dir f =
     remove_directory d_tmp;
     raise exc
 
+let link_to_module ~to_link ~files_to_link ~files ~enable_source_maps:_ ~dir =
+  let process_file ~name ~module_name file =
+    Zip.with_open_in file
+    @@ fun z ->
+    let intf =
+      let ch, pos, len, _ = Zip.get_entry z ~name in
+      Wasm_binary.read_interface (Wasm_binary.from_channel ~name ch pos len)
+    in
+    ( { Wasm_link.module_name
+      ; file
+      ; code = Some (Zip.read_entry z ~name)
+      ; opt_source_map = None
+      }
+    , intf )
+  in
+  let runtime_file = fst (List.hd files) in
+  let z = Zip.open_in runtime_file in
+  let runtime, runtime_intf =
+    process_file ~name:"runtime.wasm" ~module_name:"env" runtime_file
+  in
+  let prelude =
+    { Wasm_link.module_name = "OCaml"
+    ; file = runtime_file
+    ; code = Some (Zip.read_entry z ~name:"prelude.wasm")
+    ; opt_source_map = None
+    }
+  in
+  Zip.close_in z;
+  let lst =
+    List.tl files
+    |> List.filter_map ~f:(fun (file, _) ->
+           if StringSet.mem file files_to_link
+           then Some (process_file ~name:"code.wasm" ~module_name:"OCaml" file)
+           else None)
+  in
+  let missing_primitives =
+    if Config.Flag.genprim ()
+    then compute_missing_primitives (runtime_intf, List.map ~f:snd lst)
+    else []
+  in
+  Fs.with_intermediate_file (Filename.temp_file "start" ".wasm")
+  @@ fun start_module ->
+  generate_start_function ~to_link ~out_file:start_module;
+  let start =
+    { Wasm_link.module_name = "OCaml"
+    ; file = start_module
+    ; code = None
+    ; opt_source_map = None
+    }
+  in
+  Fs.with_intermediate_file (Filename.temp_file "stubs" ".wasm")
+  @@ fun stubs_module ->
+  generate_missing_primitives ~missing_primitives ~out_file:stubs_module;
+  let missing_primitives =
+    { Wasm_link.module_name = "env"
+    ; file = stubs_module
+    ; code = None
+    ; opt_source_map = None
+    }
+  in
+  ignore
+    (Wasm_link.f
+       (runtime :: prelude :: missing_primitives :: start :: List.map ~f:fst lst)
+       ~filter_export:(fun nm -> String.equal nm "_start" || String.equal nm "memory")
+       ~output_file:(Filename.concat dir "code.wasm"))
+
 let link ~output_file ~linkall ~enable_source_maps ~files =
   if times () then Format.eprintf "linking@.";
   let t = Timer.make () in
@@ -778,30 +940,35 @@ let link ~output_file ~linkall ~enable_source_maps ~files =
   if times () then Format.eprintf "    finding what to link: %a@." Timer.print t1;
   if times () then Format.eprintf "  scan: %a@." Timer.print t;
   let t = Timer.make () in
-  let interfaces, wasm_dir, link_spec =
+  let missing_primitives, wasm_dir, link_spec =
     let dir = Filename.chop_extension output_file ^ ".assets" in
     gen_dir dir
     @@ fun tmp_dir ->
     Sys.mkdir tmp_dir 0o777;
-    let start_module =
-      "start-"
-      ^ String.sub
-          (Digest.to_hex (Digest.string (String.concat ~sep:"/" to_link)))
-          ~pos:0
-          ~len:8
-    in
-    generate_start_function
-      ~to_link
-      ~out_file:(Filename.concat tmp_dir (start_module ^ ".wasm"));
-    let module_names, interfaces =
-      link_to_directory ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir
-    in
-    ( interfaces
-    , dir
-    , let to_link = compute_dependencies ~files_to_link ~files in
-      List.combine module_names (None :: None :: to_link) @ [ start_module, None ] )
+    if not (Config.Flag.wasi ())
+    then (
+      let start_module =
+        "start-"
+        ^ String.sub
+            (Digest.to_hex (Digest.string (String.concat ~sep:"/" to_link)))
+            ~pos:0
+            ~len:8
+      in
+      let module_names, interfaces =
+        link_to_directory ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir
+      in
+      let missing_primitives = compute_missing_primitives interfaces in
+      generate_start_function
+        ~to_link
+        ~out_file:(Filename.concat tmp_dir (start_module ^ ".wasm"));
+      ( List.map ~f:fst missing_primitives
+      , dir
+      , let to_link = compute_dependencies ~files_to_link ~files in
+        List.combine module_names (None :: None :: to_link) @ [ start_module, None ] ))
+    else (
+      link_to_module ~to_link ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir;
+      [], dir, [ "code", None ])
   in
-  let missing_primitives = compute_missing_primitives interfaces in
   if times () then Format.eprintf "    copy wasm files: %a@." Timer.print t;
   let t1 = Timer.make () in
   let js_runtime =
diff --git a/compiler/lib-wasm/link.mli b/compiler/lib-wasm/link.mli
index b52681ee92..ec7675ead1 100644
--- a/compiler/lib-wasm/link.mli
+++ b/compiler/lib-wasm/link.mli
@@ -19,9 +19,17 @@
 open Stdlib
 
 module Wasm_binary : sig
+  type importdesc =
+    | Func of int
+    | Table
+    | Mem
+    | Global
+    | Tag
+
   type import =
     { module_ : string
     ; name : string
+    ; desc : importdesc
     }
 
   val check : contents:string -> bool
diff --git a/compiler/lib-wasm/runtime.ml b/compiler/lib-wasm/runtime.ml
index f7f1df4c0d..0d6ad06459 100644
--- a/compiler/lib-wasm/runtime.ml
+++ b/compiler/lib-wasm/runtime.ml
@@ -31,7 +31,7 @@ let build ~allowed_imports ~link_options ~opt_options ~variables ~inputs ~output
       then (
         Format.eprintf "The runtime contains unknown imports:@.";
         List.iter
-          ~f:(fun { Link.Wasm_binary.module_; name } ->
+          ~f:(fun { Link.Wasm_binary.module_; name; _ } ->
             Format.eprintf "  %s %s@." module_ name)
           missing_imports;
         exit 2))
diff --git a/compiler/lib-wasm/wasm_link.ml b/compiler/lib-wasm/wasm_link.ml
index 5872f1f945..469ac3de9b 100644
--- a/compiler/lib-wasm/wasm_link.ml
+++ b/compiler/lib-wasm/wasm_link.ml
@@ -23,6 +23,10 @@ type heaptype =
   | Nofunc
   | Extern
   | Noextern
+  | Exn
+  | Noexn
+  | Cont
+  | Nocont
   | Any
   | Eq
   | I31
@@ -66,6 +70,7 @@ type comptype =
       }
   | Struct of fieldtype array
   | Array of fieldtype
+  | Cont of int
 
 type subtype =
   { final : bool
@@ -164,6 +169,8 @@ module Write = struct
 
   let heaptype st ch typ =
     match (typ : heaptype) with
+    | Nocont -> byte ch 0x75
+    | Noexn -> byte ch 0x74
     | Nofunc -> byte ch 0x73
     | Noextern -> byte ch 0x72
     | None_ -> byte ch 0x71
@@ -174,6 +181,8 @@ module Write = struct
     | I31 -> byte ch 0x6C
     | Struct -> byte ch 0x6B
     | Array -> byte ch 0x6A
+    | Exn -> byte ch 0x69
+    | Cont -> byte ch 0x68
     | Type idx -> sint ch (typeidx st idx)
 
   let reftype st ch { nullable; typ } =
@@ -219,6 +228,9 @@ module Write = struct
         byte ch 1;
         uint ch (typeidx st supertype));
     match typ with
+    | Cont idx ->
+        byte ch 0x5D;
+        sint ch (typeidx st idx)
     | Array field_type ->
         byte ch 0x5E;
         fieldtype st ch field_type
@@ -569,7 +581,9 @@ module Read = struct
   let heaptype st ch =
     let i = sint ch in
     match i + 128 with
-    | 0X73 -> Nofunc
+    | 0x75 -> Nocont
+    | 0x74 -> Noexn
+    | 0x73 -> Nofunc
     | 0x72 -> Noextern
     | 0x71 -> None_
     | 0x70 -> Func
@@ -579,6 +593,8 @@ module Read = struct
     | 0x6C -> I31
     | 0x6B -> Struct
     | 0x6A -> Array
+    | 0x69 -> Exn
+    | 0x68 -> Cont
     | _ ->
         if i < 0 then failwith (Printf.sprintf "Unknown heaptype %x@." i);
         let i =
@@ -596,7 +612,9 @@ module Read = struct
 
   let reftype' st i ch =
     match i with
-    | 0X73 -> nullable Nofunc
+    | 0x75 -> nullable Nocont
+    | 0x74 -> nullable Noexn
+    | 0x73 -> nullable Nofunc
     | 0x72 -> nullable Noextern
     | 0x71 -> nullable None_
     | 0x70 -> nullable Func
@@ -606,6 +624,8 @@ module Read = struct
     | 0x6C -> nullable I31
     | 0x6B -> nullable Struct
     | 0x6A -> nullable Array
+    | 0x69 -> nullable Exn
+    | 0x68 -> nullable Cont
     | 0x63 -> nullable (heaptype st ch)
     | 0x64 -> { nullable = false; typ = heaptype st ch }
     | _ -> failwith (Printf.sprintf "Unknown reftype %x@." i)
@@ -652,6 +672,14 @@ module Read = struct
 
   let comptype st i ch =
     match i with
+    | 0x5D ->
+        let i = sint ch in
+        let i =
+          if i >= st.type_index_count
+          then lnot (i - st.type_index_count)
+          else st.type_mapping.(i)
+        in
+        Cont i
     | 0x5E -> Array (fieldtype st ch)
     | 0x5F -> Struct (vec (fieldtype st) ch)
     | 0x60 ->
@@ -1258,6 +1286,13 @@ module Scan = struct
       | 0xD1 (* ref.is_null *) | 0xD3 (* ref.eq *) | 0xD4 (* ref.as_non_null *) ->
           pos + 1 |> instructions
       | 0xD2 (* ref.func *) -> pos + 1 |> funcidx |> instructions
+      | 0xE0 (* cont.new *) -> pos + 1 |> typeidx |> instructions
+      | 0xE1 (* cont.bind *) -> pos + 1 |> typeidx |> typeidx |> instructions
+      | 0xE2 (* suspend *) -> pos + 1 |> tagidx |> instructions
+      | 0xE3 (* resume *) -> pos + 1 |> typeidx |> vector on_clause |> instructions
+      | 0xE4 (* resume_throw *) ->
+          pos + 1 |> typeidx |> tagidx |> vector on_clause |> instructions
+      | 0xE5 (* switch *) -> pos + 1 |> typeidx |> tagidx |> instructions
       | 0xFB -> pos + 1 |> gc_instruction
       | 0xFC -> (
           if debug then Format.eprintf "  %d@." (get (pos + 1));
@@ -1392,6 +1427,11 @@ module Scan = struct
       | 0 (* catch *) | 1 (* catch_ref *) -> pos + 1 |> tagidx |> labelidx
       | 2 (* catch_all *) | 3 (* catch_all_ref *) -> pos + 1 |> labelidx
       | c -> failwith (Printf.sprintf "bad catch 0x02%d@." c)
+    and on_clause pos =
+      match get pos with
+      | 0 (* on *) -> pos + 1 |> tagidx |> labelidx
+      | 1 (* on .. switch *) -> pos + 1 |> tagidx
+      | c -> failwith (Printf.sprintf "bad on clause 0x02%d@." c)
     and block_end pos =
       if debug then Format.eprintf "0x%02X (@%d) block end@." (get pos) pos;
       match get pos with
@@ -1544,30 +1584,43 @@ let rec subtype subtyping_info (i : int) i' =
   | None -> false
   | Some s -> subtype subtyping_info s i'
 
-let heap_subtype (subtyping_info : subtype array) (ty : heaptype) (ty' : heaptype) =
+let rec heap_subtype (subtyping_info : subtype array) (ty : heaptype) (ty' : heaptype) =
   match ty, ty' with
-  | (Func | Nofunc), Func
-  | Nofunc, Nofunc
-  | (Extern | Noextern), Extern
+  | Func, Func
+  | Extern, Extern
+  | Noextern, Noextern
+  | Exn, Exn
+  | Noexn, Noexn
+  | Cont, Cont
+  | Nocont, Nocont
   | (Any | Eq | I31 | Struct | Array | None_ | Type _), Any
   | (Eq | I31 | Struct | Array | None_ | Type _), Eq
-  | (I31 | None_), I31
-  | (Struct | None_), Struct
-  | (Array | None_), Array
+  | I31, I31
+  | Struct, Struct
+  | Array, Array
   | None_, None_ -> true
   | Type i, Struct -> (
       match subtyping_info.(i).typ with
       | Struct _ -> true
-      | Array _ | Func _ -> false)
+      | Array _ | Func _ | Cont _ -> false)
   | Type i, Array -> (
       match subtyping_info.(i).typ with
       | Array _ -> true
-      | Struct _ | Func _ -> false)
+      | Struct _ | Func _ | Cont _ -> false)
   | Type i, Func -> (
       match subtyping_info.(i).typ with
       | Func _ -> true
-      | Struct _ | Array _ -> false)
+      | Struct _ | Array _ | Cont _ -> false)
+  | Type i, Cont -> (
+      match subtyping_info.(i).typ with
+      | Cont _ -> true
+      | Struct _ | Array _ | Func _ -> false)
   | Type i, Type i' -> subtype subtyping_info i i'
+  | Nofunc, _ -> heap_subtype subtyping_info ty' Func
+  | Noextern, _ -> heap_subtype subtyping_info ty' Extern
+  | Noexn, _ -> heap_subtype subtyping_info ty' Exn
+  | Nocont, _ -> heap_subtype subtyping_info ty' Cont
+  | None_, _ -> heap_subtype subtyping_info ty' Any
   | _ -> false
 
 let ref_subtype subtyping_info { nullable; typ } { nullable = nullable'; typ = typ' } =
@@ -1884,7 +1937,7 @@ type input =
   ; opt_source_map : Source_map.Standard.t option
   }
 
-let f files ~output_file =
+let f ?(filter_export = fun _ -> true) files ~output_file =
   let files =
     Array.map
       ~f:(fun { module_name; file; code; opt_source_map } ->
@@ -2138,20 +2191,28 @@ let f files ~output_file =
   Array.iter ~f:Scan.clear_position_data positions;
 
   (* 7: export *)
+  let exports =
+    Array.map
+      ~f:(fun intf ->
+        map_exportable_info
+          (fun _ exports -> List.filter ~f:(fun (nm, _) -> filter_export nm) exports)
+          intf.Read.exports)
+      intfs
+  in
   let export_count =
     Array.fold_left
-      ~f:(fun count intf ->
+      ~f:(fun count exports ->
         fold_exportable_info
           (fun _ exports count -> List.length exports + count)
           count
-          intf.Read.exports)
+          exports)
       ~init:0
-      intfs
+      exports
   in
   Write.uint buf export_count;
-  let exports = Hashtbl.create 128 in
+  let export_tbl = Hashtbl.create 128 in
   Array.iteri
-    ~f:(fun i intf ->
+    ~f:(fun i exports ->
       iter_exportable_info
         (fun kind lst ->
           let map =
@@ -2164,7 +2225,7 @@ let f files ~output_file =
           in
           List.iter
             ~f:(fun (name, idx) ->
-              match Hashtbl.find exports name with
+              match Hashtbl.find export_tbl name with
               | i' ->
                   failwith
                     (Printf.sprintf
@@ -2173,11 +2234,11 @@ let f files ~output_file =
                        files.(i').file
                        files.(i).file)
               | exception Not_found ->
-                  Hashtbl.add exports name i;
+                  Hashtbl.add export_tbl name i;
                   Write.export buf kind name map.(idx))
             lst)
-        intf.Read.exports)
-    intfs;
+        exports)
+    exports;
   add_section out_ch ~id:7 buf;
 
   (* 8: start *)
@@ -2447,7 +2508,6 @@ let f files ~output_file =
 (*
 LATER
 - testsuite : import/export matching, source maps, multiple start functions, ...
-- missing instructions ==> typed continuations (?)
 - check features?
 
 MAYBE
diff --git a/compiler/lib-wasm/wasm_link.mli b/compiler/lib-wasm/wasm_link.mli
index 0c0ed0a582..4cbd769668 100644
--- a/compiler/lib-wasm/wasm_link.mli
+++ b/compiler/lib-wasm/wasm_link.mli
@@ -23,4 +23,5 @@ type input =
   ; opt_source_map : Source_map.Standard.t option
   }
 
-val f : input list -> output_file:string -> Source_map.t
+val f :
+  ?filter_export:(string -> bool) -> input list -> output_file:string -> Source_map.t
diff --git a/compiler/lib-wasm/wat_output.ml b/compiler/lib-wasm/wat_output.ml
index 6f6e0915a7..9af9457043 100644
--- a/compiler/lib-wasm/wat_output.ml
+++ b/compiler/lib-wasm/wat_output.ml
@@ -460,19 +460,23 @@ let expression_or_instructions ctx st in_function =
             @ [ List (Atom "else" :: expression iff) ])
         ]
     | Try (ty, body, catches) ->
-        [ List
-            (Atom "try"
-            :: (block_type st ty
-               @ List (Atom "do" :: instructions body)
-                 :: List.map
-                      ~f:(fun (tag, i, ty) ->
-                        List
-                          (Atom "catch"
-                          :: index st.tag_names tag
-                          :: (instruction (Wasm_ast.Event Code_generation.hidden_location)
-                             @ instruction (Wasm_ast.Br (i + 1, Some (Pop ty))))))
-                      catches))
-        ]
+        if Config.Flag.trap_on_exception ()
+        then [ List (Atom "block" :: (block_type st ty @ instructions body)) ]
+        else
+          [ List
+              (Atom "try"
+              :: (block_type st ty
+                 @ List (Atom "do" :: instructions body)
+                   :: List.map
+                        ~f:(fun (tag, i, ty) ->
+                          List
+                            (Atom "catch"
+                            :: index st.tag_names tag
+                            :: (instruction
+                                  (Wasm_ast.Event Code_generation.hidden_location)
+                               @ instruction (Wasm_ast.Br (i + 1, Some (Pop ty))))))
+                        catches))
+          ]
     | ExternConvertAny e' -> [ List (Atom "extern.convert_any" :: expression e') ]
   and instruction i =
     match i with
@@ -516,8 +520,14 @@ let expression_or_instructions ctx st in_function =
             | None -> []
             | Some e -> expression e))
         ]
-    | Throw (tag, e) -> [ List (Atom "throw" :: index st.tag_names tag :: expression e) ]
-    | Rethrow i -> [ List [ Atom "rethrow"; Atom (string_of_int i) ] ]
+    | Throw (tag, e) ->
+        if Config.Flag.trap_on_exception ()
+        then [ List [ Atom "unreachable" ] ]
+        else [ List (Atom "throw" :: index st.tag_names tag :: expression e) ]
+    | Rethrow i ->
+        if Config.Flag.trap_on_exception ()
+        then [ List [ Atom "unreachable" ] ]
+        else [ List [ Atom "rethrow"; Atom (string_of_int i) ] ]
     | CallInstr (f, l) ->
         [ List
             (Atom "call"
diff --git a/compiler/lib-wasm/wat_preprocess.ml b/compiler/lib-wasm/wat_preprocess.ml
index c3ef91b88a..b468a550a4 100644
--- a/compiler/lib-wasm/wat_preprocess.ml
+++ b/compiler/lib-wasm/wat_preprocess.ml
@@ -416,6 +416,69 @@ let rec rewrite_list st l = List.iter ~f:(rewrite st) l
 
 and rewrite st elt =
   match elt with
+  | { desc =
+        List
+          ({ desc = Atom "try"; _ }
+          :: { desc = List ({ desc = Atom "result"; _ } :: _)
+             ; loc = pos_before_result, pos_after_result
+             }
+          :: { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+             ; loc = _, pos_after_body
+             }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(block";
+      skip st pos_before_result;
+      write st pos_after_result;
+      skip st pos_after_do;
+      rewrite_list st body;
+      write st pos_after_body;
+      skip st pos'
+  | { desc =
+        List
+          ({ desc = Atom "try"; _ }
+          :: { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+             ; loc = _, pos_after_body
+             }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(block";
+      skip st pos_after_do;
+      rewrite_list st body;
+      write st pos_after_body;
+      skip st pos'
+  | { desc = List ({ desc = Atom "throw"; _ } :: _); loc = pos, pos' }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(unreachable)";
+      skip st pos'
+  | { desc = List ({ desc = Atom "tag"; _ } :: _); loc = pos, pos' }
+  | { desc =
+        List
+          ({ desc = Atom "import"; _ }
+          :: _
+          :: _
+          :: { desc = List ({ desc = Atom "tag"; _ } :: _); _ }
+          :: _)
+    ; loc = pos, pos'
+    }
+  | { desc =
+        List
+          ({ desc = Atom "export"; _ }
+          :: _
+          :: { desc = List ({ desc = Atom "tag"; _ } :: _); _ }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      skip st pos'
   | { desc =
         List
           [ { desc = Atom "@if"; _ }
diff --git a/compiler/lib/build_info.ml b/compiler/lib/build_info.ml
index 4de9956edf..b5da01bbb4 100644
--- a/compiler/lib/build_info.ml
+++ b/compiler/lib/build_info.ml
@@ -68,6 +68,7 @@ let create kind =
     | v -> Printf.sprintf "%s+%s" Compiler_version.s v
   in
   [ "use-js-string", string_of_bool (Config.Flag.use_js_string ())
+  ; "wasi", string_of_bool (Config.Flag.wasi ())
   ; "effects", string_of_effects_backend (Config.effects ())
   ; "version", version
   ; "kind", string_of_kind kind
@@ -139,9 +140,9 @@ let merge fname1 info1 fname2 info2 =
         match k, v1, v2 with
         | "kind", v1, v2 ->
             if Option.equal String.equal v1 v2 then v1 else Some (string_of_kind `Unknown)
-        | ("effects" | "use-js-string" | "version"), Some v1, Some v2
+        | ("effects" | "use-js-string" | "wasi" | "version"), Some v1, Some v2
           when String.equal v1 v2 -> Some v1
-        | (("effects" | "use-js-string" | "version") as key), v1, v2 ->
+        | (("effects" | "use-js-string" | "wasi" | "version") as key), v1, v2 ->
             raise
               (Incompatible_build_info { key; first = fname1, v1; second = fname2, v2 })
         | _, Some v1, Some v2 when String.equal v1 v2 -> Some v1
@@ -156,7 +157,7 @@ let configure t =
   StringMap.iter
     (fun k v ->
       match k with
-      | "use-js-string" -> Config.Flag.set k (bool_of_string v)
+      | "use-js-string" | "wasi" -> Config.Flag.set k (bool_of_string v)
       | "effects" -> Config.set_effects_backend (effects_backend_of_string v)
       | _ -> ())
     t
diff --git a/compiler/lib/config.ml b/compiler/lib/config.ml
index 15488319a4..2ec1b9dd61 100644
--- a/compiler/lib/config.ml
+++ b/compiler/lib/config.ml
@@ -101,6 +101,10 @@ module Flag = struct
   let auto_link = o ~name:"auto-link" ~default:true
 
   let es6 = o ~name:"es6" ~default:false
+
+  let wasi = o ~name:"wasi" ~default:false
+
+  let trap_on_exception = o ~name:"trap-on-exception" ~default:false
 end
 
 module Param = struct
diff --git a/compiler/lib/config.mli b/compiler/lib/config.mli
index 71642430bf..a05274a2bd 100644
--- a/compiler/lib/config.mli
+++ b/compiler/lib/config.mli
@@ -76,6 +76,10 @@ module Flag : sig
 
   val es6 : unit -> bool
 
+  val wasi : unit -> bool
+
+  val trap_on_exception : unit -> bool
+
   val enable : string -> unit
 
   val disable : string -> unit
diff --git a/compiler/lib/driver.ml b/compiler/lib/driver.ml
index 484489069e..28828e6e89 100644
--- a/compiler/lib/driver.ml
+++ b/compiler/lib/driver.ml
@@ -691,8 +691,9 @@ let optimize ~profile p =
     +> map_fst
          (match Config.target (), Config.effects () with
          | `JavaScript, `Disabled -> Generate_closure.f
-         | `JavaScript, (`Cps | `Double_translation) | `Wasm, (`Jspi | `Cps) -> Fun.id
-         | `JavaScript, `Jspi | `Wasm, (`Disabled | `Double_translation) -> assert false)
+         | `JavaScript, (`Cps | `Double_translation) | `Wasm, (`Disabled | `Jspi | `Cps)
+           -> Fun.id
+         | `JavaScript, `Jspi | `Wasm, `Double_translation -> assert false)
     +> map_fst deadcode'
   in
   if times () then Format.eprintf "Start Optimizing...@.";
diff --git a/compiler/lib/inline.ml b/compiler/lib/inline.ml
index b4f5cdcd44..1d5c78f934 100644
--- a/compiler/lib/inline.ml
+++ b/compiler/lib/inline.ml
@@ -306,7 +306,7 @@ let times = Debug.find "times"
 
 let stats = Debug.find "stats"
 
-let f p live_vars =
+let f p (live_vars : Deadcode.variable_uses) =
   let inline_count = ref 0 in
   Code.invariant p;
   let t = Timer.make () in
diff --git a/compiler/tests-jsoo/dune b/compiler/tests-jsoo/dune
index a8daf4e0a9..bafb9378b5 100644
--- a/compiler/tests-jsoo/dune
+++ b/compiler/tests-jsoo/dune
@@ -11,6 +11,8 @@
  (enabled_if
   (>= %{ocaml_version} 4.14))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -22,6 +24,8 @@
  (enabled_if
   (>= %{ocaml_version} 5.1.1))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -33,6 +37,22 @@
  (enabled_if
   (>= %{ocaml_version} 5.1.1))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
+  (modes js wasm best))
+ (preprocess
+  (pps ppx_expect)))
+
+(library
+ (name jsoo_testsuite_perms)
+ (modules test_unix_perms)
+ (libraries unix)
+ ;; WASI has no notion of file permissions (it uses capabilities instead)
+ (enabled_if
+  (<> %{profile} wasi))
+ (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -47,6 +67,7 @@
    test_float16
    test_marshal_compressed
    test_parsing
+   test_unix_perms
    calc_parser
    calc_lexer))
  (libraries unix compiler-libs.common js_of_ocaml-compiler)
@@ -54,6 +75,8 @@
   (language c)
   (names bigarray_stubs jsoo_runtime_stubs))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
diff --git a/compiler/tests-jsoo/lib-effects/dune b/compiler/tests-jsoo/lib-effects/dune
index be8029f8b1..adad4af0e3 100644
--- a/compiler/tests-jsoo/lib-effects/dune
+++ b/compiler/tests-jsoo/lib-effects/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable effects))))
  (_
   (js_of_ocaml
    (flags
@@ -11,6 +15,8 @@
  (enabled_if
   (>= %{ocaml_version} 5))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (modules
   (:standard
diff --git a/compiler/tests-jsoo/test_unix.ml b/compiler/tests-jsoo/test_unix.ml
index aa25c0baad..3546260463 100644
--- a/compiler/tests-jsoo/test_unix.ml
+++ b/compiler/tests-jsoo/test_unix.ml
@@ -14,85 +14,6 @@ let%expect_test "Unix.times" =
   then Printf.printf "OK\n";
   [%expect {| OK |}]
 
-let on_windows = Sys.os_type = "Win32"
-
-let%expect_test "Unix.chmod / Unix.fchmod / Unix.access" =
-  let tmp = Filename.temp_file "a" "txt" in
-  let test ?(ok_on_windows = false) flags =
-    try
-      Unix.access tmp flags;
-      if on_windows && ok_on_windows
-      then Printf.printf "denied (success on Windows)\n"
-      else Printf.printf "success\n"
-    with
-    | Unix.Unix_error ((EPERM | EACCES), _, _) ->
-        if (not on_windows) && ok_on_windows
-        then Printf.printf "denied (success on Windows)\n"
-        else Printf.printf "denied\n"
-    | Unix.Unix_error (ENOENT, _, _) -> Printf.printf "absent\n"
-  in
-  let touch perms =
-    Unix.chmod tmp 0o600;
-    Unix.unlink tmp;
-    let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] perms in
-    Unix.close fd
-  in
-  let test_perms set =
-    set 0o200;
-    test ~ok_on_windows:true [ R_OK ];
-    test [ W_OK ];
-    test ~ok_on_windows:true [ R_OK; W_OK ];
-    [%expect
-      {|
-      denied (success on Windows)
-      success
-      denied (success on Windows)
-      |}];
-    set 0o400;
-    test [ R_OK ];
-    test [ W_OK ];
-    test [ R_OK; W_OK ];
-    [%expect {|
-    success
-    denied
-    denied |}];
-    set 0o600;
-    test [ R_OK ];
-    test [ W_OK ];
-    test [ R_OK; W_OK ];
-    [%expect {|
-    success
-    success
-    success |}];
-    set 0o000;
-    test ~ok_on_windows:true [ R_OK ];
-    test [ W_OK ];
-    test [ R_OK; W_OK ];
-    [%expect {|
-      denied (success on Windows)
-      denied
-      denied
-      |}]
-  in
-  test [ F_OK ];
-  [%expect {|
-    success |}];
-  Unix.chmod tmp 0o600;
-  Unix.unlink tmp;
-  test [ F_OK ];
-  [%expect {|
-    absent |}];
-  let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] 0o666 in
-  test [ F_OK ];
-  [%expect {|
-    success |}];
-  if not on_windows then test_perms (Unix.fchmod fd);
-  Unix.close fd;
-  test_perms (Unix.chmod tmp);
-  test_perms touch;
-  Unix.chmod tmp 0o600;
-  Unix.unlink tmp
-
 let%expect_test "Unix.link" =
   let tmp = Filename.temp_file "a" "txt" in
   let ch = open_out tmp in
diff --git a/compiler/tests-jsoo/test_unix_perms.ml b/compiler/tests-jsoo/test_unix_perms.ml
new file mode 100644
index 0000000000..8f07952db9
--- /dev/null
+++ b/compiler/tests-jsoo/test_unix_perms.ml
@@ -0,0 +1,78 @@
+let on_windows = Sys.os_type = "Win32"
+
+let%expect_test "Unix.chmod / Unix.fchmod / Unix.access" =
+  let tmp = Filename.temp_file "a" "txt" in
+  let test ?(ok_on_windows = false) flags =
+    try
+      Unix.access tmp flags;
+      if on_windows && ok_on_windows
+      then Printf.printf "denied (success on Windows)\n"
+      else Printf.printf "success\n"
+    with
+    | Unix.Unix_error ((EPERM | EACCES), _, _) ->
+        if (not on_windows) && ok_on_windows
+        then Printf.printf "denied (success on Windows)\n"
+        else Printf.printf "denied\n"
+    | Unix.Unix_error (ENOENT, _, _) -> Printf.printf "absent\n"
+  in
+  let touch perms =
+    Unix.chmod tmp 0o600;
+    Unix.unlink tmp;
+    let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] perms in
+    Unix.close fd
+  in
+  let test_perms set =
+    set 0o200;
+    test ~ok_on_windows:true [ R_OK ];
+    test [ W_OK ];
+    test ~ok_on_windows:true [ R_OK; W_OK ];
+    [%expect
+      {|
+      denied (success on Windows)
+      success
+      denied (success on Windows)
+      |}];
+    set 0o400;
+    test [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+    success
+    denied
+    denied |}];
+    set 0o600;
+    test [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+    success
+    success
+    success |}];
+    set 0o000;
+    test ~ok_on_windows:true [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+      denied (success on Windows)
+      denied
+      denied
+      |}]
+  in
+  test [ F_OK ];
+  [%expect {|
+    success |}];
+  Unix.chmod tmp 0o600;
+  Unix.unlink tmp;
+  test [ F_OK ];
+  [%expect {|
+    absent |}];
+  let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] 0o666 in
+  test [ F_OK ];
+  [%expect {|
+    success |}];
+  if not on_windows then test_perms (Unix.fchmod fd);
+  Unix.close fd;
+  test_perms (Unix.chmod tmp);
+  test_perms touch;
+  Unix.chmod tmp 0o600;
+  Unix.unlink tmp
diff --git a/compiler/tests-ocaml/basic-io-2/dune b/compiler/tests-ocaml/basic-io-2/dune
index 121f745198..e666404c1f 100644
--- a/compiler/tests-ocaml/basic-io-2/dune
+++ b/compiler/tests-ocaml/basic-io-2/dune
@@ -1,5 +1,8 @@
 (tests
  (names io)
  (modes js wasm)
+ ;; Sys.command not available
+ (enabled_if
+  (<> %{profile} wasi))
  (action
   (run node %{test} %{dep:test-file-short-lines})))
diff --git a/compiler/tests-ocaml/effect-syntax/dune b/compiler/tests-ocaml/effect-syntax/dune
index 22b9fdf7e4..d7e7e2fa3b 100644
--- a/compiler/tests-ocaml/effect-syntax/dune
+++ b/compiler/tests-ocaml/effect-syntax/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable=effects))))
  (_
   (js_of_ocaml
    (flags
diff --git a/compiler/tests-ocaml/effects/dune b/compiler/tests-ocaml/effects/dune
index 019935b596..bb7cc86051 100644
--- a/compiler/tests-ocaml/effects/dune
+++ b/compiler/tests-ocaml/effects/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable=effects))))
  (_
   (js_of_ocaml
    (flags
diff --git a/compiler/tests-ocaml/lib-channels/close_in.ml b/compiler/tests-ocaml/lib-channels/close_in.ml
index 9b3717362a..8697d78c6a 100644
--- a/compiler/tests-ocaml/lib-channels/close_in.ml
+++ b/compiler/tests-ocaml/lib-channels/close_in.ml
@@ -6,8 +6,14 @@
    between 1 and IO_BUFFER_SIZE *)
 let nb_bytes = 3
 
+let temp_file =
+  let name, ch = Filename.open_temp_file "data" ".txt" in
+  output_string ch (String.make 1024 'a');
+  close_out ch;
+  name
+
 let () =
-  let ic = open_in_bin (Filename.basename Sys.argv.(0)) in
+  let ic = open_in_bin temp_file in
   seek_in ic nb_bytes;
   close_in ic;
   assert (
@@ -21,7 +27,7 @@ let () =
 
 (* A variant of #11878, which #11965 failed to fix. *)
 let () =
-  let ic = open_in_bin (Filename.basename Sys.argv.(0)) in
+  let ic = open_in_bin temp_file in
   close_in ic;
   begin try
     seek_in ic (-1);
diff --git a/compiler/tests-ocaml/lib-digest/dune b/compiler/tests-ocaml/lib-digest/dune
index 3ba1799930..19fe2dce08 100644
--- a/compiler/tests-ocaml/lib-digest/dune
+++ b/compiler/tests-ocaml/lib-digest/dune
@@ -8,6 +8,8 @@
  (names digests)
  (libraries)
  (build_if
-  (>= %{ocaml_version} 5.2))
+  (and
+   (>= %{ocaml_version} 5.2)
+   (<> %{profile} wasi)))
  (modules digests)
  (modes js wasm))
diff --git a/compiler/tests-ocaml/lib-marshal/intext.ml b/compiler/tests-ocaml/lib-marshal/intext.ml
index 3e0477dffd..5340806495 100644
--- a/compiler/tests-ocaml/lib-marshal/intext.ml
+++ b/compiler/tests-ocaml/lib-marshal/intext.ml
@@ -4,7 +4,8 @@
 
 (* Test for output_value / input_value *)
 
-let max_data_depth = 500000
+let max_data_depth = 10000
+(* Reduced since we use a quadratic algorithm for sharing in the WASI runtime *)
 
 type t = A | B of int | C of float | D of string | E of char
        | F of t | G of t * t | H of int * t | I of t * float | J
diff --git a/compiler/tests-ocaml/lib-marshal/intext_par.ml b/compiler/tests-ocaml/lib-marshal/intext_par.ml
index f93c55c685..fbf0a8dec9 100644
--- a/compiler/tests-ocaml/lib-marshal/intext_par.ml
+++ b/compiler/tests-ocaml/lib-marshal/intext_par.ml
@@ -17,7 +17,8 @@ let test_size =
 
 let num_domains = 1 lsl test_size
 
-let max_data_depth = 500000
+let max_data_depth = 10000
+(* Reduced since we use a quadratic algorithm for sharing in the WASI runtime *)
 
 type t = A | B of int | C of float | D of string | E of char
        | F of t | G of t * t | H of int * t | I of t * float | J
diff --git a/compiler/tests-ocaml/lib-unix/isatty/dune b/compiler/tests-ocaml/lib-unix/isatty/dune
index 6740efe55b..852dd49d6a 100644
--- a/compiler/tests-ocaml/lib-unix/isatty/dune
+++ b/compiler/tests-ocaml/lib-unix/isatty/dune
@@ -6,7 +6,10 @@
 (tests
  (names isatty_tty)
  (enabled_if
-  (not %{env:CI=false}))
+  (and
+   (<> %{profile} wasi)
+   (not %{env:CI=false})))
+ ; WASI has no notion of tty
  ; isatty_tty does not work on the CI since we are not running in a tty there
  (libraries ocaml_testing unix)
  (modes js wasm))
diff --git a/dune b/dune
index a4064b14a9..21183b243c 100644
--- a/dune
+++ b/dune
@@ -30,6 +30,23 @@
   (binaries
    (tools/node_wrapper.exe as node)
    (tools/node_wrapper.exe as node.exe)))
+ (wasi
+  (wasm_of_ocaml
+   (build_runtime_flags
+    (:standard
+     --enable
+     wasi
+     (:include wasi_extra_flags)))
+   (flags
+    (:standard
+     --pretty
+     --enable
+     wasi
+     (:include wasi_extra_flags)))
+   (compilation_mode separate))
+  (binaries
+   (tools/node_wrapper.exe as node)
+   (tools/node_wrapper.exe as node.exe)))
  (bench_no_debug
   (flags
    (:standard \ -g))
@@ -55,6 +72,13 @@
     %{dep:VERSION}
     %{dep:tools/version/GIT-VERSION}))))
 
+(rule
+ (targets wasi_extra_flags)
+ (action
+  (with-stdout-to
+   %{targets}
+   (echo "(%{env:WASI_FLAGS=})"))))
+
 (data_only_dirs _wikidoc doc-dev janestreet)
 
 (vendored_dirs)
diff --git a/lib/deriving_json/tests/dune b/lib/deriving_json/tests/dune
index c1e0147b3d..b7772e347e 100644
--- a/lib/deriving_json/tests/dune
+++ b/lib/deriving_json/tests/dune
@@ -2,6 +2,8 @@
  (name deriving_expect_tests)
  (libraries unix js_of_ocaml js_of_ocaml.deriving)
  (inline_tests
+  (enabled_if
+   (<> %{profile} wasi))
   (modes js wasm))
  (preprocess
   (pps ppx_expect ppx_deriving_json)))
diff --git a/lib/tests/dune.inc b/lib/tests/dune.inc
index 7abcb2b003..2025f19ffc 100644
--- a/lib/tests/dune.inc
+++ b/lib/tests/dune.inc
@@ -2,7 +2,7 @@
 (library
  ;; lib/tests/test_css_angle.ml
  (name test_css_angle_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_angle)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -12,7 +12,7 @@
 (library
  ;; lib/tests/test_css_color.ml
  (name test_css_color_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_color)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -22,7 +22,7 @@
 (library
  ;; lib/tests/test_css_length.ml
  (name test_css_length_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_length)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -42,7 +42,7 @@
 (library
  ;; lib/tests/test_fun_call_2.ml
  (name test_fun_call_2_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_fun_call_2)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -52,7 +52,7 @@
 (library
  ;; lib/tests/test_json.ml
  (name test_json_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_json)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -62,7 +62,7 @@
 (library
  ;; lib/tests/test_nodejs_filesystem_errors.ml
  (name test_nodejs_filesystem_errors_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_nodejs_filesystem_errors)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -82,7 +82,7 @@
 (library
  ;; lib/tests/test_poly_equal.ml
  (name test_poly_equal_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_poly_equal)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -92,7 +92,7 @@
 (library
  ;; lib/tests/test_regexp.ml
  (name test_regexp_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_regexp)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -112,7 +112,7 @@
 (library
  ;; lib/tests/test_typed_array.ml
  (name test_typed_array_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_typed_array)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -122,7 +122,7 @@
 (library
  ;; lib/tests/test_unsafe_set_get.ml
  (name test_unsafe_set_get_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_unsafe_set_get)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -132,7 +132,7 @@
 (library
  ;; lib/tests/test_url.ml
  (name test_url_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_url)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
diff --git a/lib/tests/gen-rules/gen.ml b/lib/tests/gen-rules/gen.ml
index 8cc26a522b..d928746e96 100644
--- a/lib/tests/gen-rules/gen.ml
+++ b/lib/tests/gen-rules/gen.ml
@@ -81,7 +81,8 @@ let () =
            basename
            (Hashtbl.hash prefix mod 100)
            (match enabled_if basename with
-           | Any | Not_wasm -> "true"
+           | Any -> "(<> %{profile} wasi)"
+           | Not_wasm -> "true"
            | GE5 -> "(>= %{ocaml_version} 5)"
            | No_effects_not_wasm -> "(<> %{profile} with-effects)")
            basename
diff --git a/runtime/wasm/backtrace.wat b/runtime/wasm/backtrace.wat
index 9b63e4e554..25282323a4 100644
--- a/runtime/wasm/backtrace.wat
+++ b/runtime/wasm/backtrace.wat
@@ -16,12 +16,22 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "fail" "caml_invalid_argument"
+      (func $caml_invalid_argument (param (ref eq))))
+(@if wasi
+(@then
+   (global $backtrace_status (mut (ref eq)) (ref.i31 (i32.const 0)))
+   (func $backtrace_status (result (ref eq))
+      (global.get $backtrace_status))
+   (func $record_backtrace (param $b (ref eq))
+      (global.set $backtrace_status (local.get $b)))
+)
+(@else
    (import "bindings" "backtrace_status"
       (func $backtrace_status (result (ref eq))))
    (import "bindings" "record_backtrace"
       (func $record_backtrace (param (ref eq))))
-   (import "fail" "caml_invalid_argument"
-      (func $caml_invalid_argument (param (ref eq))))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
diff --git a/runtime/wasm/bigarray.wat b/runtime/wasm/bigarray.wat
index 2322ccf192..c468ada3f6 100644
--- a/runtime/wasm/bigarray.wat
+++ b/runtime/wasm/bigarray.wat
@@ -16,6 +16,497 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "fail" "caml_bound_error" (func $caml_bound_error))
+   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
+   (import "fail" "caml_invalid_argument"
+      (func $caml_invalid_argument (param (ref eq))))
+   (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
+   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
+   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
+   (import "int32" "caml_copy_int32"
+      (func $caml_copy_int32 (param i32) (result (ref eq))))
+   (import "int32" "Int32_val"
+      (func $Int32_val (param (ref eq)) (result i32)))
+   (import "int32" "caml_copy_nativeint"
+      (func $caml_copy_nativeint (param i32) (result (ref eq))))
+   (import "int64" "caml_copy_int64"
+      (func $caml_copy_int64 (param i64) (result (ref eq))))
+   (import "int64" "Int64_val"
+      (func $Int64_val (param (ref eq)) (result i64)))
+   (import "obj" "double_array_tag" (global $double_array_tag i32))
+   (import "compare" "unordered" (global $unordered i32))
+   (import "hash" "caml_hash_mix_int"
+      (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
+   (import "hash" "caml_hash_mix_int64"
+      (func $caml_hash_mix_int64 (param i32) (param i64) (result i32)))
+   (import "hash" "caml_hash_mix_double"
+      (func $caml_hash_mix_double (param i32) (param f64) (result i32)))
+   (import "hash" "caml_hash_mix_float"
+      (func $caml_hash_mix_float (param i32) (param f32) (result i32)))
+   (import "hash" "caml_hash_mix_float16"
+      (func $caml_hash_mix_float16 (param i32) (param i32) (result i32)))
+   (import "marshal" "caml_serialize_int_1"
+      (func $caml_serialize_int_1 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_2"
+      (func $caml_serialize_int_2 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_4"
+      (func $caml_serialize_int_4 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_8"
+      (func $caml_serialize_int_8 (param (ref eq)) (param i64)))
+   (import "marshal" "caml_deserialize_uint_1"
+      (func $caml_deserialize_uint_1 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_sint_1"
+      (func $caml_deserialize_sint_1 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_uint_2"
+      (func $caml_deserialize_uint_2 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_sint_2"
+      (func $caml_deserialize_sint_2 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_int_4"
+      (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_int_8"
+      (func $caml_deserialize_int_8 (param (ref eq)) (result i64)))
+
+(@if wasi
+(@then
+   (type $i32_array (array (mut i32)))
+   (type $i16_array (array (mut i16)))
+   (type $f32_array (array (mut f32)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func $ta_create (export "ta_create")
+      (param $kind i32) (param $sz i32) (result (ref extern))
+      (local $a (ref array))
+      (local.set $a
+       (block $cont (result (ref array))
+        (block $f32
+         (block $f64
+          (block $i8
+           (block $i16
+            (block $i32
+             (br_table
+                $f32 $f64 $i8 $i8 $i16 $i16 $i32
+                $i32 $i32 $i32 $f32 $f64 $i8 $i16
+                (local.get $kind)))
+            ;; i32
+            (br $cont (array.new $i32_array (i32.const 0) (local.get $sz))))
+           ;; i16
+           (br $cont (array.new $i16_array (i32.const 0) (local.get $sz))))
+          ;; i8
+          (br $cont (array.new $bytes (i32.const 0) (local.get $sz))))
+         ;; f64
+         (br $cont (array.new $float_array (f64.const 0) (local.get $sz))))
+        ;; f32
+        (array.new $f32_array (f32.const 0) (local.get $sz))))
+      (extern.convert_any
+         (struct.new $data (local.get $a) (i32.const 0) (local.get $sz))))
+
+   (func $ta_length (export "ta_length") (param $b (ref extern)) (result i32)
+      (struct.get $data $len
+         (ref.cast (ref $data) (any.convert_extern (local.get $b)))))
+
+   (func $ta_get_f64 (param $b (ref extern)) (param $i i32) (result f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get $float_array
+         (ref.cast (ref $float_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_f32 (param $b (ref extern)) (param $i i32) (result f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (f64.promote_f32
+         (array.get $f32_array
+            (ref.cast (ref $f32_array) (struct.get $data $array (local.get $d)))
+            (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))))
+
+   (func $ta_get_i32 (export "ta_get_i32")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get $i32_array
+         (ref.cast (ref $i32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_i16 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_s $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_ui16 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_u $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_i8 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_s $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_ui8 (export "ta_get_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_u $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get32_ui8 (export "ta_get32_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (i32.or
+         (i32.or
+            (array.get_u $bytes (local.get $s) (local.get $i))
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 1)))
+               (i32.const 8)))
+         (i32.or
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 2)))
+               (i32.const 16))
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 3)))
+               (i32.const 24)))))
+
+   (func $ta_get16_ui8 (export "ta_get16_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (i32.or
+         (array.get_u $bytes (local.get $s) (local.get $i))
+         (i32.shl
+            (array.get_u $bytes (local.get $s)
+               (i32.add (local.get $i) (i32.const 1)))
+            (i32.const 8))))
+
+   (func $ta_set_f64 (param $b (ref extern)) (param $i i32) (param $v f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $float_array
+         (ref.cast (ref $float_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (local.get $v)))
+
+   (func $ta_set_f32 (param $b (ref extern)) (param $i i32) (param $v f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $f32_array
+         (ref.cast (ref $f32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (f32.demote_f64 (local.get $v))))
+
+   (func $ta_set_i32 (export "ta_set_i32")
+      (param $b (ref extern)) (param $i i32) (param $v i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i32_array
+         (ref.cast (ref $i32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (local.get $v)))
+
+   (func $ta_set_i16 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_ui16 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_i8 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_ui8 (export "ta_set_ui8")
+      (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set16_ui8
+      (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data)) (local $s (ref $bytes)) (local $j i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $j (i31.get_u (local.get $v)))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (array.set $bytes (local.get $s) (local.get $i) (local.get $j))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 1))
+         (i32.shr_u (local.get $j) (i32.const 8))))
+
+   (func $ta_set32_ui8 (param $b (ref extern)) (param $i i32) (param $v i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (array.set $bytes (local.get $s) (local.get $i) (local.get $v))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 1))
+         (i32.shr_u (local.get $v) (i32.const 8)))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 2))
+         (i32.shr_u (local.get $v) (i32.const 16)))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 3))
+         (i32.shr_u (local.get $v) (i32.const 24))))
+
+   (func $ta_fill_int (param $b (ref extern)) (param $v i32)
+      (local $d (ref $data))
+      (local $a (ref array))
+      (local $a32 (ref $i32_array)) (local $a16 (ref $i16_array))
+      (local $a8 (ref $bytes))
+      (local $ofs i32) (local $i i32) (local $len i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $a (struct.get $data $array (local.get $d)))
+      (local.set $ofs (struct.get $data $offset (local.get $d)))
+      (local.set $len (struct.get $data $len (local.get $d)))
+      (if (ref.test (ref $i32_array) (local.get $a))
+         (then
+            (local.set $a32 (ref.cast (ref $i32_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i32_array (local.get $a32)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $v))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+      (else (if (ref.test (ref $i16_array) (local.get $a))
+         (then
+            (local.set $a16 (ref.cast (ref $i16_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i16_array (local.get $a16)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $v))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+      (else
+         (local.set $a8 (ref.cast (ref $bytes) (local.get $a)))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $len))
+               (then
+                  (array.set $bytes (local.get $a8)
+                     (i32.add (local.get $ofs) (local.get $i))
+                     (local.get $v))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop)))))))))
+
+   (func $ta_fill_float (param $b (ref extern)) (param $f f64)
+      (local $d (ref $data))
+      (local $a (ref array))
+      (local $a64 (ref $float_array)) (local $a32 (ref $f32_array))
+      (local $f32 f32)
+      (local $ofs i32) (local $i i32) (local $len i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $a (struct.get $data $array (local.get $d)))
+      (local.set $ofs (struct.get $data $offset (local.get $d)))
+      (local.set $len (struct.get $data $len (local.get $d)))
+      (if (ref.test (ref $float_array) (local.get $a))
+         (then
+            (local.set $a64 (ref.cast (ref $float_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $float_array (local.get $a64)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $f))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+         (else
+            (local.set $a32 (ref.cast (ref $f32_array) (local.get $a)))
+            (local.set $f32 (f32.demote_f64 (local.get $f)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $f32_array (local.get $a32)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $f32))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))))
+
+   (func $ta_set (export "ta_set")
+      (param $d (ref extern)) (param $s (ref extern)) (param $do i32)
+      (local $sd (ref $data)) (local $sa (ref array)) (local $so i32)
+      (local $dd (ref $data)) (local $da (ref array))
+      (local $i i32) (local $len i32)
+      (local $sf64 (ref $float_array)) (local $df64 (ref $float_array))
+      (local $sf32 (ref $f32_array)) (local $df32 (ref $f32_array))
+      (local $si32 (ref $i32_array)) (local $di32 (ref $i32_array))
+      (local $si16 (ref $i16_array)) (local $di16 (ref $i16_array))
+      (local $si8 (ref $bytes)) (local $di8 (ref $bytes))
+      (local.set $sd (ref.cast (ref $data) (any.convert_extern (local.get $s))))
+      (local.set $sa (struct.get $data $array (local.get $sd)))
+      (local.set $so (struct.get $data $offset (local.get $sd)))
+      (local.set $len (struct.get $data $len (local.get $sd)))
+      (local.set $dd (ref.cast (ref $data) (any.convert_extern (local.get $d))))
+      (local.set $da (struct.get $data $array (local.get $dd)))
+      (local.set $do
+         (i32.add (struct.get $data $offset (local.get $dd)) (local.get $do)))
+      (if (ref.test (ref $float_array) (local.get $sa))
+         (then
+            (local.set $sf64 (ref.cast (ref $float_array) (local.get $sa)))
+            (local.set $df64 (ref.cast (ref $float_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $float_array (local.get $df64)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $float_array (local.get $sf64)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $f32_array) (local.get $sa))
+         (then
+            (local.set $sf32 (ref.cast (ref $f32_array) (local.get $sa)))
+            (local.set $df32 (ref.cast (ref $f32_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $f32_array (local.get $df32)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $f32_array (local.get $sf32)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $i32_array) (local.get $sa))
+         (then
+            (local.set $si32 (ref.cast (ref $i32_array) (local.get $sa)))
+            (local.set $di32 (ref.cast (ref $i32_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i32_array (local.get $di32)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $i32_array (local.get $si32)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $i16_array) (local.get $sa))
+         (then
+            (local.set $si16 (ref.cast (ref $i16_array) (local.get $sa)))
+            (local.set $di16 (ref.cast (ref $i16_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i16_array (local.get $di16)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $i16_array (local.get $si16)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $bytes) (local.get $sa))
+         (then
+            (local.set $si8 (ref.cast (ref $bytes) (local.get $sa)))
+            (local.set $di8 (ref.cast (ref $bytes) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $bytes (local.get $di8)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $bytes (local.get $si8)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))))
+
+   (func $ta_blit (param $s (ref extern)) (param $d (ref extern))
+      (return_call $ta_set (local.get $d) (local.get $s) (i32.const 0)))
+
+   (func $ta_subarray (export "ta_subarray")
+      (param $b (ref extern)) (param $s i32) (param $e i32) (result (ref extern))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (extern.convert_any
+         (struct.new $data
+            (struct.get $data $array (local.get $d))
+            (i32.add (struct.get $data $offset (local.get $d)) (local.get $s))
+            (i32.sub (local.get $e) (local.get $s)))))
+
+   (func $ta_blit_from_bytes (export "ta_blit_from_bytes")
+      (param $s (ref $bytes)) (param $so i32)
+      (param $b (ref extern)) (param $do i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local $d (ref $bytes))
+      (local $i i32)
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $d
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $do
+         (i32.add (local.get $do (struct.get $data $offset (local.get $data)))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $d)
+                  (i32.add (local.get $do) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $so) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $ta_blit_to_bytes (export "ta_blit_to_bytes")
+      (param $b (ref extern)) (param $so i32)
+      (param $d (ref $bytes)) (param $do i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local $s (ref $bytes))
+      (local $i i32)
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $so
+         (i32.add (local.get $so (struct.get $data $offset (local.get $data)))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $d)
+                  (i32.add (local.get $do) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $so) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+)
+(@else
    (import "bindings" "ta_create"
       (func $ta_create (param i32) (param i32) (result (ref extern))))
    (import "bindings" "ta_normalize"
@@ -77,55 +568,7 @@
       (func $ta_blit_to_bytes
          (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
          (param i32)))
-   (import "fail" "caml_bound_error" (func $caml_bound_error))
-   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
-   (import "fail" "caml_invalid_argument"
-      (func $caml_invalid_argument (param (ref eq))))
-   (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
-   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
-   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
-   (import "int32" "caml_copy_int32"
-      (func $caml_copy_int32 (param i32) (result (ref eq))))
-   (import "int32" "Int32_val"
-      (func $Int32_val (param (ref eq)) (result i32)))
-   (import "int32" "caml_copy_nativeint"
-      (func $caml_copy_nativeint (param i32) (result (ref eq))))
-   (import "int64" "caml_copy_int64"
-      (func $caml_copy_int64 (param i64) (result (ref eq))))
-   (import "int64" "Int64_val"
-      (func $Int64_val (param (ref eq)) (result i64)))
-   (import "obj" "double_array_tag" (global $double_array_tag i32))
-   (import "compare" "unordered" (global $unordered i32))
-   (import "hash" "caml_hash_mix_int"
-      (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
-   (import "hash" "caml_hash_mix_int64"
-      (func $caml_hash_mix_int64 (param i32) (param i64) (result i32)))
-   (import "hash" "caml_hash_mix_double"
-      (func $caml_hash_mix_double (param i32) (param f64) (result i32)))
-   (import "hash" "caml_hash_mix_float"
-      (func $caml_hash_mix_float (param i32) (param f32) (result i32)))
-   (import "hash" "caml_hash_mix_float16"
-      (func $caml_hash_mix_float16 (param i32) (param i32) (result i32)))
-   (import "marshal" "caml_serialize_int_1"
-      (func $caml_serialize_int_1 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_2"
-      (func $caml_serialize_int_2 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_4"
-      (func $caml_serialize_int_4 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_8"
-      (func $caml_serialize_int_8 (param (ref eq)) (param i64)))
-   (import "marshal" "caml_deserialize_uint_1"
-      (func $caml_deserialize_uint_1 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_sint_1"
-      (func $caml_deserialize_sint_1 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_uint_2"
-      (func $caml_deserialize_uint_2 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_sint_2"
-      (func $caml_deserialize_sint_2 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_int_4"
-      (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_int_8"
-      (func $caml_deserialize_int_8 (param (ref eq)) (result i64)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -831,6 +1274,8 @@
    (@string $ta_unsupported_kind "Typed_array.to_genarray: unsupported kind")
    (@string $ta_too_large "Typed_array.to_genarray: too large")
 
+(@if (not wasi)
+(@then
    (func (export "caml_ba_from_typed_array") (param (ref eq)) (result (ref eq))
       (local $data (ref extern))
       (local $kind i32)
@@ -859,6 +1304,7 @@
          (any.convert_extern
             (struct.get $bigarray $ba_data
                (ref.cast (ref $bigarray) (local.get 0))))))
+))
 
    (func $caml_ba_get_at_offset
       (param $ba (ref $bigarray)) (param $i i32) (result (ref eq))
@@ -2121,6 +2567,8 @@
          (i32.wrap_i64 (i64.shr_u (local.get $d) (i64.const 32))))
       (ref.i31 (i32.const 0)))
 
+(@if (not wasi)
+(@then
    (export "caml_bytes_of_uint8_array" (func $caml_string_of_uint8_array))
    (func $caml_string_of_uint8_array (export "caml_string_of_uint8_array")
       (param (ref eq)) (result (ref eq))
@@ -2152,6 +2600,7 @@
          (local.get $s) (i32.const 0) (local.get $ta) (i32.const 0)
          (local.get $len))
       (call $wrap (any.convert_extern (local.get $ta))))
+))
 
    (func (export "caml_ba_get_kind") (param (ref eq)) (result i32)
       (struct.get $bigarray $ba_kind (ref.cast (ref $bigarray) (local.get 0))))
diff --git a/runtime/wasm/bigstring.wat b/runtime/wasm/bigstring.wat
index 1d9afd2ae9..ec3b903b02 100644
--- a/runtime/wasm/bigstring.wat
+++ b/runtime/wasm/bigstring.wat
@@ -31,6 +31,31 @@
       (func $caml_ba_fill (param (ref eq)) (param (ref eq)) (result (ref eq))))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
+(@if wasi
+(@then
+   (import "bigarray" "ta_get_ui8"
+      (func $ta_get_ui8 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_get32_ui8"
+      (func $ta_get32_ui8 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_set_ui8"
+      (func $ta_set_ui8 (param (ref extern)) (param i32) (param (ref i31))))
+   (import "bigarray" "ta_subarray"
+      (func $ta_subarray
+         (param (ref extern)) (param i32) (param i32) (result (ref extern))))
+   (import "bigarray" "ta_set"
+      (func $ta_set (param (ref extern)) (param (ref extern)) (param i32)))
+   (import "bigarray" "ta_length"
+      (func $ta_length (param (ref extern)) (result i32)))
+   (import "bigarray" "ta_blit_from_bytes"
+      (func $ta_blit_from_bytes
+         (param (ref $bytes)) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
+   (import "bigarray" "ta_blit_to_bytes"
+      (func $ta_blit_to_bytes
+         (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
+         (param i32)))
+)
+(@else
    (import "bindings" "ta_create"
       (func $ta_create (param i32) (param anyref) (result anyref)))
    (import "bindings" "ta_get_ui8"
@@ -56,6 +81,7 @@
       (func $ta_blit_to_bytes
          (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
          (param i32)))
+))
    (import "hash" "caml_hash_mix_int"
       (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
 
@@ -98,6 +124,8 @@
          (local.set $h (call $caml_hash_mix_int (local.get $h) (local.get $w))))
       (i32.xor (local.get $h) (local.get $len)))
 
+(@if (not wasi)
+(@then
    (@string $buffer "buffer")
 
    (func (export "bigstring_to_array_buffer")
@@ -116,6 +144,7 @@
    (func (export "bigstring_of_typed_array") (param (ref eq)) (result (ref eq))
        (return_call $caml_ba_from_typed_array
           (call $wrap (call $ta_bytes (call $unwrap (local.get 0))))))
+))
 
    (func (export "caml_bigstring_memset")
       (param $s (ref eq)) (param $pos (ref eq)) (param $len (ref eq))
diff --git a/runtime/wasm/blake2.wat b/runtime/wasm/blake2.wat
index 25ad007838..214f941b67 100644
--- a/runtime/wasm/blake2.wat
+++ b/runtime/wasm/blake2.wat
@@ -1,5 +1,5 @@
 (module
-(@if (>= ocaml_version (5 2 0))
+(@if (and (>= ocaml_version (5 2 0)) (not wasi))
 (@then
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
diff --git a/runtime/wasm/compare.wat b/runtime/wasm/compare.wat
index b6a48a62b7..4b8805a4a7 100644
--- a/runtime/wasm/compare.wat
+++ b/runtime/wasm/compare.wat
@@ -479,6 +479,8 @@
                   (call $clear_compare_stack)
                   (call $caml_invalid_argument (global.get $abstract_value))
                   (ref.i31 (i32.const 0))))
+(@if (not wasi)
+(@then
                (drop (block $v1_not_js (result (ref eq))
                   (local.set $js1
                      (struct.get $js 0
@@ -506,6 +508,7 @@
                            (call $equals (local.get $js1) (local.get $js2)))
                         (return (global.get $unordered))))
                   (br $heterogeneous (ref.i31 (i32.const 0)))))
+))
                (if (call $caml_is_closure (local.get $v1))
                   (then
                      (drop (br_if $heterogeneous (ref.i31 (i32.const 0))
diff --git a/runtime/wasm/deps-wasi.json b/runtime/wasm/deps-wasi.json
new file mode 100644
index 0000000000..0a49660901
--- /dev/null
+++ b/runtime/wasm/deps-wasi.json
@@ -0,0 +1,15 @@
+[
+  {
+    "name": "root",
+    "reaches": ["start", "memory"],
+    "root": true
+  },
+  {
+    "name": "start",
+    "export": "_start"
+  },
+  {
+    "name": "memory",
+    "export": "memory"
+  }
+]
diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index a9305e7a41..521dba7637 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -17,6 +17,7 @@
    --binaryen=-g
    --binaryen-opt=-O3
    --set=effects=jspi
+   --disable=wasi
    --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
    %{target}
    %{read-lines:args})))
@@ -32,10 +33,46 @@
    --binaryen=-g
    --binaryen-opt=-O3
    --set=effects=cps
+   --disable=wasi
    --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
    %{target}
    %{read-lines:args})))
 
+(rule
+ (target runtime-wasi.wasm)
+ (deps
+  args
+  (glob_files *.wat)
+  libc.wasm)
+ (action
+  (run
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --set=effects=disabled
+   --enable=wasi
+   --allowed-imports=wasi_snapshot_preview1,OCaml
+   %{target}
+   libc:libc.wasm
+   %{read-lines:args})))
+
+(rule
+ (target runtime-cps-wasi.wasm)
+ (deps
+  args
+  (glob_files *.wat)
+  libc.wasm)
+ (action
+  (run
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --set=effects=cps
+   --enable=wasi
+   %{target}
+   libc:libc.wasm
+   %{read-lines:args})))
+
 (rule
  (target args)
  (deps
@@ -45,3 +82,34 @@
   (with-stdout-to
    %{target}
    (run ocaml %{deps}))))
+
+(rule
+ (target libc.new.wasm)
+ (deps libc.c)
+ (enabled_if
+  (not %{env:CI=false}))
+ (mode promote)
+ (action
+  (with-stdout-to
+   %{target}
+   (pipe-stdout
+    (run
+     docker
+     run
+     -v
+     .:/src
+     -w
+     /src
+     ghcr.io/webassembly/wasi-sdk
+     /opt/wasi-sdk/bin/clang
+     -O2
+     libc.c
+     -flto
+     -o
+     -)
+    (run wasm-opt -Oz --strip-debug --strip-dwarf - -o -)))))
+
+(rule
+ (alias recompile-libc)
+ (action
+  (cmp libc.wasm libc.new.wasm)))
diff --git a/runtime/wasm/effect-native.wat b/runtime/wasm/effect-native.wat
new file mode 100644
index 0000000000..b5ecb5254a
--- /dev/null
+++ b/runtime/wasm/effect-native.wat
@@ -0,0 +1,213 @@
+(module
+(@if (and wasi (<> effects "cps"))
+(@then
+   (import "fail" "caml_raise_constant"
+      (func $caml_raise_constant (param (ref eq))))
+   (import "fail" "caml_raise_with_arg"
+      (func $caml_raise_with_arg (param $tag (ref eq)) (param $arg (ref eq))))
+   (import "obj" "caml_fresh_oo_id"
+     (func $caml_fresh_oo_id (param (ref eq)) (result (ref eq))))
+   (import "obj" "cont_tag" (global $cont_tag i32))
+   (import "stdlib" "caml_named_value"
+      (func $caml_named_value (param (ref eq)) (result (ref null eq))))
+   (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
+   (import "jslib" "caml_wrap_exception"
+      (func $caml_wrap_exception (param externref) (result (ref eq))))
+   (import "stdlib" "caml_main_wrapper"
+      (global $caml_main_wrapper (mut (ref null $wrapper_func))))
+   (import "effect" "effect_allowed" (global $effect_allowed (mut i32)))
+
+   (type $block (array (mut (ref eq))))
+   (type $bytes (array (mut i8)))
+   (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
+   (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
+   (type $function_3
+      (func (param (ref eq) (ref eq) (ref eq) (ref eq)) (result (ref eq))))
+   (type $closure_3
+      (sub $closure
+         (struct (field (ref $function_1)) (field (ref $function_3)))))
+
+   ;; Effect types
+
+   (tag $effect (param (ref eq)) (result (ref eq) (ref eq)))
+
+   (type $cont_function (func (param (ref eq) (ref eq)) (result (ref eq))))
+
+   (type $cont (cont $cont_function))
+
+   (type $generic_fiber
+      (sub
+         (struct
+            (field $value (mut (ref eq)))
+            (field $exn (mut (ref eq)))
+            (field $effect (mut (ref eq))))))
+
+   (type $fiber
+      (sub final $generic_fiber
+         (struct
+            (field $value (mut (ref eq)))
+            (field $exn (mut (ref eq)))
+            (field $effect (mut (ref eq)))
+            (field $cont (mut (ref $cont))))))
+
+   ;; Unhandled effects
+
+   (@string $effect_unhandled "Effect.Unhandled")
+
+   (func $raise_unhandled
+      (param $eff (ref eq)) (param (ref eq)) (result (ref eq))
+      (block $null
+         (call $caml_raise_with_arg
+            (br_on_null $null
+               (call $caml_named_value (global.get $effect_unhandled)))
+            (local.get $eff)))
+      (call $caml_raise_constant
+         (array.new_fixed $block 3 (ref.i31 (i32.const 248))
+            (global.get $effect_unhandled)
+            (call $caml_fresh_oo_id (ref.i31 (i32.const 0)))))
+      (ref.i31 (i32.const 0)))
+
+   (global $raise_unhandled (ref $closure)
+      (struct.new $closure (ref.func $raise_unhandled)))
+
+   (type $func (func (result (ref eq))))
+   (type $wrapper_func (func (param (ref $func))))
+   (type $func_closure (struct (field (ref $func))))
+
+   (func $wrapper_cont
+      (param $f (ref eq)) (param (ref eq)) (result (ref eq))
+      (return_call_ref $func
+         (local.get $f)
+         (struct.get $func_closure 0
+            (ref.cast (ref $func_closure) (local.get $f)))))
+
+   (func $unhandled_effect_wrapper (param $start (ref $func))
+      (local $cont (ref $cont))
+      (local $f (ref eq)) (local $v (ref eq))
+      (local $resume_res (tuple (ref eq) (ref $cont)))
+      (local.set $cont (cont.new $cont (ref.func $wrapper_cont)))
+      (local.set $f (struct.new $func_closure (local.get $start)))
+      (local.set $v (ref.i31 (i32.const 0)))
+      (loop $loop
+         (local.set $resume_res
+            (block $handle_effect (result (ref eq) (ref $cont))
+               (resume $cont (on $effect $handle_effect)
+                  (local.get $f) (local.get $v) (local.get $cont))
+               (return)))
+         (local.set $cont (tuple.extract 2 1 (local.get $resume_res)))
+         (local.set $v (tuple.extract 2 0 (local.get $resume_res)))
+         (local.set $f (global.get $raise_unhandled))
+         (br $loop)))
+
+   (func $init
+      (global.set $caml_main_wrapper (ref.func $unhandled_effect_wrapper)))
+
+   (start $init)
+
+   ;; Resume
+
+   (@string $already_resumed "Effect.Continuation_already_resumed")
+
+   (func $resume (export "%resume")
+      (param $vfiber (ref eq)) (param $f (ref eq)) (param $v (ref eq))
+      (param $tail (ref eq)) (result (ref eq))
+      (local $fiber (ref $fiber))
+      (local $res (ref eq))
+      (local $exn (ref eq))
+      (local $resume_res (tuple (ref eq) (ref $cont)))
+      (if (ref.eq (local.get $vfiber) (ref.i31 (i32.const 0)))
+         (then
+            (call $caml_raise_constant
+               (ref.as_non_null
+                  (call $caml_named_value (global.get $already_resumed))))))
+      (local.set $fiber (ref.cast (ref $fiber) (local.get $vfiber)))
+      (local.set $exn
+         (block $handle_exception (result (ref eq))
+            (local.set $resume_res
+               (block $handle_effect (result (ref eq) (ref $cont))
+                  (local.set $res
+                     (try (result (ref eq))
+                        (do
+                           (resume $cont
+                               (on $effect $handle_effect)
+                               (local.get $f) (local.get $v)
+                               (struct.get $fiber $cont (local.get $fiber))))
+(@if (not wasi)
+(@then
+                        (catch $javascript_exception
+                           (br $handle_exception
+                              (call $caml_wrap_exception (pop externref))))
+))
+                        (catch $ocaml_exception
+                           (br $handle_exception (pop (ref eq))))))
+                  ;; handle return
+                  (return_call_ref $function_1 (local.get $res)
+                     (local.tee $f
+                        (struct.get $fiber $value (local.get $fiber)))
+                     (struct.get $closure 0
+                        (ref.cast (ref $closure) (local.get $f))))))
+            ;; handle effect
+            (struct.set $fiber $cont (local.get $fiber)
+               (tuple.extract 2 1 (local.get $resume_res)))
+            (return_call_ref $function_3
+               (tuple.extract 2 0 (local.get $resume_res))
+               (array.new_fixed $block 3 (ref.i31 (global.get $cont_tag))
+                  (local.get $fiber)
+                  (local.get $fiber))
+               (local.get $tail)
+               (local.tee $f
+                  (struct.get $fiber $effect (local.get $fiber)))
+               (struct.get $closure_3 1
+                  (ref.cast (ref $closure_3) (local.get $f))))))
+      ;; handle exception
+      (return_call_ref $function_1 (local.get $exn)
+         (local.tee $f
+            (struct.get $fiber $exn (local.get $fiber)))
+         (struct.get $closure 0 (ref.cast (ref $closure) (local.get $f)))))
+
+   ;; Perform
+
+   (func (export "%reperform")
+      (param $eff (ref eq)) (param $cont (ref eq)) (param $tail (ref eq))
+      (result (ref eq))
+      (local $res (tuple (ref eq) (ref eq)))
+      (local.set $res (suspend $effect (local.get $eff)))
+      (return_call $resume
+         (ref.as_non_null
+            (array.get $block
+               (ref.cast (ref $block) (local.get $cont))
+               (i32.const 1)))
+         (tuple.extract 2 0 (local.get $res))
+         (tuple.extract 2 1 (local.get $res))
+         (local.get $tail)))
+
+   (func (export "%perform") (param $eff (ref eq)) (result (ref eq))
+      (local $res (tuple (ref eq) (ref eq)))
+      (if (i32.eqz (global.get $effect_allowed))
+         (then
+            (return_call $raise_unhandled
+               (local.get $eff) (ref.i31 (i32.const 0)))))
+      (local.set $res (suspend $effect (local.get $eff)))
+      (return_call_ref $function_1 (tuple.extract 2 1 (local.get $res))
+         (tuple.extract 2 0 (local.get $res))
+         (struct.get $closure 0
+            (ref.cast (ref $closure) (tuple.extract 2 0 (local.get $res))))))
+
+   ;; Allocate a stack
+
+   (func $initial_cont
+      (param $f (ref eq)) (param $x (ref eq)) (result (ref eq))
+      (return_call_ref $function_1 (local.get $x)
+         (local.get $f)
+         (struct.get $closure 0 (ref.cast (ref $closure) (local.get $f)))))
+
+   (func (export "caml_alloc_stack")
+      (param $hv (ref eq)) (param $hx (ref eq)) (param $hf (ref eq))
+      (result (ref eq))
+      (struct.new $fiber
+         (local.get $hv) (local.get $hx) (local.get $hf)
+         (cont.new $cont (ref.func $initial_cont))))
+))
+)
diff --git a/runtime/wasm/effect.wat b/runtime/wasm/effect.wat
index 05bc0ad9c2..896cbe22ef 100644
--- a/runtime/wasm/effect.wat
+++ b/runtime/wasm/effect.wat
@@ -25,11 +25,20 @@
      (func $caml_fresh_oo_id (param (ref eq)) (result (ref eq))))
    (import "obj" "cont_tag" (global $cont_tag i32))
    (import "obj" "object_tag" (global $object_tag i32))
+   (import "obj" "caml_callback_1"
+      (func $caml_callback_1
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
    (import "stdlib" "caml_named_value"
       (func $caml_named_value (param (ref eq)) (result (ref null eq))))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
+(@if wasi
+(@then
+   (func $caml_wrap_exception (param externref) (result (ref eq))
+      (unreachable))
+)
+(@else
    (import "jslib" "caml_wrap_exception"
       (func $caml_wrap_exception (param externref) (result (ref eq))))
    (import "bindings" "start_fiber" (func $start_fiber (param (ref eq))))
@@ -38,9 +47,7 @@
          (param $f funcref) (param $env eqref) (result anyref)))
    (import "bindings" "resume_fiber"
       (func $resume_fiber (param externref) (param (ref eq))))
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -81,7 +88,7 @@
    (global $raise_unhandled (ref $closure)
       (struct.new $closure (ref.func $raise_unhandled)))
 
-   (global $effect_allowed (mut i32) (i32.const 1))
+   (global $effect_allowed (export "effect_allowed") (mut i32) (i32.const 1))
 
    (func $caml_continuation_use_noexc (export "caml_continuation_use_noexc")
       (param (ref eq)) (result (ref eq))
@@ -132,7 +139,7 @@
                (ref.i31 (global.get $cont_tag))))))
       (i32.const 0))
 
-(@if (= effects "jspi")
+(@if (and (not wasi) (= effects "jspi"))
 (@then
    ;; Apply a function f to a value v, both contained in a pair (f, v)
 
diff --git a/runtime/wasm/fail.wat b/runtime/wasm/fail.wat
index 04a6092a0e..2aa44adf42 100644
--- a/runtime/wasm/fail.wat
+++ b/runtime/wasm/fail.wat
@@ -18,7 +18,13 @@
 (module
    (import "stdlib" "caml_global_data"
       (global $caml_global_data (mut (ref $block))))
+(@if wasi
+(@then
+   (tag $javascript_exception (param externref))
+)
+(@else
    (import "bindings" "jstag" (tag $javascript_exception (param externref)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
diff --git a/runtime/wasm/float.wat b/runtime/wasm/float.wat
index b0bf76e609..18ee32cc19 100644
--- a/runtime/wasm/float.wat
+++ b/runtime/wasm/float.wat
@@ -16,12 +16,35 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if wasi
+(@then
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "format_float"
+      (func $format_float (param i32 i32 i32 f64) (result i32)))
+   (import "libc" "strtod" (func $strtod (param i32) (param i32) (result f64)))
+   (import "libc" "exp" (func $exp (param f64) (result f64)))
+   (import "wasi_memory" "checked_malloc"
+     (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "release_memory"
+      (func $release_memory (param i32 i32)))
+   (import "wasi_memory" "blit_string_to_memory"
+      (func $blit_string_to_memory (param i32 (ref $bytes))))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+)
+(@else
    (import "bindings" "format_float"
       (func $format_float
          (param i32) (param i32) (param i32) (param f64) (result anyref)))
    (import "bindings" "identity"
       (func $parse_float (param anyref) (result f64)))
    (import "Math" "exp" (func $exp (param f64) (result f64)))
+))
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
@@ -247,6 +270,49 @@
    (global $inf (ref $chars)
       (array.new_fixed $chars 3 (@char "i") (@char "n") (@char "f")))
 
+(@if wasi
+(@then
+   (func (export "caml_format_float")
+      (param $vfmt (ref eq)) (param $arg (ref eq)) (result (ref eq))
+      (local $fmt (ref $bytes)) (local $res (ref $bytes))
+      (local $d f64)
+      (local $buffer i32) (local $out_buffer i32)
+      (local $fmt_len i32) (local $avail i32) (local $len i32)
+      (local.set $fmt (ref.cast (ref $bytes) (local.get $vfmt)))
+      (local.set $d
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $arg))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $fmt_len (array.len (local.get $fmt)))
+      (call $blit_string_to_memory (local.get $buffer) (local.get $fmt))
+      (i32.store8
+         (i32.add (local.get $buffer) (local.get $fmt_len)) (i32.const 0))
+      (local.set $out_buffer
+         (i32.add (local.get $buffer)
+            (i32.add (local.get $fmt_len) (i32.const 1))))
+      (local.set $avail
+         (i32.sub (global.get $IO_BUFFER_SIZE) (local.get $fmt_len)))
+      (local.set $len
+         (call $format_float
+            (local.get $out_buffer) (local.get $avail)
+            (local.get $buffer) (local.get $d)))
+      (if (i32.ge_u (local.get $len) (local.get $avail))
+         (then
+            (local.set $out_buffer
+               (call $checked_malloc (i32.add (local.get $len) (i32.const 1))))
+            (drop
+               (call $format_float
+                  (local.get $out_buffer)
+                  (i32.add (local.get $len) (i32.const 1))
+                  (local.get $buffer) (local.get $d)))))
+      (local.set $res
+         (call $blit_memory_to_string (local.get $out_buffer) (local.get $len)))
+      (if (i32.ge_u (local.get $len) (local.get $avail))
+         (then
+            (call $free (local.get $out_buffer))))
+      (local.get $res)
+  )
+)
+(@else
    (func (export "caml_format_float")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (local $f f64) (local $b i64) (local $format (tuple i32 i32 i32 i32))
@@ -329,6 +395,7 @@
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $uppercase (i32.lt_u (local.get $i) (local.get $len))))))
       (local.get $s))
+))
 
    (@string $float_of_string "float_of_string")
 
@@ -485,6 +552,7 @@
       (local $s' (ref $bytes))
       (local $negative i32) (local $c i32)
       (local $f f64)
+      (local $buffer i32) (local $buf i32)
       (local.set $s (ref.cast (ref $bytes) (local.get 0)))
       (local.set $len (array.len (local.get $s)))
       (loop $count
@@ -651,9 +719,26 @@
                                                 (f64.const inf)
                                                 (local.get $negative))))
                                        ))))))))))))))))))
+(@if wasi
+(@then
+         (local.set $buffer (call $get_buffer))
+         (local.set $buf
+            (call $write_string_to_memory
+               (i32.add (local.get $buffer) (i32.const 4))
+               (global.get $IO_BUFFER_SIZE)
+               (local.get $s)))
+         (local.set $f (call $strtod (local.get $buf) (local.get $buffer)))
+         (call $release_memory (i32.add (local.get $buffer) (i32.const 4))
+            (local.get $buf))
+         (br_if $error
+            (i32.ne (i32.load (local.get $buffer))
+              (i32.add (local.get $buf) (local.get $len))))
+)
+(@else
          (local.set $f
             (call $parse_float (call $jsstring_of_bytes (local.get $s))))
          (br_if $error (f64.ne (local.get $f) (local.get $f)))
+))
          (return (struct.new $float (local.get $f))))
       (call $caml_failwith (global.get $float_of_string))
       (return (ref.i31 (i32.const 0))))
diff --git a/runtime/wasm/fs.wat b/runtime/wasm/fs.wat
index 89903e2c92..2d92f8e192 100644
--- a/runtime/wasm/fs.wat
+++ b/runtime/wasm/fs.wat
@@ -16,6 +16,41 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if wasi
+(@then
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_snapshot_preview1" "fd_prestat_get"
+      (func $fd_prestat_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_prestat_dir_name"
+      (func $fd_prestat_dir_name (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_rename"
+      (func $path_rename (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_unlink_file"
+      (func $path_unlink_file (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_create_directory"
+      (func $path_create_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_remove_directory"
+      (func $path_remove_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_get"
+      (func $path_filestat_get (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_readdir"
+      (func $fd_readddir (param i32 i32 i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param (ref eq) i32)))
+)
+(@else
    (import "bindings" "getcwd" (func $getcwd (result anyref)))
    (import "bindings" "chdir" (func $chdir (param anyref)))
    (import "bindings" "mkdir" (func $mkdir (param anyref) (param i32)))
@@ -38,17 +73,300 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_js_to_string_array"
       (func $caml_js_to_string_array (param $a (ref extern)) (result (ref eq))))
-   (import "fail" "caml_raise_sys_error"
-      (func $caml_raise_sys_error (param (ref eq))))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
    (import "sys" "caml_handle_sys_error"
       (func $caml_handle_sys_error (param externref)))
+))
    (import "string" "caml_string_concat"
       (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
+   (import "fail" "caml_raise_sys_error"
+      (func $caml_raise_sys_error (param (ref eq))))
 
    (type $bytes (array (mut i8)))
+   (type $block (array (mut (ref eq))))
+
+(@if wasi
+(@then
+   (type $preopen
+      (struct
+         (field $prefix (ref $bytes))
+         (field $fd i32)
+         (field $next (ref null $preopen))))
+
+   (global $preopens (mut (ref null $preopen)) (ref.null $preopen))
+
+   (global $preopens_initialized (mut i32) (i32.const 0))
+
+   (func $normalize_prefix (param $prefix (ref $bytes)) (result (ref $bytes))
+      (local $i i32) (local $len i32) (local $c i32) (local $res (ref $bytes))
+      (local.set $len (array.len (local.get $prefix)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (local.set $c
+                  (array.get $bytes (local.get $prefix) (local.get $i)))
+               (if (i32.eq (local.get $c) (i32.const 47)) ;; '/'
+                  (then
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))
+               (if (i32.eq (local.get $c) (i32.const 46)) ;; '.'
+                  (then
+                     (if (i32.eq (local.get $i)
+                            (i32.sub (local.get $len) (i32.const 1)))
+                        (then
+                           (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                           (br $loop))
+                        (else
+                           (local.set $c
+                              (array.get $bytes (local.get $prefix)
+                                 (i32.add (local.get $i) (i32.const 1))))
+                           (if (i32.eq (local.get $c) (i32.const 47)) ;; '/'
+                              (then
+                                 (local.set $i
+                                    (i32.add (local.get $i) (i32.const 2)))
+                                 (br $loop))))))))))
+      (if (i32.eq (local.get $i) (local.get $len))
+         (then (return (@string ""))))
+      (local.set $i (i32.sub (local.get $i) (i32.const 1)))
+      (if (i32.gt_u (local.get $i) (i32.const 0))
+         (then
+            (local.set $res
+               (array.new $bytes (i32.const 0)
+                  (i32.sub (local.get $len) (local.get $i))))
+            (array.copy $bytes $bytes
+               (local.get $res) (i32.const 0)
+               (local.get $prefix) (local.get $i)
+               (i32.sub (local.get $len) (local.get $i)))
+            (return (local.get $res))))
+      (return (local.get $prefix)))
+
+   (func $get_preopens (result (ref null $preopen))
+      (local $fd i32) (local $buffer i32) (local $res i32) (local $len i32)
+      (if $done (i32.eqz (global.get $preopens_initialized))
+         (then
+            (local.set $buffer (call $get_buffer))
+            (local.set $fd (i32.const 3))
+            (loop $loop
+               (local.set $res
+                  (call $fd_prestat_get (local.get $fd) (local.get $buffer)))
+               (br_if $done (i32.eq (local.get $res) (i32.const 8))) ;; EBADF
+               (block $skip
+                  (br_if $skip
+                     (i32.eqz
+                        (i32.and (i32.eqz (local.get $res))
+                           (i32.eqz (i32.load8_u (local.get $buffer))))))
+                  (local.set $len (i32.load offset=4 (local.get $buffer)))
+                  (local.set $res
+                     (call $fd_prestat_dir_name
+                        (local.get $fd) (local.get $buffer) (local.get $len)))
+                  (br_if $skip (local.get $res))
+                  (global.set $preopens
+                     (struct.new $preopen
+                        (call $normalize_prefix
+                           (call $blit_memory_to_string
+                              (local.get $buffer) (local.get $len)))
+                        (local.get $fd)
+                        (global.get $preopens))))
+               (local.set $fd (i32.add (local.get $fd) (i32.const 1)))
+               (br $loop))
+            (global.set $preopens_initialized (i32.const 1))))
+      (global.get $preopens))
+
+   (global $current_dir (mut (ref $bytes)) (@string ""))
+
+   (@string $root_dir "/")
+
+   (func $make_absolute
+      (param $path (ref $bytes)) (result (ref $bytes))
+      (local $need_slash i32) (local $i i32) (local $abs_path (ref $bytes))
+      (if (i32.eqz (array.len (local.get $path)))
+         (then ;; empty path
+            (return (global.get $current_dir))))
+      (if (i32.eq (i32.const 47) ;; '/'
+             (array.get_u $bytes (local.get $path) (i32.const 0)))
+         (then ;; absolute path
+            (return (local.get $path))))
+      (if (i32.and
+             (i32.eq (i32.const 46) ;; '.'
+                (array.get_u $bytes (local.get $path) (i32.const 0)))
+             (i32.eq (array.len (local.get $path)) (i32.const 1)))
+         (then
+            ;; "."
+            (return (global.get $current_dir))))
+      (if (i32.ge_u (array.len (local.get $path)) (i32.const 2))
+         (then
+            (if (i32.and
+                   (i32.eq (i32.const 46) ;; '.'
+                      (array.get_u $bytes (local.get $path) (i32.const 0)))
+                   (i32.eq (i32.const 47) ;; '/'
+                      (array.get_u $bytes (local.get $path) (i32.const 1))))
+               (then ;; starts with "./"
+                  (local.set $i (i32.const 2))))))
+      (if (i32.eq (local.get $i) (array.len (local.get $path)))
+         (then ;; "./"
+            (return (global.get $current_dir))))
+      (local.set $need_slash
+         (if (result i32) (array.len (global.get $current_dir))
+            (then
+               (i32.ne (i32.const 47) ;; '/'
+                  (array.get_u $bytes (global.get $current_dir)
+                     (i32.sub (array.len (global.get $current_dir))
+                        (i32.const 1)))))
+            (else
+               (i32.const 1))))
+      (local.set $abs_path
+         (array.new $bytes (i32.const 0)
+            (i32.add (array.len (global.get $current_dir))
+               (i32.add (i32.sub (local.get $need_slash) (local.get $i))
+                  (array.len (local.get $path))))))
+      (array.copy $bytes $bytes
+         (local.get $abs_path) (i32.const 0)
+         (global.get $current_dir) (i32.const 0)
+         (array.len (global.get $current_dir)))
+      (array.set $bytes (local.get $abs_path)
+         (array.len (global.get $current_dir))
+         (i32.const 47)) ;; '/'
+      (array.copy $bytes $bytes
+         (local.get $abs_path)
+         (i32.add (array.len (global.get $current_dir))
+            (local.get $need_slash))
+         (local.get $path) (local.get $i)
+         (i32.sub (array.len (local.get $path)) (local.get $i)))
+      (local.get $abs_path))
 
+   (func $wasi_chdir (export "wasi_chdir") (param $name (ref eq))
+      (local $abs_path (ref $bytes)) (local $path (ref $bytes)) (local $i i32)
+      (local.set $abs_path
+         (call $make_absolute (ref.cast (ref $bytes) (local.get $name))))
+      (local.set $i (i32.sub (array.len (local.get $abs_path)) (i32.const 1)))
+      ;; remove trailing slashes
+      (loop $loop
+         (if (i32.ge_s (local.get $i) (i32.const 0))
+            (then
+               (if (i32.eq (i32.const 47) ;; '/'
+                      (array.get $bytes (local.get $abs_path) (local.get $i)))
+                  (then
+                     (local.set $i (i32.sub (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (local.set $i (i32.add (local.get $i) (i32.const 1)))
+      (if (i32.lt_u (local.get $i) (array.len (local.get $abs_path)))
+         (then
+            (local.set $path (array.new $bytes (i32.const 0) (local.get $i)))
+            (array.copy $bytes $bytes
+               (local.get $path) (i32.const 0)
+               (local.get $abs_path) (i32.const 0)
+               (local.get $i))
+            (local.set $abs_path (local.get $path))))
+      (global.set $current_dir (local.get $abs_path)))
+
+   (func $prefix_match
+      (param $prefix (ref $bytes)) (param $path (ref $bytes)) (result i32)
+      (local $i i32) (local $len i32)
+      (local.set $len (array.len (local.get $prefix)))
+      (if (i32.lt_u (array.len (local.get $path)) (local.get $len))
+         (then (return (i32.const 0))))
+      (if (i32.gt_u (array.len (local.get $path)) (local.get $len))
+         (then
+            (if (i32.ne (array.get_u $bytes (local.get $path) (local.get $i))
+                   (i32.const 47))
+               (then (return (i32.const 0))))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (if (i32.ne (array.get_u $bytes (local.get $path) (local.get $i))
+                      (array.get_u $bytes (local.get $prefix) (local.get $i)))
+                  (then (return (i32.const 0))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (return (i32.const 1)))
+
+   (func $resolve_abs_path
+      (param $path (ref $bytes)) (result i32 (ref $bytes))
+      (local $fd i32) (local $len i32) (local $i i32)
+      (local $preopens (ref null $preopen)) (local $current (ref $preopen))
+      (local $prefix (ref $bytes)) (local $rel_path (ref $bytes))
+      (local.set $preopens (call $get_preopens))
+      (local.set $i (i32.const -1))
+      (block $done
+         (loop $loop
+            (local.set $current (br_on_null $done (local.get $preopens)))
+            (local.set $prefix
+               (struct.get $preopen $prefix (local.get $current)))
+            (if (i32.and
+                   (i32.gt_s (array.len (local.get $prefix)) (local.get $i))
+                   (call $prefix_match (local.get $prefix) (local.get $path)))
+               (then
+                  (local.set $fd (struct.get $preopen $fd (local.get $current)))
+                  (local.set $i (array.len (local.get $prefix)))))
+            (local.set $preopens
+               (struct.get $preopen $next (local.get $current)))
+            (br $loop)))
+      (if (i32.eq (local.get $i) (i32.const -1))
+         (then ;; not found
+            (return (tuple.make 2 (i32.const -1) (@string "")))))
+      ;; skip leading slashes
+      (local.set $len (local.get $i))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (array.len (local.get $path)))
+            (then
+               (if (i32.eq (array.get_u $bytes (local.get $path) (local.get $i))
+                      (i32.const 47)) ;; 47
+                  (then
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))
+               (local.set $rel_path
+                  (array.new $bytes (i32.const 0)
+                     (i32.sub (array.len (local.get $path)) (local.get $i))))
+               (array.copy $bytes $bytes
+                  (local.get $rel_path) (i32.const 0)
+                  (local.get $path) (local.get $i)
+                  (i32.sub (array.len (local.get $path)) (local.get $i)))
+               (return
+                  (tuple.make 2 (local.get $fd) (local.get $rel_path))))))
+      (return (tuple.make 2 (local.get $fd) (@string "."))))
+
+   (func (export "wasi_resolve_path")
+      (param $vpath (ref eq))
+      (result (;fd;) i32 (;address;) i32 (;length;) i32)
+      (local $res (tuple i32 (ref $bytes)))
+      (local $p i32)
+      (local.set $res
+         (call $resolve_abs_path
+            (call $make_absolute
+               (ref.cast (ref $bytes) (local.get $vpath)))))
+      (if (i32.ge_u (tuple.extract 2 0 (local.get $res)) (i32.const 0))
+         (then
+            (local.set $p
+               (call $write_string_to_memory
+                  (i32.const 0) (i32.const 0)
+                  (tuple.extract 2 1 (local.get $res))))))
+      (return
+         (tuple.make 3
+            (tuple.extract 2 0 (local.get $res))
+            (local.get $p)
+            (array.len (tuple.extract 2 1 (local.get $res))))))
+
+   (func $caml_sys_resolve_path (export "caml_sys_resolve_path")
+      (param $path (ref eq)) (result i32 i32 i32)
+      (local $res (tuple i32 i32 i32))
+      (local.set $res (call $wasi_resolve_path (local.get $path)))
+      (if (i32.lt_s (tuple.extract 3 0 (local.get $res)) (i32.const 0))
+         (then ;; ENOENT
+            (call $caml_handle_sys_error (local.get $path) (i32.const 44))))
+      (local.get $res))
+))
+
+(@if wasi
+(@then
+   (func (export "caml_sys_getcwd")
+      (export "unix_getcwd") (export "caml_unix_getcwd")
+      (param (ref eq)) (result (ref eq))
+      (if (array.len (global.get $current_dir))
+         (then (return (global.get $current_dir))))
+      (global.get $root_dir))
+)
+(@else
    (func (export "caml_sys_getcwd")
       (param (ref eq)) (result (ref eq))
       (try (result (ref eq))
@@ -57,7 +375,34 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))
             (ref.i31 (i32.const 0)))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_chdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $kind i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (if (i32.ne (local.get $kind) (i32.const 3))
+         (then
+            (call $caml_handle_sys_error
+               (local.get $name) (i32.const 54)))) ;; ENOTDIR
+      (call $wasi_chdir (local.get $name))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_chdir")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -67,7 +412,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_mkdir")
+      (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_create_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_mkdir")
       (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
       (try
@@ -78,7 +442,128 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_read_directory")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $fd i32)
+      (local $buf i32) (local $new_buf i32)
+      (local $size i32) (local $pos i32) (local $available i32)
+      (local $left i32) (local $namelen i32)
+      (local $entry i32) (local $entry_size i32)
+      (local $cookie i64) (local $tbl (ref $block)) (local $new_tbl (ref $block))
+      (local $i i32) (local $s (ref $bytes))
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (i32.const 2) ;; O_DIRECTORY
+            (i64.const 0x4000) ;; allow fd_readdir
+            (i64.const 0)
+            (i32.const 0)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (local.set $fd (i32.load (local.get $buffer)))
+      (local.set $buf (call $checked_malloc (i32.const 512)))
+      (local.set $size (i32.const 512))
+      (local.set $tbl (array.new $block (ref.i31 (i32.const 0)) (i32.const 50)))
+      (local.set $i (i32.const 1))
+      (loop $loop
+         (block $refill
+            (local.set $left (i32.sub (local.get $available) (local.get $pos)))
+            (br_if $refill (i32.lt_u (local.get $left) (i32.const 24)))
+            (local.set $entry (i32.add (local.get $buf) (local.get $pos)))
+            (local.set $namelen (i32.load offset=16 (local.get $entry)))
+            (local.set $entry_size (i32.add (local.get $namelen) (i32.const 24)))
+            (br_if $refill (i32.lt_u (local.get $left) (local.get $entry_size)))
+            (local.set $pos (i32.add (local.get $pos) (local.get $entry_size)))
+            (local.set $cookie (i64.load (local.get $entry)))
+            (if (i32.eq (local.get $i) (array.len (local.get $tbl)))
+               (then
+                  (local.set $new_tbl
+                     (array.new $block (ref.i31 (i32.const 0))
+                        (i32.shl (local.get $i) (i32.const 1))))
+                  (array.copy $block $block
+                     (local.get $new_tbl) (i32.const 0)
+                     (local.get $tbl) (i32.const 0) (local.get $i))
+                  (local.set $tbl (local.get $new_tbl))))
+            (local.set $s
+               (call $blit_memory_to_string
+                  (i32.add (local.get $entry) (i32.const 24))
+                  (local.get $namelen)))
+            ;; skip "." and ".."
+            (if (i32.eq (local.get $namelen) (i32.const 2))
+               (then
+                  (br_if $loop
+                     (i32.and
+                        (i32.eq (i32.const 46)
+                           (array.get_u $bytes (local.get $s) (i32.const 0)))
+                        (i32.eq (i32.const 46)
+                           (array.get_u $bytes (local.get $s) (i32.const 1))))))
+               (else
+                  (if (i32.eq (local.get $namelen) (i32.const 2))
+                     (then
+                        (br_if $loop
+                           (i32.eq
+                              (array.get_u $bytes (local.get $s) (i32.const 0))
+                              (i32.const 46)))))))
+            (array.set $block (local.get $tbl) (local.get $i) (local.get $s))
+            (local.set $i (i32.add (local.get $i) (i32.const 1)))
+            (br $loop))
+         ;; refill
+         (if (i32.lt_u (local.get $size) (local.get $entry_size))
+            (then
+               ;; the entry does not fit
+               (local.set $new_buf
+                  (call $checked_malloc (local.get $entry_size)))
+               (call $free (local.get $buf))
+               (local.set $buf (local.get $new_buf))
+               (local.set $size (local.get $entry_size))))
+         (block $done
+            (br_if $done
+               (i32.and
+                  (i32.ne (i32.const 0) (local.get $available))
+                  (i32.lt_u (local.get $available) (local.get $size))))
+            (local.set $res
+               (call $fd_readddir
+                  (local.get $fd)
+                  (local.get $buffer)
+                  (local.get $size)
+                  (local.get $cookie)
+                  (local.get $buffer)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (local.get $name) (local.get $res))))
+            (local.set $available (i32.load (local.get $buffer)))
+            (br_if $done (i32.eqz (local.get $available)))
+            (local.set $pos (i32.const 0))
+            (br $loop)))
+      ;; done
+      (call $free (local.get $buf))
+      (local.set $res (call $fd_close (local.get $fd)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (if (i32.eq (local.get $i) (array.len (local.get $tbl)))
+         (then (return (local.get $tbl))))
+      (local.set $new_tbl
+         (array.new $block (ref.i31 (i32.const 0)) (local.get $i)))
+      (array.copy $block $block
+         (local.get $new_tbl) (i32.const 0)
+         (local.get $tbl) (i32.const 0) (local.get $i))
+      (local.get $new_tbl))
+)
+(@else
    (func (export "caml_sys_read_directory")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -91,7 +576,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))
             (return (ref.i31 (i32.const 0))))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_rmdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_remove_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_rmdir")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -101,7 +605,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_remove")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_unlink_file
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_remove")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -111,7 +634,32 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_rename")
+      (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $op (call $caml_sys_resolve_path (local.get $o)))
+      (local.set $np (call $caml_sys_resolve_path (local.get $n)))
+      (local.set $res
+         (call $path_rename
+            (tuple.extract 3 0 (local.get $op))
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $o) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_rename")
       (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
       (try
@@ -122,11 +670,31 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_file_exists")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (ref.i31 (i32.eqz (local.get $res))))
+)
+(@else
    (func (export "caml_sys_file_exists")
       (param $name (ref eq)) (result (ref eq))
       (return_call $file_exists
          (call $unwrap (call $caml_jsstring_of_string (local.get $name)))))
+))
 
    (@string $no_such_file ": No such file or directory")
 
@@ -148,6 +716,30 @@
    (func (export "caml_fs_init") (result (ref eq))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $caml_sys_file_mode (param $name (ref eq)) (result i32)
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (i32.load8_u offset=16 (local.get $buffer)))
+
+   (func (export "caml_sys_is_directory")
+      (param $name (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.eq (call $caml_sys_file_mode (local.get $name)) (i32.const 3))))
+)
+(@else
    (func (export "caml_sys_is_directory")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -159,7 +751,16 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))
             (return (ref.i31 (i32.const 0))))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_is_regular_file")
+      (param $name (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.eq (call $caml_sys_file_mode (local.get $name)) (i32.const 4))))
+)
+(@else
    (func (export "caml_sys_is_regular_file")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -175,4 +776,5 @@
    (func (export "caml_mount_autoload")
       (param (ref eq) (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
+))
 )
diff --git a/runtime/wasm/hash.wat b/runtime/wasm/hash.wat
index a7a78c9e49..ad9fd4d628 100644
--- a/runtime/wasm/hash.wat
+++ b/runtime/wasm/hash.wat
@@ -304,6 +304,8 @@
                                           (local.get $v))))))))
                      (local.set $num (i32.sub (local.get $num) (i32.const 1)))
                      (br $loop)))
+(@if (not wasi)
+(@then
                   (drop (block $not_jsstring (result anyref)
                      (local.set $str
                         (struct.get $js 0
@@ -315,6 +317,7 @@
                      (local.set $h
                         (call $jsstring_hash (local.get $h) (local.get $str)))
                      (ref.i31 (i32.const 0))))
+))
                   ;; closures and continuations and other js values are ignored
                   (br $loop)))))
       ;; clear the queue to avoid a memory leak
diff --git a/runtime/wasm/io.wat b/runtime/wasm/io.wat
index deff2a6d40..422c57c373 100644
--- a/runtime/wasm/io.wat
+++ b/runtime/wasm/io.wat
@@ -25,6 +25,31 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_list_of_js_array"
       (func $caml_list_of_js_array (param (ref eq)) (result (ref eq))))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_read"
+      (func $fd_read (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_seek"
+      (func $fd_seek (param i32 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "blit_memory_to_substring"
+      (func $blit_memory_to_substring (param i32 (ref $bytes) i32 i32)))
+   (import "wasi_memory" "blit_substring_to_memory"
+      (func $blit_substring_to_memory (param i32 (ref $bytes) i32 i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param (ref eq) i32)))
+   (import "fs" "caml_sys_resolve_path"
+      (func $caml_sys_resolve_path (param (ref eq)) (result i32 i32 i32)))
+)
+(@else
    (import "bindings" "open"
       (func $open (param anyref) (param i32) (param i32) (result i32)))
    (import "bindings" "close" (func $close (param i32)))
@@ -70,6 +95,11 @@
          (param (ref extern)) (param i32) (param i32) (result (ref extern))))
    (import "bindings" "ta_set"
       (func $ta_set (param (ref extern)) (param (ref extern)) (param i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param externref)))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
+))
    (import "custom" "custom_compare_id"
       (func $custom_compare_id
         (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
@@ -80,13 +110,129 @@
       (func $caml_copy_int64 (param i64) (result (ref eq))))
    (import "int64" "Int64_val"
       (func $Int64_val (param (ref eq)) (result i64)))
-   (import "fail" "javascript_exception"
-      (tag $javascript_exception (param externref)))
-   (import "sys" "caml_handle_sys_error"
-      (func $caml_handle_sys_error (param externref)))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
 
+(@if wasi
+(@then
+   (func $ta_new (param $sz i32) (result (ref extern))
+      (extern.convert_any (array.new $bytes (i32.const 0) (local.get $sz))))
+
+   (func $ta_copy
+      (param $buf (ref extern))
+      (param $dst i32) (param $src i32) (param $end i32)
+      (local $b (ref $bytes))
+      (local.set $b
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (array.copy $bytes $bytes
+         (local.get $b) (local.get $dst)
+         (local.get $b) (local.get $src)
+         (i32.sub (local.get $end) (local.get $src))))
+
+   (func $ta_set_ui8 (param $buf (ref extern)) (param $i i32) (param $c i32)
+      (array.set $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i) (local.get $c)))
+
+   (func $ta_get_ui8 (param $buf (ref extern)) (param $i i32) (result i32)
+      (array.get_u $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i)))
+
+   (func $ta_blit_from_bytes
+      (param $s (ref $bytes)) (param $i i32) (param $buf (ref extern))
+      (param $j i32) (param $l i32)
+      (array.copy $bytes $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $j)
+         (local.get $s) (local.get $i)
+         (local.get $l)))
+
+   (func $ta_blit_to_bytes
+      (param $buf (ref extern)) (param $i i32) (param $s (ref $bytes))
+      (param $j i32) (param $l i32)
+      (array.copy $bytes $bytes
+         (local.get $s) (local.get $j)
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i)
+         (local.get $l)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func $ta_blit_from_buffer
+      (param $buf (ref extern)) (param $i i32)
+      (param $ta (ref extern)) (param $j i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $ta))))
+      (call $ta_blit_to_bytes
+         (local.get $buf)
+         (local.get $i)
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data)))
+         (i32.add (struct.get $data $offset (local.get $data)) (local.get $j))
+         (local.get $len)))
+
+   (func $ta_blit_to_buffer
+      (param $ta (ref extern)) (param $i i32)
+      (param $buf (ref extern)) (param $j i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $ta))))
+      (call $ta_blit_from_bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data)))
+         (i32.add (struct.get $data $offset (local.get $data)) (local.get $i))
+         (local.get $buf)
+         (local.get $j)
+         (local.get $len)))
+
+   (global $caml_stdout
+      (mut (ref eq)) (ref.i31 (i32.const 0)))
+
+   (func $register_channel (param $ch (ref eq))
+      (if (i32.eq
+             (struct.get $channel $fd (ref.cast (ref $channel) (local.get $ch)))
+             (i32.const 1))
+         (then
+            (global.set $caml_stdout (local.get $ch)))))
+
+   (func $unregister_channel (param (ref eq)))
+   (func $map_new (result (ref extern))
+      (extern.convert_any (ref.i31 (i32.const 0))))
+   (func $map_get (param (ref extern)) (param i32) (result (ref $fd_offset))
+      (struct.new $fd_offset (i64.const 0) (i32.const 0)))
+   (func $map_set (param (ref extern)) (param i32) (param (ref $fd_offset)))
+   (func $map_delete (param (ref extern)) (param i32))
+
+   (func $file_size (param $fd i32) (result i64)
+      (local $cur i64) (local $end i64) (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (block $error
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (i64.const 0) (i32.const 1) (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (local.set $cur (i64.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (i64.const 0) (i32.const 2) (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (local.set $end (i64.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (local.get $cur) (i32.const 0)
+               (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (return (local.get $end)))
+      (call $caml_handle_sys_error (ref.i31 (i32.const 0)) (local.get $res))
+      (i64.const 0))
+)
+(@else
    (import "bindings" "map_new" (func $map_new (result (ref extern))))
    (import "bindings" "map_get"
       (func $map_get
@@ -97,6 +243,26 @@
    (import "bindings" "map_delete"
       (func $map_delete (param (ref extern)) (param i32)))
 
+   (func $ta_blit_from_buffer
+      (param $buf (ref extern)) (param $i i32)
+      (param $ta (ref extern)) (param $j i32)
+      (param $len i32)
+      (call $ta_set
+         (local.get $ta)
+         (call $ta_subarray (local.get $buf) (local.get $i)
+            (i32.add (local.get $i) (local.get $len)))
+         (local.get $j)))
+
+   (func $ta_blit_to_buffer
+      (param $ta (ref extern)) (param $i i32)
+      (param $buf (ref extern)) (param $j i32)
+      (param $len i32)
+      (call $ta_set (local.get $buf)
+         (call $ta_subarray (local.get $ta) (local.get $i)
+            (i32.add (local.get $i) (local.get $len)))
+         (local.get $j)))
+))
+
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $offset_array (array (mut i64)))
@@ -193,7 +359,24 @@
 
    (global $IO_BUFFER_SIZE (export "IO_BUFFER_SIZE") i32 (i32.const 65536))
 
-   (type $open_flags (array i8))
+   (type $open_flags (array i16))
+
+(@if wasi
+(@then
+   ;;      1 O_RDONLY
+   ;;      2 O_WRONLY
+   ;;   0x10 O_CREAT
+   ;;   0x40 O_EXCL
+   ;;   0x80 O_TRUNC
+   ;;  0x100 O_APPEND
+   ;;  0x400 O_NONBLOCK
+   (global $sys_open_flags (ref $open_flags)
+      (array.new_fixed $open_flags 9
+         (i32.const 1) (i32.const 2) (i32.const 0x102) (i32.const 0x10)
+         (i32.const 0x80) (i32.const 0x40) (i32.const 0) (i32.const 0)
+         (i32.const 0x400)))
+)
+(@else
    ;;   1 O_RDONLY
    ;;   2 O_WRONLY
    ;;   4 O_RDWR
@@ -206,6 +389,7 @@
       (array.new_fixed $open_flags 9
          (i32.const 1) (i32.const 2) (i32.const 10) (i32.const 16) (i32.const 32)
          (i32.const 64) (i32.const 0) (i32.const 0) (i32.const 128)))
+))
 
    (func $convert_flag_list (export "convert_flag_list")
       (param $tbl (ref $open_flags)) (param $vflags (ref eq)) (result i32)
@@ -227,6 +411,41 @@
             (br $loop))))
       (local.get $flags))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_open")
+      (param $vpath (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
+      (result (ref eq))
+      (local $fd i32) (local $flags i32) (local $offset i64)
+      (local $path (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $path (call $caml_sys_resolve_path (local.get $vpath)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $flags
+         (call $convert_flag_list
+            (global.get $sys_open_flags) (local.get $vflags)))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $path))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $path))
+            (tuple.extract 3 2 (local.get $path))
+            (i32.and (i32.shr_u (local.get $flags) (i32.const 4))
+              (i32.const 0xF))
+            (select (i64.const 0x860007c) (i64.const 0x820003e)
+               (i32.and (local.get $flags) (i32.const 2)))
+            (i64.const 0)
+            (i32.shr_u (local.get $flags) (i32.const 8))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $path)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error (local.get $vpath) (local.get $res))))
+      (local.set $fd (i32.load (local.get $buffer)))
+      (call $initialize_fd_offset (local.get $fd) (local.get $offset))
+      (ref.i31 (local.get $fd)))
+)
+(@else
    (func (export "caml_sys_open")
       (param $path (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
       (result (ref eq))
@@ -243,14 +462,30 @@
                   (local.get $flags)
                   (i31.get_u (ref.cast (ref i31) (local.get $perm)))))
             (if (i32.and (local.get $flags) (i32.const 4)) ;; O_APPEND
-               (then (local.set $offset (call $file_size (local.get $fd))))))
+               (then (local.set $offset (call $file_size (local.get $fd)))))
+         )
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (call $initialize_fd_offset (local.get $fd) (local.get $offset))
       (ref.i31 (local.get $fd)))
+))
 
+(@if wasi
+(@then
    (func (export "caml_sys_close") (param (ref eq)) (result (ref eq))
-      (local $fd i32)
+      (local $fd i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get 0))))
+      (call $release_fd_offset (local.get $fd))
+      (local.set $res (call $fd_close (local.get $fd)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_sys_close") (param (ref eq)) (result (ref eq))
+      (local $fd i32) (local $res i32)
       (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get 0))))
       (call $release_fd_offset (local.get $fd))
       (try
@@ -259,14 +494,40 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
    (func (export "caml_ml_set_channel_name")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $push_channel (param $l (ref eq)) (param $ch (ref eq)) (result (ref eq))
+      (local $c (ref $channel))
+      (block $continue
+         (br_if $continue (i32.eqz (ref.test (ref $channel) (local.get $ch))))
+         (local.set $c (ref.cast (ref $channel) (local.get $ch)))
+         (br_if $continue
+            (i32.eq (struct.get $channel $fd (local.get $c)) (i32.const -1)))
+         (local.set $l
+            (array.new_fixed $block 3
+               (ref.i31 (i32.const 0)) (local.get $ch) (local.get $l))))
+      (local.get $l))
+))
+
+(@if wasi
+(@then
+   (func (export "caml_ml_out_channels_list")
+      (param (ref eq)) (result (ref eq))
+      (call $push_channel
+         (call $push_channel (ref.i31 (i32.const 0)) (global.get $caml_stdout))
+         (global.get $caml_stderr)))
+)
+(@else
    (func (export "caml_ml_out_channels_list")
       (param (ref eq)) (result (ref eq))
       (return_call $caml_list_of_js_array (call $wrap (call $channel_list))))
+))
 
    (func (export "caml_ml_open_descriptor_in")
       (param $fd (ref eq)) (result (ref eq))
@@ -322,7 +583,7 @@
    (func (export "caml_ml_close_channel")
       (param (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
-      (local $fd i32)
+      (local $fd i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get 0)))
       ;; output channels: any output will trigger a flush since the
       ;; buffer is non-empty (curr > 0) and full (curr = size)
@@ -337,14 +598,56 @@
             (struct.set $channel $fd (local.get $ch) (i32.const -1))
             (call $unregister_channel (local.get $ch))
             (call $release_fd_offset (local.get $fd))
+(@if wasi
+(@then
+            (local.set $res (call $fd_close (local.get $fd)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
             (try
                (do
                   (call $close (local.get $fd)))
                (catch $javascript_exception
                   ;; ignore exception
-                  (drop (pop externref))))))
+                  (drop (pop externref))))
+))
+      ))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $read
+      (param $fd i32) (param $buf (ref extern)) (param $pos i32) (param $n i32)
+      (result i32)
+      (local $buffer i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nread i32)
+      (local $s (ref $bytes)) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $n))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (local.set $n (i32.load (local.get $nread)))
+      (local.set $s
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (call $blit_memory_to_substring
+         (local.get $buffer) (local.get $s) (local.get $pos) (local.get $n))
+      (local.get $n))
+))
+
    (func $caml_do_read
       (param $ch (ref $channel)) (param $pos i32) (param $len i32) (result i32)
       (local $fd i32)
@@ -352,6 +655,16 @@
       (local $offset i64)
       (local $n i32)
       (local.set $fd (struct.get $channel $fd (local.get $ch)))
+(@if wasi
+(@then
+      (local.set $n
+         (call $read
+            (local.get $fd)
+            (struct.get $channel $buffer (local.get $ch))
+            (local.get $pos)
+            (local.get $len)))
+)
+(@else
       (local.set $fd_offset (call $get_fd_offset (local.get $fd)))
       (local.set $offset (struct.get $fd_offset $offset (local.get $fd_offset)))
       (try
@@ -378,6 +691,7 @@
       (struct.set $fd_offset $offset
          (local.get $fd_offset)
          (i64.add (local.get $offset) (i64.extend_i32_u (local.get $n))))
+))
       (local.get $n))
 
    (func $caml_refill (param $ch (ref $channel)) (result i32)
@@ -450,12 +764,12 @@
          (then
             (if (i32.gt_u (local.get $len) (local.get $avail))
                (then (local.set $len (local.get $avail))))
-            (call $ta_set (local.get $d)
-               (call $ta_subarray (struct.get $channel $buffer (local.get $ch))
-                  (struct.get $channel $curr (local.get $ch))
-                  (i32.add (struct.get $channel $curr (local.get $ch))
-                     (local.get $len)))
-               (local.get $pos))
+            (call $ta_blit_from_buffer
+               (struct.get $channel $buffer (local.get $ch))
+               (struct.get $channel $curr (local.get $ch))
+               (local.get $d)
+               (local.get $pos)
+               (local.get $len))
             (struct.set $channel $curr (local.get $ch)
                (i32.add (struct.get $channel $curr (local.get $ch))
                   (local.get $len)))
@@ -466,10 +780,12 @@
       (struct.set $channel $max (local.get $ch) (local.get $nread))
       (if (i32.gt_u (local.get $len) (local.get $nread))
          (then (local.set $len (local.get $nread))))
-      (call $ta_set (local.get $d)
-         (call $ta_subarray (struct.get $channel $buffer (local.get $ch))
-            (i32.const 0) (local.get $len))
-         (local.get $pos))
+      (call $ta_blit_from_buffer
+         (struct.get $channel $buffer (local.get $ch))
+         (i32.const 0)
+         (local.get $d)
+         (local.get $pos)
+         (local.get $len))
       (struct.set $channel $curr (local.get $ch) (local.get $len))
       (local.get $len))
 
@@ -564,10 +880,7 @@
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (ref.i31
          (i32.sub
-            (i32.wrap_i64
-               (struct.get $fd_offset $offset
-                  (call $get_fd_offset
-                     (struct.get $channel $fd (local.get $ch)))))
+            (i32.wrap_i64 (call $caml_ml_get_channel_offset (local.get $ch)))
             (i32.sub
               (struct.get $channel $max (local.get $ch))
               (struct.get $channel $curr (local.get $ch))))))
@@ -577,10 +890,7 @@
       (local $ch (ref $channel))
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_copy_int64
-         (i64.sub
-            (struct.get $fd_offset $offset
-               (call $get_fd_offset
-                  (struct.get $channel $fd (local.get $ch))))
+         (i64.sub (call $caml_ml_get_channel_offset (local.get $ch))
             (i64.extend_i32_s
                (i32.sub
                   (struct.get $channel $max (local.get $ch))
@@ -592,10 +902,7 @@
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (ref.i31
          (i32.add
-            (i32.wrap_i64
-               (struct.get $fd_offset $offset
-                  (call $get_fd_offset
-                     (struct.get $channel $fd (local.get $ch)))))
+            (i32.wrap_i64 (call $caml_ml_get_channel_offset (local.get $ch)))
             (struct.get $channel $curr (local.get $ch)))))
 
    (func (export "caml_ml_pos_out_64")
@@ -603,12 +910,29 @@
       (local $ch (ref $channel))
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_copy_int64
-         (i64.add
-            (struct.get $fd_offset $offset
-               (call $get_fd_offset
-                  (struct.get $channel $fd (local.get $ch))))
+         (i64.add (call $caml_ml_get_channel_offset (local.get $ch))
             (i64.extend_i32_s (struct.get $channel $curr (local.get $ch))))))
 
+(@if wasi
+(@then
+   (func $caml_seek_in
+      (param $ch (ref $channel)) (param $dest i64) (result (ref eq))
+      (local $fd i32) (local $buffer i32) (local $res i32)
+      (local.set $fd (struct.get $channel $fd (local.get $ch)))
+      (local.set $buffer (call $get_buffer))
+      ;; ZZZ store current offset in channel do avoid some syscalls?
+      (local.set $res
+         (call $fd_seek
+            (local.get $fd) (local.get $dest) (i32.const 0) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (struct.set $channel $curr (local.get $ch) (i32.const 0))
+      (struct.set $channel $max (local.get $ch) (i32.const 0))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func $caml_seek_in
       (param $ch (ref $channel)) (param $dest i64) (result (ref eq))
       (local $fd i32) (local $offset i64)
@@ -641,6 +965,7 @@
             (struct.set $channel $curr (local.get $ch) (i32.const 0))
             (struct.set $channel $max (local.get $ch) (i32.const 0))))
       (ref.i31 (i32.const 0)))
+))
 
    (func (export "caml_ml_seek_in")
       (param $ch (ref eq)) (param $dest (ref eq)) (result (ref eq))
@@ -657,8 +982,26 @@
       (param $vch (ref eq)) (param $voffset (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
       (local $fd_offset (ref $fd_offset)) (local $offset i64)
+      (local $buffer i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_flush (local.get $ch))
+(@if wasi
+(@then
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (struct.get $channel $fd (local.get $ch))
+            (i64.extend_i32_s
+               (i31.get_s (ref.cast (ref i31) (local.get $voffset))))
+            (i32.const 0)
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
+      ;; ZZZ Check for error
       (local.set $fd_offset
          (call $get_fd_offset (struct.get $channel $fd (local.get $ch))))
       (local.set $offset
@@ -668,14 +1011,32 @@
          (then (call $caml_raise_sys_error (@string "Invalid argument"))))
       (struct.set $fd_offset $offset (local.get $fd_offset) (local.get $offset))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
+))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_seek_out_64")
       (param $vch (ref eq)) (param $voffset (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
       (local $fd_offset (ref $fd_offset)) (local $offset i64)
+      (local $buffer i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_flush (local.get $ch))
+(@if wasi
+(@then
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (struct.get $channel $fd (local.get $ch))
+            (call $Int64_val (local.get $voffset))
+            (i32.const 0)
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
+      ;; ZZZ Check for error
       (local.set $fd_offset
          (call $get_fd_offset (struct.get $channel $fd (local.get $ch))))
       (local.set $offset (call $Int64_val (local.get $voffset)))
@@ -683,6 +1044,7 @@
          (then (call $caml_raise_sys_error (@string "Invalid argument"))))
       (struct.set $fd_offset $offset (local.get $fd_offset) (local.get $offset))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
+))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_input_scan_line")
@@ -759,6 +1121,36 @@
          (then (call $caml_flush (local.get $ch))))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $write
+      (param $fd i32) (param $buf (ref extern)) (param $pos i32) (param $n i32)
+      (result i32)
+      (local $buffer i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $s (ref $bytes)) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $n))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $s
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (call $blit_substring_to_memory
+         (local.get $buffer) (local.get $s) (local.get $pos) (local.get $n))
+      (local.set $res
+         (call $fd_write
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nwritten)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (i32.load (local.get $nwritten)))
+))
+
    (func $caml_flush_partial (param $ch (ref $channel)) (result i32)
       (local $towrite i32) (local $written i32) (local $fd i32)
       (local $fd_offset (ref $fd_offset))
@@ -768,6 +1160,16 @@
          (then
             (local.set $buf (struct.get $channel $buffer (local.get $ch)))
             (local.set $fd (struct.get $channel $fd (local.get $ch)))
+(@if wasi
+(@then
+            (local.set $written
+               (call $write
+                  (local.get $fd)
+                  (local.get $buf)
+                  (i32.const 0)
+                  (local.get $towrite)))
+)
+(@else
             (local.set $fd_offset (call $get_fd_offset (local.get $fd)))
             (local.set $offset
                (struct.get $fd_offset $offset (local.get $fd_offset)))
@@ -797,6 +1199,7 @@
                (i64.add
                   (local.get $offset)
                   (i64.extend_i32_u (local.get $written))))
+))
             (if (i32.gt_u (local.get $towrite) (local.get $written))
                (then
                   (call $ta_copy (local.get $buf)
@@ -837,10 +1240,10 @@
       (if (i32.ge_u (local.get $len) (local.get $free))
          (then (local.set $len (local.get $free))))
       (local.set $buf (struct.get $channel $buffer (local.get $ch)))
-      (call $ta_set (local.get $buf)
-         (call $ta_subarray (local.get $d)
-            (local.get $pos) (i32.add (local.get $pos) (local.get $len)))
-         (local.get $curr))
+      (call $ta_blit_to_buffer
+         (local.get $d) (local.get $pos)
+         (local.get $buf) (local.get $curr)
+         (local.get $len))
       (struct.set $channel $curr (local.get $ch)
          (i32.add (local.get $curr) (local.get $len)))
       (if (i32.ge_u (local.get $len) (local.get $free))
@@ -967,11 +1370,31 @@
       (struct.set $channel $fd
          (ref.cast (ref $channel) (local.get 0)) (local.get 1)))
 
-   (func (export "caml_ml_get_channel_offset") (param $ch (ref eq)) (result i64)
+(@if wasi
+(@then
+   (func $caml_ml_get_channel_offset (export "caml_ml_get_channel_offset")
+      (param $ch (ref eq)) (result i64)
+      (local $fd i32) (local $buffer i32) (local $res i32)
+      (local.set $fd
+         (struct.get $channel $fd (ref.cast (ref $channel) (local.get $ch))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (local.get $fd) (i64.const 0) (i32.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (i64.load (local.get $buffer)))
+)
+(@else
+   (func $caml_ml_get_channel_offset (export "caml_ml_get_channel_offset")
+      (param $ch (ref eq)) (result i64)
       (struct.get $fd_offset $offset
          (call $get_fd_offset
             (struct.get $channel $fd
                (ref.cast (ref $channel) (local.get $ch))))))
+))
 
    (func (export "caml_ml_output_bigarray")
       (param $ch (ref eq)) (param $a (ref eq)) (param $vpos (ref eq))
diff --git a/runtime/wasm/jslib.wat b/runtime/wasm/jslib.wat
index 84b8690151..906768ebfb 100644
--- a/runtime/wasm/jslib.wat
+++ b/runtime/wasm/jslib.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "bindings" "identity" (func $to_float (param anyref) (result f64)))
    (import "bindings" "identity" (func $from_float (param f64) (result anyref)))
    (import "bindings" "identity" (func $to_bool (param anyref) (result i32)))
@@ -686,6 +688,7 @@
                   (return
                      (array.get $block (local.get $exn) (i32.const 2)))))))
       (call $wrap (ref.null any)))
+))
 
    (func (export "caml_jsoo_flags_use_js_string")
       (param (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/jslib_js_of_ocaml.wat b/runtime/wasm/jslib_js_of_ocaml.wat
index 5f3c4c14e0..ec69833df0 100644
--- a/runtime/wasm/jslib_js_of_ocaml.wat
+++ b/runtime/wasm/jslib_js_of_ocaml.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "caml_js_global"
@@ -56,4 +58,5 @@
             (call $caml_js_global (ref.i31 (i32.const 0)))
             (global.get $XMLHttpRequest))
          (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))
+))
 )
diff --git a/runtime/wasm/jsstring.wat b/runtime/wasm/jsstring.wat
index de0780d990..c769ea514a 100644
--- a/runtime/wasm/jsstring.wat
+++ b/runtime/wasm/jsstring.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "wasm:js-string" "compare"
       (func $compare_strings (param externref externref) (result i32)))
    (import "wasm:js-string" "test"
@@ -257,4 +259,5 @@
       (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
       (call $read_from_buffer (local.get $s) (i32.const 0) (local.get $len))
       (global.set $stack (struct.new $stack (local.get $s) (global.get $stack))))
+))
 )
diff --git a/runtime/wasm/libc.c b/runtime/wasm/libc.c
new file mode 100644
index 0000000000..3b0c44bd04
--- /dev/null
+++ b/runtime/wasm/libc.c
@@ -0,0 +1,175 @@
+/*
+Primitives implemented by the WASI libc. Use 'dune build @recompile-libc'
+to update libc.wasm.
+
+clang -O2  --target=wasm32-wasi --sysroot=/path/to/wasi-libc/sysroot -nodefaultlibs -lc libc.c -o libc.wasm
+*/
+
+#include <stdio.h>
+#include <math.h>
+#include <stdlib.h>
+#include <time.h>
+#include <string.h>
+
+__attribute__((export_name("cos")))
+double libc_cos (double x) {
+  return cos(x);
+}
+
+__attribute__((export_name("sin")))
+double libc_sin (double x) {
+  return sin(x);
+}
+
+__attribute__((export_name("tan")))
+double libc_tan (double x) {
+  return tan(x);
+}
+
+__attribute__((export_name("acos")))
+double libc_acos (double x) {
+  return acos(x);
+}
+
+__attribute__((export_name("asin")))
+double libc_asin (double x) {
+  return asin(x);
+}
+
+__attribute__((export_name("atan")))
+double libc_atan (double x) {
+  return atan(x);
+}
+
+__attribute__((export_name("cosh")))
+double libc_cosh (double x) {
+  return cosh(x);
+}
+
+__attribute__((export_name("sinh")))
+double libc_sinh (double x) {
+  return sinh(x);
+}
+
+__attribute__((export_name("tanh")))
+double libc_tanh (double x) {
+  return tanh(x);
+}
+
+__attribute__((export_name("acosh")))
+double libc_acosh (double x) {
+  return acosh(x);
+}
+
+__attribute__((export_name("asinh")))
+double libc_asinh (double x) {
+  return asinh(x);
+}
+
+__attribute__((export_name("atanh")))
+double libc_atanh (double x) {
+  return atanh(x);
+}
+
+__attribute__((export_name("cbrt")))
+double libc_cbrt (double x) {
+  return cbrt(x);
+}
+
+__attribute__((export_name("exp")))
+double libc_exp (double x) {
+  return exp(x);
+}
+
+__attribute__((export_name("expm1")))
+double libc_expm1 (double x) {
+  return expm1(x);
+}
+
+__attribute__((export_name("log")))
+double libc_log (double x) {
+  return log(x);
+}
+
+__attribute__((export_name("log1p")))
+double libc_log1p (double x) {
+  return log1p(x);
+}
+
+__attribute__((export_name("log2")))
+double libc_log2 (double x) {
+  return log2(x);
+}
+
+__attribute__((export_name("log10")))
+double libc_log10 (double x) {
+  return log10(x);
+}
+
+__attribute__((export_name("atan2")))
+double libc_atan2 (double x, double y) {
+  return atan2(x, y);
+}
+
+__attribute__((export_name("hypot")))
+double libc_hypot (double x, double y) {
+  return hypot(x, y);
+}
+
+__attribute__((export_name("pow")))
+double libc_pow (double x, double y) {
+  return pow(x, y);
+}
+
+__attribute__((export_name("fmod")))
+double libc_fmod (double x, double y) {
+  return fmod(x, y);
+}
+
+__attribute__((export_name("strtod")))
+double libc_strtod (const char * buf, char ** end) {
+  return strtod(buf, end);
+}
+
+__attribute__((export_name("format_float")))
+int format_float (char * buf, size_t len, const char * fmt, double f) {
+  return snprintf(buf, len, fmt, f);
+}
+
+__attribute__((export_name("malloc")))
+void * libc_malloc (size_t len) {
+  return malloc(len);
+}
+
+__attribute__((export_name("free")))
+void libc_free (void * ptr) {
+  return free(ptr);
+}
+
+__attribute__((export_name("strlen")))
+size_t libc_strlen (const char * s) {
+  return strlen(s);
+}
+
+
+__attribute__((export_name("gmtime")))
+struct tm * libc_gmtime (const time_t * timep) {
+  return gmtime(timep);
+}
+
+__attribute__((export_name("localtime")))
+struct tm * libc_localtime (const time_t * timep) {
+  return localtime(timep);
+}
+
+__attribute__((export_name("mktime")))
+time_t libc_mktime(struct tm *tm) {
+  return mktime(tm);
+}
+
+__attribute__((import_module("OCaml"), import_name("_initialize")))
+void start(void);
+
+int main () {
+  start();
+}
diff --git a/runtime/wasm/libc.wasm b/runtime/wasm/libc.wasm
new file mode 100644
index 0000000000..5e3f34061d
Binary files /dev/null and b/runtime/wasm/libc.wasm differ
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 4aa53e5936..553d7016e1 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -30,11 +30,6 @@
       (func $caml_is_closure (param (ref eq)) (result i32)))
    (import "effect" "caml_is_continuation"
       (func $caml_is_continuation (param (ref eq)) (result i32)))
-   (import "bindings" "map_new" (func $map_new (result (ref any))))
-   (import "bindings" "map_get"
-      (func $map_get (param (ref any)) (param (ref eq)) (result i31ref)))
-   (import "bindings" "map_set"
-      (func $map_set (param (ref any)) (param (ref eq)) (param (ref i31))))
    (import "io" "caml_really_putblock"
       (func $caml_really_putblock
          (param (ref eq)) (param (ref $bytes)) (param i32) (param i32)))
@@ -49,6 +44,76 @@
    (import "custom" "caml_find_custom_operations"
       (func $caml_find_custom_operations
          (param (ref $bytes)) (result (ref null $custom_operations))))
+   (type $block (array (mut (ref eq))))
+(@if wasi
+(@then
+   (type $map
+      (struct
+         (field $size (mut i32))
+         (field $keys (mut (ref $block)))
+         (field $values (mut (ref $block)))))
+   (func $map_new (result (ref any))
+      (struct.new $map
+         (i32.const 0)
+         (array.new $block (ref.i31 (i32.const 0)) (i32.const 2))
+         (array.new $block (ref.i31 (i32.const 0)) (i32.const 2))))
+   (func $map_get (param $map (ref any)) (param $k (ref eq)) (result i31ref)
+      (local $m (ref $map)) (local $keys (ref $block))
+      (local $i i32) (local $size i32)
+      (local.set $m (ref.cast (ref $map) (local.get $map)))
+      (local.set $size (struct.get $map $size (local.get $m)))
+      (local.set $keys (struct.get $map $keys (local.get $m)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $size))
+            (then
+               (if (ref.eq (array.get $block (local.get $keys) (local.get $i))
+                      (local.get $k))
+                  (then
+                     (return
+                        (ref.cast (ref i31)
+                           (array.get $block
+                              (struct.get $map $values (local.get $m))
+                              (local.get $i))))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (ref.null i31))
+   (func $map_set (param $map (ref any)) (param $k (ref eq)) (param $v (ref i31))
+      (local $m (ref $map)) (local $i i32) (local $size i32)
+      (local $keys (ref $block)) (local $a (ref $block))
+      (local.set $m (ref.cast (ref $map) (local.get $map)))
+      (local.set $i (struct.get $map $size (local.get $m)))
+      (local.set $keys (struct.get $map $keys (local.get $m)))
+      (if (i32.eq (local.get $i) (array.len (local.get $keys)))
+         (then
+            (local.set $size (i32.shl (local.get $i) (i32.const 1)))
+            (local.set $a
+               (array.new $block (ref.i31 (i32.const 0)) (local.get $size)))
+            (array.copy $block $block
+               (local.get $a) (i32.const 0)
+               (local.get $keys) (i32.const 0)
+               (local.get $i))
+            (struct.set $map $keys (local.get $m) (local.get $a))
+            (local.set $keys (local.get $a))
+            (local.set $a
+               (array.new $block (ref.i31 (i32.const 0)) (local.get $size)))
+            (array.copy $block $block
+               (local.get $a) (i32.const 0)
+               (struct.get $map $values (local.get $m)) (i32.const 0)
+               (local.get $i))
+            (struct.set $map $values (local.get $m) (local.get $a))))
+      (array.set $block (local.get $keys) (local.get $i) (local.get $k))
+      (array.set $block (struct.get $map $values (local.get $m))
+         (local.get $i) (local.get $v))
+      (struct.set $map $size (local.get $m)
+         (i32.add (local.get $i) (i32.const 1))))
+)
+(@else
+   (import "bindings" "map_new" (func $map_new (result (ref any))))
+   (import "bindings" "map_get"
+      (func $map_get (param (ref any)) (param (ref eq)) (result i31ref)))
+   (import "bindings" "map_set"
+      (func $map_set (param (ref any)) (param (ref eq)) (param (ref i31))))
+))
 
    (@string $input_val_from_string "input_value_from_string")
 
@@ -130,7 +195,6 @@
          (global.get $input_value))
       (return_call $intern_rec (local.get $s) (local.get $h)))
 
-   (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
diff --git a/runtime/wasm/prng.wat b/runtime/wasm/prng.wat
index 4918eaa0bf..08e242056e 100644
--- a/runtime/wasm/prng.wat
+++ b/runtime/wasm/prng.wat
@@ -16,10 +16,20 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+
+(@if wasi
+(@then
+   (import "bigarray" "ta_get_i32"
+      (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_set_i32"
+      (func $ta_set_i32 (param (ref extern)) (param i32) (param i32)))
+)
+(@else
    (import "bindings" "ta_get_i32"
       (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
    (import "bindings" "ta_set_i32"
       (func $ta_set_i32 (param (ref extern)) (param i32) (param i32)))
+))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
 
diff --git a/runtime/wasm/runtime-wasi.js b/runtime/wasm/runtime-wasi.js
new file mode 100644
index 0000000000..b892b15866
--- /dev/null
+++ b/runtime/wasm/runtime-wasi.js
@@ -0,0 +1,84 @@
+// Wasm_of_ocaml runtime support
+// http://www.ocsigen.org/js_of_ocaml/
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, with linking exception;
+// either version 2.1 of the License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+(js) => async (args) => {
+  // biome-ignore lint/suspicious/noRedundantUseStrict:
+  "use strict";
+
+  const emitWarning = globalThis.process.emitWarning;
+  globalThis.process.emitWarning = function (...args) {
+    if (args[1] !== "ExperimentalWarning") emitWarning(...args);
+  };
+
+  const { link, src, generated } = args;
+
+  const { argv, env } = require("node:process");
+  const { WASI } = require("node:wasi");
+  const wasi = new WASI({
+    version: "preview1",
+    args: argv.slice(1),
+    env,
+    preopens: { ".": ".", "/tmp": "/tmp" },
+    returnOnExit: false,
+  });
+  const imports = wasi.getImportObject();
+  function loadRelative(src) {
+    const path = require("node:path");
+    const f = path.join(path.dirname(require.main.filename), src);
+    return require("node:fs/promises").readFile(f);
+  }
+  async function instantiateModule(code) {
+    return WebAssembly.instantiate(await code, imports);
+  }
+  async function instantiateFromDir() {
+    imports.env = {};
+    imports.OCaml = {};
+    const deps = [];
+    async function loadModule(module, isRuntime) {
+      const sync = module[1].constructor !== Array;
+      async function instantiate() {
+        const code = loadRelative(src + "/" + module[0] + ".wasm");
+        await Promise.all(sync ? deps : module[1].map((i) => deps[i]));
+        const wasmModule = await instantiateModule(code);
+        Object.assign(
+          isRuntime ? imports.env : imports.OCaml,
+          wasmModule.instance.exports,
+        );
+      }
+      const promise = instantiate();
+      deps.push(promise);
+      return promise;
+    }
+    async function loadModules(lst) {
+      for (const module of lst) {
+        await loadModule(module);
+      }
+    }
+    await loadModule(link[0], 1);
+    if (link.length > 1) {
+      await loadModule(link[1]);
+      const workers = new Array(20)
+        .fill(link.slice(2).values())
+        .map(loadModules);
+      await Promise.all(workers);
+    }
+    return { instance: { exports: Object.assign(imports.env, imports.OCaml) } };
+  }
+  const wasmModule = await instantiateFromDir();
+
+  wasi.start(wasmModule.instance);
+};
diff --git a/runtime/wasm/stdlib.wat b/runtime/wasm/stdlib.wat
index 62ff000f26..51daffaa93 100644
--- a/runtime/wasm/stdlib.wat
+++ b/runtime/wasm/stdlib.wat
@@ -31,7 +31,6 @@
    (import "obj" "caml_callback_2"
       (func $caml_callback_2
          (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "bindings" "write" (func $write (param i32) (param anyref)))
    (import "string" "caml_string_concat"
       (func $caml_string_concat
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
@@ -39,9 +38,25 @@
       (func $caml_format_exception (param (ref eq)) (result (ref eq))))
    (import "sys" "ocaml_exit" (tag $ocaml_exit))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "proc_exit" (func $exit (param i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "release_memory"
+      (func $release_memory (param i32 i32)))
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "libc" "memory" (memory 2))
+)
+(@else
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
+   (import "bindings" "write" (func $write (param i32) (param anyref)))
    (import "bindings" "exit" (func $exit (param i32)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -187,6 +202,8 @@
 
    (global $uncaught_exception (mut externref) (ref.null extern))
 
+(@if (not wasi)
+(@then
    (func $reraise_exception (result (ref eq))
       (throw $javascript_exception (global.get $uncaught_exception))
       (ref.i31 (i32.const 0)))
@@ -194,11 +211,30 @@
    (func (export "caml_handle_uncaught_exception") (param $exn externref)
       (global.set $uncaught_exception (local.get $exn))
       (call $caml_main (ref.func $reraise_exception)))
+))
+
+   (type $wrapper_func (func (param (ref $func))))
+   (global $caml_main_wrapper (export "caml_main_wrapper")
+      (mut (ref null $wrapper_func))
+      (ref.null $wrapper_func))
 
    (func $caml_main (export "caml_main") (param $start (ref func))
       (local $exn (ref eq))
+      (local $msg (ref eq))
+(@if wasi
+(@then
+      (local $buffer i32) (local $i i32) (local $len i32)
+      (local $buf i32) (local $remaining i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $res i32)
+))
       (try
          (do
+            (block $fallback
+               (call_ref $wrapper_func
+                  (ref.cast (ref $func) (local.get $start))
+                  (br_on_null $fallback (global.get $caml_main_wrapper)))
+               (return))
             (drop (call_ref $func (ref.cast (ref $func) (local.get $start)))))
          (catch $ocaml_exit)
          (catch $ocaml_exception
@@ -223,13 +259,49 @@
                         (br_on_null $null
                            (call $caml_named_value (global.get $do_at_exit)))
                         (ref.i31 (i32.const 0)))))
+               (local.set $msg
+                  (call $caml_string_concat
+                     (global.get $fatal_error)
+                     (call $caml_string_concat
+                        (call $caml_format_exception (local.get $exn))
+                        (@string "\n"))))
+(@if wasi
+(@then
+               (local.set $len
+                  (array.len (ref.cast (ref $bytes) (local.get $msg))))
+               (local.set $buffer (call $get_buffer))
+               (local.set $nwritten (local.get $buffer))
+               (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+               (local.set $iovs_len (i32.const 1))
+               (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+               (local.set $buf
+                  (call $write_string_to_memory
+                     (local.get $buf) (global.get $IO_BUFFER_SIZE)
+                     (local.get $msg)))
+               (local.set $remaining (local.get $buf))
+               (loop $write
+                  (i32.store (local.get $iovs) (local.get $remaining))
+                  (i32.store offset=4 (local.get $iovs) (local.get $len))
+                  (local.set $res
+                     (call $fd_write
+                        (i32.const 2) (local.get $iovs) (local.get $iovs_len)
+                        (local.get $nwritten)))
+                  (if (i32.eqz (local.get $res))
+                     (then
+                        (local.set $len
+                           (i32.sub (local.get $len)
+                              (i32.load (local.get $nwritten))))
+                        (local.set $remaining
+                           (i32.add (local.get $remaining)
+                              (i32.load (local.get $nwritten))))
+                        (br_if $write (local.get $len)))))
+               (call $release_memory (local.get $buffer) (local.get $buf))
+)
+(@else
                (call $write (i32.const 2)
                   (call $unwrap
-                     (call $caml_jsstring_of_string
-                        (call $caml_string_concat
-                           (global.get $fatal_error)
-                           (call $caml_string_concat
-                              (call $caml_format_exception (local.get $exn))
-                              (@string "\n")))))))
+                     (call $caml_jsstring_of_string (local.get $msg))))
+))
+            )
             (call $exit (i32.const 2)))))
 )
diff --git a/runtime/wasm/sys.wat b/runtime/wasm/sys.wat
index 788e0ee478..98cd53016d 100644
--- a/runtime/wasm/sys.wat
+++ b/runtime/wasm/sys.wat
@@ -16,6 +16,40 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "fail" "caml_raise_sys_error"
+      (func $caml_raise_sys_error (param (ref eq))))
+   (import "fail" "caml_raise_not_found" (func $caml_raise_not_found))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "random_get"
+      (func $random_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "clock_time_get"
+      (func $clock_time_get (param i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "args_get"
+      (func $args_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "args_sizes_get"
+      (func $args_sizes_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "environ_get"
+      (func $environ_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "environ_sizes_get"
+      (func $environ_sizes_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "proc_exit" (func $exit (param i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "strlen" (func $strlen (param i32) (result i32)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_errors" "error_messages" (global $error_messages (ref $block)))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "fail" "caml_invalid_argument"
+      (func $caml_invalid_argument (param (ref eq))))
+)
+(@else
    (import "bindings" "ta_length"
       (func $ta_length (param (ref extern)) (result i32)))
    (import "bindings" "ta_get_i32"
@@ -32,9 +66,6 @@
    (import "jslib" "caml_js_meth_call"
       (func $caml_js_meth_call
          (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "fail" "caml_raise_sys_error"
-      (func $caml_raise_sys_error (param (ref eq))))
-   (import "fail" "caml_raise_not_found" (func $caml_raise_not_found))
    (import "bindings" "argv" (func $argv (result (ref extern))))
    (import "bindings" "on_windows" (global $on_windows i32))
    (import "bindings" "isatty"
@@ -42,15 +73,12 @@
    (import "bindings" "system" (func $system (param anyref) (result (ref eq))))
    (import "bindings" "getenv" (func $getenv (param anyref) (result anyref)))
    (import "bindings" "time" (func $time (result f64)))
-   (import "bindings" "array_length"
-      (func $array_length (param (ref extern)) (result i32)))
-   (import "bindings" "array_get"
-      (func $array_get (param (ref extern)) (param i32) (result anyref)))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
    (import "jsstring" "jsstring_test"
       (func $jsstring_test (param anyref) (result i32)))
-   (import "bindings" "exit" (func $exit (param (ref eq))))
+   (import "bindings" "exit" (func $exit (param i32)))
+))
    (import "io" "caml_channel_descriptor"
       (func $caml_channel_descriptor (param (ref eq)) (result (ref eq))))
 
@@ -62,12 +90,100 @@
 
    (func (export "caml_sys_exit") (export "unix_exit") (export "caml_unix_exit")
       (param $code (ref eq)) (result (ref eq))
-      (call $exit (local.get $code))
+      (call $exit (i31.get_s (ref.cast (ref i31) (local.get $code))))
       ;; Fallback: try to exit through an exception
       (throw $ocaml_exit))
 
-   (export "caml_sys_unsafe_getenv" (func $caml_sys_getenv))
-   (func $caml_sys_getenv (export "caml_sys_getenv")
+(@if wasi
+(@then
+   (global $environment (mut i32) (i32.const 0))
+   (global $environment_count (mut i32) (i32.const 0))
+   (global $environment_data (mut i32) (i32.const 0))
+
+   (func $initialize_env
+      (local $buffer i32) (local $res i32) (local $env i32) (local $data i32)
+      (if (i32.eqz (global.get $environment))
+         (then
+            (local.set $buffer (call $get_buffer))
+            (local.set $res
+               (call $environ_sizes_get
+                  (local.get $buffer)
+                  (i32.add (local.get $buffer) (i32.const 4))))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+            (local.set $env
+               (call $checked_malloc
+                  (i32.shl (i32.load (local.get $buffer)) (i32.const 2))))
+            (local.set $data
+               (call $checked_malloc (i32.load offset=4 (local.get $buffer))))
+            (local.set $res
+               (call $environ_get (local.get $env) (local.get $data)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+            (global.set $environment (local.get $env))
+            (global.set $environment_data (local.get $data))
+            (global.set $environment_count (i32.load (local.get $buffer))))))
+
+   (func (export "caml_sys_getenv") (export "caml_sys_unsafe_getenv")
+      (param $name (ref eq)) (result (ref eq))
+      (local $var (ref $bytes)) (local $i i32) (local $j i32)
+      (local $len i32) (local $s i32) (local $c i32)
+      (call $initialize_env)
+      (local.set $var (ref.cast (ref $bytes) (local.get $name)))
+      (local.set $len (array.len (local.get $var)))
+      (block $not_found
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $len))
+               (then
+                  (br_if $not_found
+                     (i32.eq (i32.const 61) ;; '='
+                         (array.get_u $bytes (local.get $var) (local.get $i))))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop))))
+         (local.set $i (i32.const 0))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (global.get $environment_count))
+               (then
+                  (local.set $s
+                     (i32.load
+                        (i32.add (global.get $environment)
+                           (i32.shl (local.get $i) (i32.const 2)))))
+                  (local.set $j (i32.const 0))
+                  (block $next
+                     (loop $scan
+                        (if (i32.lt_u (local.get $j) (local.get $len))
+                           (then
+                              (local.set $c
+                                 (i32.load8_u
+                                    (i32.add (local.get $s) (local.get $j))))
+                              (br_if $next (i32.eqz (local.get $c)))
+                              (br_if $next
+                                  (i32.ne (local.get $c)
+                                     (array.get $bytes
+                                        (local.get $var) (local.get $j))))
+                              (local.set $j
+                                 (i32.add (local.get $j) (i32.const 1)))
+                              (br $scan))))
+                     (br_if $next
+                        (i32.ne (i32.const 61) ;; '='
+                           (i32.load8_u
+                              (i32.add (local.get $s) (local.get $j)))))
+                     (local.set $s
+                        (i32.add (local.get $s)
+                           (i32.add (local.get $j) (i32.const 1))))
+                     (return_call $blit_memory_to_string
+                        (local.get $s) (call $strlen (local.get $s))))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop)))))
+      (call $caml_raise_not_found)
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_sys_getenv") (export "caml_sys_unsafe_getenv")
       (param (ref eq)) (result (ref eq))
       (local $res anyref)
       (local.set $res
@@ -77,7 +193,65 @@
          (then
             (call $caml_raise_not_found)))
       (return_call $caml_string_of_jsstring (call $wrap (local.get $res))))
+))
+
+(@if wasi
+(@then
+   (global $argv (mut (ref null $block)) (ref.null $block))
+
+   (func $caml_sys_argv (export "caml_sys_argv")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $argc i32) (local $argv i32) (local $argv_buf i32)
+      (local $args (ref $block)) (local $arg i32) (local $i i32)
+      (block $init
+         (return (br_on_null $init (global.get $argv))))
+      (block $error
+         (local.set $buffer (call $get_buffer))
+         (local.set $res
+            (call $args_sizes_get
+               (local.get $buffer)
+               (i32.add (local.get $buffer) (i32.const 4))))
+         (br_if $error (local.get $res))
+         (local.set $argc (i32.load (local.get $buffer)))
+         (local.set $argv
+            (call $checked_malloc (i32.shl (local.get $argc) (i32.const 2))))
+         (local.set $argv_buf
+            (call $checked_malloc (i32.load offset=4 (local.get $buffer))))
+         (local.set $res
+            (call $args_get (local.get $argv) (local.get $argv_buf)))
+         (br_if $error (local.get $res))
+         (local.set $args
+            (array.new $block (ref.i31 (i32.const 0))
+               (i32.add (local.get $argc) (i32.const 1))))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $argc))
+               (then
+                  (local.set $arg
+                     (i32.load
+                        (i32.add (local.get $argv)
+                           (i32.shl (local.get $i) (i32.const 2)))))
+                  (array.set $block (local.get $args)
+                     (i32.add (local.get $i) (i32.const 1))
+                     (call $blit_memory_to_string
+                        (local.get $arg) (call $strlen (local.get $arg))))
+                  (local.set $i
+                     (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop))))
+         (global.set $argv (local.get $args))
+         (call $free (local.get $argv))
+         (call $free (local.get $argv_buf))
+         (return (local.get $args)))
+      (call $caml_handle_sys_error (ref.i31 (i32.const 0)) (local.get $res))
+      (array.new_fixed $block 0))
 
+   (func (export "caml_sys_executable_name")
+      (param (ref eq)) (result (ref eq))
+      (array.get $block
+         (ref.cast (ref $block) (call $caml_sys_argv (ref.i31 (i32.const 0))))
+         (i32.const 1)))
+)
+(@else
    (func (export "caml_sys_argv") (param (ref eq)) (result (ref eq))
       ;; ZZZ
       (call $caml_js_to_string_array (call $argv)))
@@ -87,24 +261,83 @@
       (array.get $block
          (ref.cast (ref $block) (call $caml_js_to_string_array (call $argv)))
          (i32.const 1)))
+))
 
-   (export "caml_sys_time_include_children" (func $caml_sys_time))
-   (func $caml_sys_time (export "caml_sys_time")
+(@if wasi
+(@then
+   (func (export "caml_sys_time") (export "caml_sys_time_include_children")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get (i32.const 2) (i64.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (struct.new $float
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))))
+)
+(@else
+   (func (export "caml_sys_time") (export "caml_sys_time_include_children")
       (param (ref eq)) (result (ref eq))
       (struct.new $float (f64.mul (call $time) (f64.const 0.001))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_system_command")
+      (param (ref eq)) (result (ref eq))
+      (call $caml_invalid_argument (@string "Sys.command not implemented"))
+      (return (ref.i31 (i32.const 0))))
+)
+(@else
    (func (export "caml_sys_system_command")
       (param (ref eq)) (result (ref eq))
-      ;; ZZZ
       (try
          (do
             (return
                (call $system
                   (call $unwrap (call $caml_jsstring_of_string (local.get 0))))))
          (catch $javascript_exception
-            (call $caml_handle_sys_error (pop externref))
-            (return (ref.i31 (i32.const 0))))))
+            (call $caml_handle_sys_error (pop externref))))
+      (return (ref.i31 (i32.const 0))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_random_seed")
+      (param (ref eq)) (result (ref eq))
+      (local $r (ref extern))
+      (local $a (ref $block))
+      (local $i i32) (local $n i32)
+      (local $buffer i32) (local $res i32)
+      (local.set $n (i32.const 12))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res (call $random_get (local.get $buffer) (i32.const 96)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (local.set $a
+         (array.new $block (ref.i31 (i32.const 0))
+            (i32.add (local.get $n) (i32.const 1))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $n))
+            (then
+               (array.set $block
+                  (local.get $a) (i32.add (local.get $i) (i32.const 1))
+                  (ref.i31
+                     (i32.load
+                        (i32.add
+                           (local.get $buffer
+                             (i32.shl (local.get $i) (i32.const 2)))))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $a))
+)
+(@else
    (func (export "caml_sys_random_seed")
       (param (ref eq)) (result (ref eq))
       (local $r (ref extern))
@@ -115,7 +348,6 @@
       (local.set $a
          (array.new $block (ref.i31 (i32.const 0))
             (i32.add (local.get $n) (i32.const 1))))
-      (local.set $i (i32.const 0))
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $n))
             (then
@@ -125,6 +357,7 @@
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (local.get $a))
+))
 
    (func (export "caml_sys_const_bigendian")
       (param (ref eq)) (result (ref eq))
@@ -142,6 +375,11 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0xfffffff)))
 
+(@if wasi
+(@then
+   (global $on_windows i32 (i32.const 0))
+))
+
    (func (export "caml_sys_const_ostype_unix")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.eqz (global.get $on_windows))))
@@ -165,9 +403,17 @@
          (ref.i31 (i32.const 32))
          (ref.i31 (i32.const 0))))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_isatty")
+      (param $ch (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_isatty")
       (param $ch (ref eq)) (result (ref eq))
       (return_call $isatty (call $caml_channel_descriptor (local.get $ch))))
+))
 
    (func (export "caml_runtime_variant") (param (ref eq)) (result (ref eq))
       (@string ""))
@@ -193,6 +439,28 @@
 
    (@string $toString "toString")
 
+(@if wasi
+(@then
+   (func $caml_handle_sys_error (export "caml_handle_sys_error")
+      (param $arg (ref eq)) (param $errno i32)
+      (local $msg (ref eq))
+      (local.set $msg
+         (if (result (ref eq)) (i32.gt_u (local.get $errno)
+                (array.len (global.get $error_messages)))
+            (then
+               (@string "unknown system error"))
+            (else
+               (array.get $block (global.get $error_messages)
+                  (local.get $errno)))))
+      (if (ref.test (ref $bytes) (local.get $arg))
+         (then
+            (local.set $msg
+               (call $caml_string_concat (local.get $arg)
+                  (call $caml_string_concat (@string ": ") (local.get $msg))))))
+      (call $caml_raise_sys_error (local.get $msg))
+   )
+)
+(@else
    (func $caml_handle_sys_error (export "caml_handle_sys_error")
       (param $exn externref)
       (call $caml_raise_sys_error
@@ -201,4 +469,5 @@
                (call $wrap (any.convert_extern (local.get $exn)))
                (global.get $toString)
                (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))))
+))
 )
diff --git a/runtime/wasm/unix.wat b/runtime/wasm/unix.wat
index 0b4a9229ae..748ce5656c 100644
--- a/runtime/wasm/unix.wat
+++ b/runtime/wasm/unix.wat
@@ -16,6 +16,73 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "clock_time_get"
+      (func $clock_time_get (param i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_get"
+      (func $path_filestat_get (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_rename"
+      (func $path_rename (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_link"
+      (func $path_link (param i32 i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_symlink"
+      (func $path_symlink (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_readlink"
+      (func $path_readlink (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_create_directory"
+      (func $path_create_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_unlink_file"
+      (func $path_unlink_file (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_remove_directory"
+      (func $path_remove_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_set_times"
+      (func $path_filestat_set_times
+         (param i32 i32 i32 i32 i64 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_filestat_get"
+      (func $fd_filestat_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_filestat_set_size"
+      (func $fd_filestat_set_size (param i32 i64) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_read"
+      (func $fd_read (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_seek"
+      (func $fd_seek (param i32 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_sync"
+      (func $fd_sync (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_readdir"
+      (func $fd_readddir (param i32 i32 i32 i64 i32) (result i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "gmtime" (func $gmtime (param i32) (result i32)))
+   (import "libc" "localtime" (func $localtime (param i32) (result i32)))
+   (import "libc" "mktime" (func $mktime (param i32) (result i64)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_memory" "blit_memory_to_substring"
+      (func $blit_memory_to_substring (param i32 (ref $bytes) i32 i32)))
+   (import "wasi_memory" "blit_substring_to_memory"
+      (func $blit_substring_to_memory (param i32 (ref $bytes) i32 i32)))
+   (import "fs" "wasi_resolve_path"
+      (func $wasi_resolve_path (param (ref eq)) (result i32 i32 i32)))
+   (import "fs" "wasi_chdir" (func $wasi_chdir (param (ref eq))))
+   (import "wasi_errors" "error_messages" (global $error_messages (ref $block)))
+   (import "ints" "caml_format_int"
+      (func $caml_format_int (param (ref eq) (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
+)
+(@else
    (import "bindings" "gettimeofday" (func $gettimeofday (result f64)))
    (import "bindings" "times" (func $times (result (ref eq))))
    (import "bindings" "gmtime" (func $gmtime (param f64) (result (ref eq))))
@@ -80,6 +147,7 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jsstring" "jsstring_test"
       (func $jsstring_test (param anyref) (result i32)))
+))
    (import "stdlib" "caml_named_value"
       (func $caml_named_value (param (ref eq)) (result (ref null eq))))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
@@ -149,6 +217,102 @@
 
    (@string $no_arg "")
 
+(@if wasi
+(@then
+   (func $unix_resolve_path (export "unix_resolve_path")
+      (param $cmd (ref eq)) (param $path (ref eq)) (result i32 i32 i32)
+      (local $res (tuple i32 i32 i32))
+      (local.set $res (call $wasi_resolve_path (local.get $path)))
+      (if (i32.lt_s (tuple.extract 3 0 (local.get $res)) (i32.const 0))
+         (then
+            (call $caml_unix_error
+               (i32.const 44) ;; ENOENT
+               (local.get $cmd) (local.get $path))))
+      (local.get $res))
+
+   (type $constr_table (array i8))
+   (global $error_codes (ref $constr_table)
+      (array.new_fixed $constr_table 77
+         (i32.const -1)
+         (i32.const  0) (i32.const  1) (i32.const 50) (i32.const 51)
+         (i32.const 49) (i32.const  2) (i32.const 39) (i32.const  3)
+         (i32.const -1) (i32.const  4) (i32.const -1) (i32.const  5)
+         (i32.const 55) (i32.const 63) (i32.const 56) (i32.const  6)
+         (i32.const 41) (i32.const  7) (i32.const -1) (i32.const  8)
+         (i32.const  9) (i32.const 10) (i32.const 65) (i32.const -1)
+         (i32.const -1) (i32.const 38) (i32.const 11) (i32.const 12)
+         (i32.const 13) (i32.const 58) (i32.const 14) (i32.const 66)
+         (i32.const 15) (i32.const 16) (i32.const 42) (i32.const -1)
+         (i32.const 17) (i32.const 52) (i32.const 54) (i32.const 53)
+         (i32.const 18) (i32.const 57) (i32.const 19) (i32.const 20)
+         (i32.const 21) (i32.const 22) (i32.const -1) (i32.const 23)
+         (i32.const -1) (i32.const 44) (i32.const 24) (i32.const 25)
+         (i32.const 59) (i32.const 26) (i32.const 27) (i32.const -1)
+         (i32.const 40) (i32.const 47) (i32.const 28) (i32.const 29)
+         (i32.const 67) (i32.const -1) (i32.const 30) (i32.const 31)
+         (i32.const -1) (i32.const 45) (i32.const 43) (i32.const 32)
+         (i32.const 33) (i32.const 34) (i32.const 35) (i32.const -1)
+         (i32.const 62) (i32.const -1) (i32.const 36) (i32.const -1)))
+
+   (func $caml_unix_error_of_code (param $errcode i32) (result (ref eq))
+      (local $err i32)
+      (if (i32.le_u (local.get $errcode) (i32.const 76))
+         (then
+            (local.set $err
+               (array.get_s $constr_table (global.get $error_codes)
+                  (local.get $errcode)))
+            (if (i32.ne (local.get $err) (i32.const -1))
+               (then
+                  (return (ref.i31 (local.get $err)))))))
+      (array.new_fixed $block 2
+         (ref.i31 (i32.const 0)) (ref.i31 (local.get $errcode))))
+
+   (func $caml_unix_error
+      (param $errcode i32) (param $cmd_name (ref eq)) (param $cmd_arg (ref eq))
+      (throw $ocaml_exception
+         (array.new_fixed $block 5
+            (ref.i31 (i32.const 0))
+            (call $get_unix_error_exn)
+            (call $caml_unix_error_of_code (local.get $errcode))
+            (local.get $cmd_name)
+            (local.get $cmd_arg))))
+
+   (func (export "unix_error_message") (export "caml_unix_error_message")
+      (param $err (ref eq)) (result (ref eq))
+      (local $errcode i32) (local $i i32) (local $n i32)
+      (if (ref.test (ref i31) (local.get $err))
+         (then
+            (local.set $n (i31.get_u (ref.cast (ref i31) (local.get $err))))
+            (loop $loop
+               (if (i32.lt_u (local.get $errcode)
+                      (array.len (global.get $error_codes)))
+                  (then
+                     (if (i32.ne (local.get $n)
+                            (array.get $constr_table (global.get $error_codes)
+                               (local.get $errcode)))
+                        (then
+                           (local.set $errcode
+                              (i32.add (local.get $errcode) (i32.const 1)))
+                           (br $loop))))
+                  (else
+                     (local.set $errcode (i32.const -1))))))
+         (else
+            (local.set $errcode
+               (i31.get_u
+                  (ref.cast (ref i31)
+                     (array.get $block
+                        (ref.cast (ref $block) (local.get $err))
+                        (i32.const 1)))))))
+      (if (i32.gt_u (local.get $errcode)
+             (array.len (global.get $error_messages)))
+         (then
+            (return_call $caml_string_concat
+               (@string "Unknown error ")
+               (call $caml_format_int (@string "%d")
+                  (ref.i31 (local.get $errcode))))))
+      (array.get $block (global.get $error_messages) (local.get $errcode)))
+)
+(@else
    (global $unix_error (ref eq) (struct.new $js (global.get $unix_error_js)))
 
    (func $ensure_string (param $s (ref eq)) (result (ref eq))
@@ -228,11 +392,52 @@
                            (i32.const 1))))))))
       (return_call $caml_string_of_jsstring
          (call $wrap (call $caml_strerror (local.get $errno)))))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_gettimeofday") (export "caml_unix_gettimeofday")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 0) (i64.const 1000) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "gettimeofday") (global.get $no_arg))))
+      (struct.new $float
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))))
+)
+(@else
    (func (export "unix_gettimeofday") (export "caml_unix_gettimeofday")
       (param (ref eq)) (result (ref eq))
       (struct.new $float (call $gettimeofday)))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_times") (export "caml_unix_times")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 2) (i64.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res)
+               (@string "time")
+               (global.get $no_arg))))
+      (array.new_fixed $float_array 4
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))
+         (f64.const 0) (f64.const 0) (f64.const 0)))
+)
+(@else
    (func (export "caml_alloc_times")
       (param $u f64) (param $s f64) (result (ref eq))
       (array.new_fixed $float_array 4
@@ -241,7 +446,24 @@
    (func (export "unix_times") (export "caml_unix_times")
       (param (ref eq)) (result (ref eq))
       (return_call $times))
+))
 
+(@if wasi
+(@then
+  (func $alloc_tm (param $tm i32) (result (ref eq))
+      (array.new_fixed $block 10 (ref.i31 (i32.const 0))
+         (ref.i31 (i32.load (local.get $tm)))
+         (ref.i31 (i32.load offset=4 (local.get $tm)))
+         (ref.i31 (i32.load offset=8 (local.get $tm)))
+         (ref.i31 (i32.load offset=12 (local.get $tm)))
+         (ref.i31 (i32.load offset=16 (local.get $tm)))
+         (ref.i31 (i32.load offset=20 (local.get $tm)))
+         (ref.i31 (i32.load offset=24 (local.get $tm)))
+         (ref.i31 (i32.load offset=28 (local.get $tm)))
+         (ref.i31 (select (i32.const 1) (i32.const 0)
+            (i32.load offset=32 (local.get $tm))))))
+)
+(@else
    (func (export "caml_alloc_tm")
       (param $sec i32) (param $min i32) (param $hour i32) (param $mday i32)
       (param $mon i32) (param $year i32) (param $wday i32) (param $yday i32)
@@ -256,22 +478,132 @@
          (ref.i31 (local.get $wday))
          (ref.i31 (local.get $yday))
          (ref.i31 (local.get $isdst))))
-
-   (func $unix_gmtime (export "unix_gmtime") (export "caml_unix_gmtime")
+))
+
+(@if wasi
+(@then
+   (func (export "caml_unix_gmtime") (export "unix_gmtime")
+      (param $t (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $tm i32)
+      (local.set $buffer (call $get_buffer))
+      (i64.store (local.get $buffer)
+         (i64.trunc_sat_f64_s
+            (struct.get $float 0 (ref.cast (ref $float) (local.get $t)))))
+      (local.set $tm (call $gmtime (local.get $buffer)))
+      (if (i32.eqz (local.get $tm))
+         (then
+            (call $caml_unix_error (i32.const 28) (; EINVAL ;)
+               (@string "gmtime") (global.get $no_arg))))
+      (return_call $alloc_tm (local.get $tm)))
+)
+(@else
+   (func (export "caml_unix_gmtime") (export "unix_gmtime")
       (param (ref eq)) (result (ref eq))
       (call $gmtime
          (struct.get $float 0 (ref.cast (ref $float) (local.get 0)))))
-
-   (func $unix_localtime (export "unix_localtime") (export "caml_unix_localtime")
+))
+
+(@if wasi
+(@then
+   (func (export "caml_unix_localtime") (export "unix_localtime")
+      (param $t (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $tm i32)
+      (local.set $buffer (call $get_buffer))
+      (i64.store (local.get $buffer)
+         (i64.trunc_sat_f64_s
+            (struct.get $float 0 (ref.cast (ref $float) (local.get $t)))))
+      (local.set $tm (call $localtime (local.get $buffer)))
+      (if (i32.eqz (local.get $tm))
+         (then
+            (call $caml_unix_error (i32.const 28) (; EINVAL ;)
+               (@string "localtime") (global.get $no_arg))))
+      (return_call $alloc_tm (local.get $tm)))
+)
+(@else
+   (func (export "caml_unix_localtime") (export "unix_localtime")
       (param (ref eq)) (result (ref eq))
       (call $localtime
          (struct.get $float 0 (ref.cast (ref $float) (local.get 0)))))
+))
 
-   (func $unix_time (export "unix_time") (export "caml_unix_time")
+(@if wasi
+(@then
+   (func (export "caml_unix_time") (export "unix_time") (param (ref eq))
+      (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 0) (i64.const 1000) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "time") (global.get $no_arg))))
+      (struct.new $float
+         (f64.floor
+            (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+               (f64.const 1e-9)))))
+)
+(@else
+   (func (export "caml_unix_time") (export "unix_time")
       (param (ref eq)) (result (ref eq))
       (struct.new $float (f64.floor (call $gettimeofday))))
-
-   (func $unix_mktime (export "unix_mktime") (export "caml_unix_mktime")
+))
+
+(@if wasi
+(@then
+   (func (export "caml_unix_mktime") (export "unix_mktime")
+      (param $v (ref eq)) (result (ref eq))
+      (local $t (ref $block)) (local $tm i32) (local $time i64)
+      (local.set $t (ref.cast (ref $block) (local.get $v)))
+      (local.set $tm (call $get_buffer))
+      (i32.store (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 1)))))
+      (i32.store offset=4 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 2)))))
+      (i32.store offset=8 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 3)))))
+      (i32.store offset=12 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 4)))))
+      (i32.store offset=16 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 5)))))
+      (i32.store offset=20 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 6)))))
+      (i32.store offset=24 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 7)))))
+      (i32.store offset=28 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 8)))))
+      (i32.store offset=32 (local.get $tm)
+         (i32.const -1))
+      (local.set $time (call $mktime (local.get $tm)))
+      (if (i64.eq (local.get $time) (i64.const -1))
+         (then
+            (call $caml_unix_error
+               (i32.const 68) (; ERANGE ;)
+               (@string "mktime") (global.get $no_arg))))
+      (array.new_fixed $block 3
+         (ref.i31 (i32.const 0))
+         (struct.new $float (f64.convert_i64_s (local.get $time)))
+         (call $alloc_tm (local.get $tm))))
+)
+(@else
+   (func (export "caml_unix_mktime") (export "unix_mktime")
       (param (ref eq)) (result (ref eq))
       (local $tm (ref $block)) (local $t f64)
       (local.set $tm (ref.cast (ref $block) (local.get 0)))
@@ -302,7 +634,53 @@
       (array.new_fixed $block 3 (ref.i31 (i32.const 0))
          (struct.new $float (local.get $t))
          (call $localtime (local.get $t))))
+))
+
+(@if wasi
+(@then
+   (@string $utimes "utimes")
 
+   (func (export "unix_utimes") (export "caml_unix_utimes")
+      (param $path (ref eq)) (param $atime (ref eq)) (param $mtime (ref eq))
+      (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $atim i64) (local $mtim i64)
+      (local $set_to_now i32) (local $res i32)
+      (local $at f64) (local $mt f64)
+      (local.set $p
+         (call $unix_resolve_path (global.get $utimes) (local.get $path)))
+      (local.set $at
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $atime))))
+      (local.set $mt
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $mtime))))
+      (local.set $set_to_now
+         (i32.and (f64.eq (local.get $at) (f64.const 0))
+            (f64.eq (local.get $mt) (f64.const 0))))
+      (if (i32.eqz (local.get $set_to_now))
+         (then
+            (local.set $atim
+               (i64.trunc_sat_f64_s
+                  (f64.mul (local.get $at) (f64.const 1e9))))
+            (local.set $mtim
+               (i64.trunc_sat_f64_s
+                  (f64.mul (local.get $mt) (f64.const 1e9))))))
+      (local.set $res
+         (call $path_filestat_set_times
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 0)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $atim)
+            (local.get $mtim)
+            (i32.shl (i32.const 5) (local.get $set_to_now))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $utimes) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_utimes") (export "caml_unix_utimes")
       (param $path (ref eq)) (param $atime (ref eq)) (param $mtime (ref eq))
       (result (ref eq))
@@ -324,6 +702,48 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (global $file_kinds (ref $constr_table)
+      (array.new_fixed $constr_table 8
+         (i32.const 3)
+         (i32.const 3)
+         (i32.const 2)
+         (i32.const 1)
+         (i32.const 0)
+         (i32.const 6)
+         (i32.const 6)
+         (i32.const 4)))
+
+   (func $alloc_stat (param $large i32) (param $p i32) (result (ref eq))
+      (array.new_fixed $block 13 (ref.i31 (i32.const 0))
+         (ref.i31 (i32.wrap_i64 (i64.load (local.get $p))))
+         (ref.i31 (i32.wrap_i64 (i64.load offset=8 (local.get $p))))
+         (ref.i31
+            (array.get $constr_table
+               (global.get $file_kinds) (i32.load8_u offset=16 (local.get $p))))
+         (ref.i31 (i32.const 384 (;0600;)))
+         (ref.i31 (i32.wrap_i64 (i64.load offset=24 (local.get $p))))
+         (ref.i31 (i32.const 1))
+         (ref.i31 (i32.const 1))
+         (ref.i31 (i32.wrap_i64 (i64.load (local.get $p))))
+         (if (result (ref eq)) (local.get $large)
+            (then
+               (call $caml_copy_int64 (i64.load offset=32 (local.get $p))))
+            (else
+               (ref.i31 (i32.wrap_i64 (i64.load offset=32 (local.get $p))))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=40 (local.get $p)))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=48 (local.get $p)))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=56 (local.get $p)))))))
+))
 
    (func (export "caml_alloc_stat")
       (param $large i32)
@@ -349,6 +769,76 @@
          (struct.new $float (local.get $mtime))
          (struct.new $float (local.get $ctime))))
 
+(@if wasi
+(@then
+   (func $stat
+      (param $path (ref eq)) (param $large i32) (param $follow i32)
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (local.get $name) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (local.get $follow) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (local.get $name) (local.get $path))))
+      (return_call $alloc_stat (local.get $large) (local.get $buffer)))
+
+   (@string $stat "stat")
+
+   (func (export "unix_stat") (export "caml_unix_stat")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 0) (i32.const 1) (global.get $stat)))
+
+   (func (export "unix_stat_64") (export "caml_unix_stat_64")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 1) (i32.const 1) (global.get $stat)))
+
+   (@string $lstat "lstat")
+
+   (func (export "unix_lstat") (export "caml_unix_lstat")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 0) (i32.const 0) (global.get $lstat)))
+
+   (func (export "unix_lstat_64") (export "caml_unix_lstat_64")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 1) (i32.const 0) (global.get $lstat)))
+
+   (func $fstat (param $fd (ref eq)) (param $large i32) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_filestat_get
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "fstat") (global.get $no_arg))))
+      (return_call $alloc_stat (local.get $large) (local.get $buffer)))
+
+   (func (export "unix_fstat") (export "caml_unix_fstat")
+      (param $fd (ref eq)) (result (ref eq))
+      (return_call $fstat (local.get $fd) (i32.const 0)))
+
+   (func (export "unix_fstat_64") (export "caml_unix_fstat_64")
+      (param $fd (ref eq)) (result (ref eq))
+      (return_call $fstat (local.get $fd) (i32.const 1)))
+)
+(@else
    (func (export "unix_stat") (export "caml_unix_stat")
       (param $path (ref eq)) (result (ref eq))
       (try (result (ref eq))
@@ -410,7 +900,16 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))
             (ref.i31 (i32.const 0)))))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_chmod") (export "caml_unix_chmod")
+      (param (ref eq) (ref eq)) (result (ref eq))
+      ;; no notion of permissions in WASI
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_chmod") (export "caml_unix_chmod")
       (param $path (ref eq)) (param $perms (ref eq)) (result (ref eq))
       (try
@@ -421,7 +920,16 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_fchmod") (export "caml_unix_fchmod")
+      (param (ref eq) (ref eq)) (result (ref eq))
+      ;; no notion of permissions in WASI
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_fchmod") (export "caml_unix_fchmod")
       (param $fd (ref eq)) (param $perms (ref eq)) (result (ref eq))
       (try
@@ -430,7 +938,38 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $rename "rename")
 
+   (func (export "unix_rename") (export "caml_unix_rename")
+      (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $op
+         (call $unix_resolve_path (global.get $rename)  (local.get $o)))
+      (local.set $np
+         (call $unix_resolve_path (global.get $rename) (local.get $n)))
+      (local.set $res
+         (call $path_rename
+            (tuple.extract 3 0 (local.get $op))
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $rename) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_rename") (export "caml_unix_rename")
       (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
       (try
@@ -441,7 +980,39 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (@string $chdir "chdir")
+
+   (func (export "unix_chdir") (export "caml_unix_chdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $kind i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $chdir) (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $chdir) (local.get $name))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (if (i32.ne (local.get $kind) (i32.const 3))
+         (then
+            (call $caml_unix_error (i32.const 54) ;; ENOTDIR
+               (global.get $chdir) (local.get $name))))
+      (call $wasi_chdir (local.get $name))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_getcwd") (export "caml_unix_getcwd")
       (param (ref eq)) (result (ref eq))
       (try (result (ref eq))
@@ -460,7 +1031,31 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $mkdir "mkdir")
 
+   (func (export "unix_mkdir") (export "caml_unix_mkdir")
+      (param $path (ref eq)) (param $perm (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $mkdir) (local.get $path)))
+      (local.set $res
+         (call $path_create_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $mkdir) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_mkdir") (export "caml_unix_mkdir")
       (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
       (try
@@ -471,7 +1066,147 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (type $directory
+      (struct
+         (field $fd i32)
+         (field $buffer (mut i32))
+         (field $size (mut i32))
+         (field $pos (mut i32))
+         (field $available (mut i32))
+         (field $cookie (mut i64))))
+
+   (@string $opendir "opendir")
+
+   (func $unix_opendir (export "unix_opendir") (export "caml_unix_opendir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $opendir) (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (i32.const 2) ;; O_DIRECTORY
+            (i64.const 0x4000) ;; allow fd_readdir
+            (i64.const 0)
+            (i32.const 0)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $opendir) (local.get $name))))
+      (struct.new $directory
+         (i32.load (local.get $buffer))
+         (call $checked_malloc (i32.const 512))
+         (i32.const 512)
+         (i32.const 0)
+         (i32.const 0)
+         (i64.const 0)))
+
+   (func $readdir_helper
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory)) (local $buf i32) (local $res i32)
+      (local $buffer i32) (local $available i32) (local $left i32)
+      (local $namelen i32) (local $entry i32) (local $entry_size i32)
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (loop $loop
+         (block $refill
+            (local.set $left
+               (i32.sub (struct.get $directory $available (local.get $dir))
+                  (struct.get $directory $pos (local.get $dir))))
+            (br_if $refill (i32.lt_u (local.get $left) (i32.const 24)))
+            (local.set $entry
+               (i32.add (struct.get $directory $buffer (local.get $dir))
+                  (struct.get $directory $pos (local.get $dir))))
+            (local.set $namelen (i32.load offset=16 (local.get $entry)))
+            (local.set $entry_size (i32.add (local.get $namelen) (i32.const 24)))
+            (br_if $refill (i32.lt_u (local.get $left) (local.get $entry_size)))
+            (struct.set $directory $pos (local.get $dir)
+               (i32.add (struct.get $directory $pos (local.get $dir))
+                  (local.get $entry_size)))
+            (struct.set $directory $cookie (local.get $dir)
+               (i64.load (local.get $entry)))
+            (return_call $blit_memory_to_string
+                (i32.add (local.get $entry) (i32.const 24))
+                (local.get $namelen)))
+         ;; refill
+         (if (i32.lt_u (struct.get $directory $size (local.get $dir))
+                (local.get $entry_size))
+            (then
+               ;; the entry does not fit
+               (local.set $buf (call $checked_malloc (local.get $entry_size)))
+               (call $free (struct.get $directory $buffer (local.get $dir)))
+               (struct.set $directory $buffer (local.get $dir) (local.get $buf))
+               (struct.set $directory $size (local.get $dir)
+                  (local.get $entry_size))))
+         (block $done
+            (br_if $done
+               (i32.and
+                  (i32.ne (i32.const 0)
+                     (struct.get $directory $available (local.get $dir))
+                  (i32.lt_u (struct.get $directory $available (local.get $dir))
+                     (struct.get $directory $size (local.get $dir))))))
+            (local.set $buffer (call $get_buffer))
+            (local.set $res
+               (call $fd_readddir
+                  (struct.get $directory $fd (local.get $dir))
+                  (struct.get $directory $buffer (local.get $dir))
+                  (struct.get $directory $size (local.get $dir))
+                  (struct.get $directory $cookie (local.get $dir))
+                  (local.get $buffer)))
+            (if (local.get $res)
+               (then
+                  (call $caml_unix_error
+                     (local.get $res) (@string "readdir") (global.get $no_arg))))
+            (local.set $available (i32.load (local.get $buffer)))
+            (br_if $done (i32.eqz (local.get $available)))
+            (struct.set $directory $pos (local.get $dir) (i32.const 0))
+            (struct.set $directory $available (local.get $dir)
+               (local.get $available))
+            (br $loop)))
+      ;; done
+      (call $caml_raise_end_of_file)
+      (ref.i31 (i32.const 0)))
+
+   (func $unix_closedir (export "unix_closedir") (export "caml_unix_closedir")
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory)) (local $buf i32) (local $res i32)
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (local.set $buf (struct.get $directory $buffer (local.get $dir)))
+      (block $error
+         (if (i32.eqz (local.get $buf))
+            (then
+               (local.set $res (i32.const 8)) ;; EBADF
+               (br $error)))
+         (call $free (local.get $buf))
+         (struct.set $directory $buffer (local.get $dir) (i32.const 0))
+         (local.set $res
+            (call $fd_close (struct.get $directory $fd (local.get $dir))))
+         (br_if $error (local.get $res))
+         (return (ref.i31 (i32.const 0))))
+      (call $caml_unix_error
+         (local.get $res) (@string "closedir") (global.get $no_arg))
+      (ref.i31 (i32.const 0)))
 
+   (func (export "unix_rewinddir") (export "caml_unix_rewinddir")
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory))
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (struct.set $directory $cookie (local.get $dir) (i64.const 0))
+      (struct.set $directory $pos (local.get $dir) (i32.const 0))
+      (struct.set $directory $available (local.get $dir) (i32.const 0))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func $unix_opendir (export "unix_opendir") (export "caml_unix_opendir")
       (param $name (ref eq)) (result (ref eq))
       (try (result (ref eq))
@@ -520,6 +1255,12 @@
             (call $throw_ebadf (@string "closedir"))))
       (ref.i31 (i32.const 0)))
 
+   (func (export "unix_rewinddir") (export "caml_unix_rewinddir")
+      (param (ref eq)) (result (ref eq))
+      (call $caml_invalid_argument (@string "rewinddir not implemented"))
+      (ref.i31 (i32.const 0)))
+))
+
    (func (export "unix_readdir") (export "caml_unix_readdir")
       (param $dir (ref eq)) (result (ref eq))
       (block $return (result (ref eq))
@@ -551,11 +1292,29 @@
          (call $win_find_next (local.get $dir))
          (local.get $dir)))
 
-   (func (export "unix_rewinddir") (export "caml_unix_rewinddir")
-      (param (ref eq)) (result (ref eq))
-      (call $caml_invalid_argument (@string "rewinddir not implemented"))
-      (ref.i31 (i32.const 0)))
+(@if wasi
+(@then
+   (@string $unlink "unlink")
 
+   (func (export "unix_unlink") (export "caml_unix_unlink")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $unlink) (local.get $path)))
+      (local.set $res
+         (call $path_unlink_file
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $unlink) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_unlink") (export "caml_unix_unlink")
       (param $p (ref eq)) (result (ref eq))
       (try
@@ -565,7 +1324,31 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $rmdir "rmdir")
 
+   (func (export "unix_rmdir") (export "caml_unix_rmdir")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $rmdir) (local.get $path)))
+      (local.set $res
+         (call $path_remove_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $rmdir) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_rmdir") (export "caml_unix_rmdir")
       (param $p (ref eq)) (result (ref eq))
       (try
@@ -575,7 +1358,47 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $link "link")
 
+   (func (export "unix_link") (export "caml_unix_link")
+      (param $follow (ref eq)) (param $o (ref eq)) (param $n (ref eq))
+      (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $flags i32)
+      (local $res i32)
+      (local.set $op (call $unix_resolve_path (global.get $link) (local.get $o)))
+      (local.set $np (call $unix_resolve_path (global.get $link) (local.get $n)))
+      (if (ref.test (ref $block) (local.get $follow))
+         (then
+            (local.set $flags
+               (i31.get_u
+                  (ref.cast (ref i31)
+                     (array.get $block
+                        (ref.cast (ref $block) (local.get $follow))
+                        (i32.const 1)))))))
+      (local.set $res
+         (call $path_link
+            (tuple.extract 3 0 (local.get $op))
+            (local.get $flags)
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $link) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_link") (export "caml_unix_link")
       (param $follow (ref eq)) (param $d (ref eq)) (param $s (ref eq))
       (result (ref eq))
@@ -596,11 +1419,48 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
    (func (export "unix_has_symlink") (export "caml_unix_has_symlink")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 1)))
 
+(@if wasi
+(@then
+   (@string $symlink "symlink")
+
+   (func (export "unix_symlink") (export "caml_unix_symlink")
+      (param $to_dir (ref eq)) (param $o (ref eq)) (param $n (ref eq))
+      (result (ref eq))
+      (local $path (ref $bytes))
+      (local $len i32)
+      (local $op i32)
+      (local $np (tuple i32 i32 i32))
+      (local $flags i32)
+      (local $res i32)
+      (local.set $path (ref.cast (ref $bytes) (local.get $o)))
+      (local.set $len (array.len (local.get $path)))
+      (local.set $op
+         (call $write_string_to_memory
+            (i32.const 0) (i32.const 0) (local.get $path)))
+      (local.set $np
+         (call $unix_resolve_path (global.get $symlink) (local.get $n)))
+      (local.set $res
+         (call $path_symlink
+            (local.get $op)
+            (local.get $len)
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (local.get $op))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $symlink) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_symlink") (export "caml_unix_symlink")
       (param $to_dir (ref eq)) (param $t (ref eq)) (param $p (ref eq))
       (result (ref eq))
@@ -623,7 +1483,37 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (@string $readlink "readlink")
+
+   (func (export "unix_readlink") (export "caml_unix_readlink")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $buf i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $readlink) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $buf (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $res
+         (call $path_readlink
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buf)
+            (global.get $IO_BUFFER_SIZE)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $readlink) (local.get $path))))
+      (return_call $blit_memory_to_string
+         (local.get $buf) (i32.load (local.get $buffer))))
+)
+(@else
    (func (export "unix_readlink") (export "caml_unix_readlink")
       (param $path (ref eq)) (result (ref eq))
       (try
@@ -636,7 +1526,60 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $truncate "truncate")
+
+   (func $truncate (param $path (ref eq)) (param $len i64) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $fd i32) (local $res i32) (local $buffer i32)
+      (block $error
+         (local.set $p
+            (call $unix_resolve_path (global.get $truncate) (local.get $path)))
+         (local.set $buffer (call $get_buffer))
+         (local.set $res
+            (call $path_open
+               (tuple.extract 3 0 (local.get $p))
+               (i32.const 1) ;; symlink_follow
+               (tuple.extract 3 1 (local.get $p))
+               (tuple.extract 3 2 (local.get $p))
+               (i32.const 0)
+               (i64.const 0x400000) ;; allow fd_filestat_set_size
+               (i64.const 0)
+               (i32.const 0)
+               (local.get $buffer)))
+         (call $free (tuple.extract 3 1 (local.get $p)))
+         (br_if $error (local.get $res))
+         (local.set $fd (i32.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_filestat_set_size (local.get $fd) (local.get $len)))
+         (if (local.get $res)
+            (then
+               (drop (call $fd_close (local.get $fd)))
+               (br $error)))
+         (local.set $res (call $fd_close (local.get $fd)))
+         (br_if $error (local.get $res))
+         (return (ref.i31 (i32.const 0))))
+      (call $caml_unix_error
+         (local.get $res) (global.get $truncate) (local.get $path))
+      (return (ref.i31 (i32.const 0))))
+
+   (func (export "unix_truncate") (export "caml_unix_truncate")
+      (param $path (ref eq)) (param $len (ref eq))
+      (result (ref eq))
+      (return_call $truncate (local.get $path)
+         (i64.extend_i32_s
+            (i31.get_s (ref.cast (ref i31) (local.get $len))))))
 
+   (func (export "unix_truncate_64") (export "caml_unix_truncate_64")
+      (param $path (ref eq)) (param $len (ref eq))
+      (result (ref eq))
+      (return_call $truncate (local.get $path)
+         (call $Int64_val (local.get $len))))
+)
+(@else
    (func (export "unix_truncate") (export "caml_unix_truncate")
       (param $path (ref eq)) (param $len (ref eq))
       (result (ref eq))
@@ -662,7 +1605,33 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func $ftruncate (param $vfd (ref eq)) (param $len i64) (result (ref eq))
+      (local $fd i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $res
+         (call $fd_filestat_set_size (local.get $fd) (local.get $len)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "ftruncate") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "unix_ftruncate") (export "caml_unix_ftruncate")
+      (param $fd (ref eq)) (param $len (ref eq)) (result (ref eq))
+      (return_call $ftruncate (local.get $fd)
+         (i64.extend_i32_s
+            (i31.get_s (ref.cast (ref i31) (local.get $len))))))
+
+   (func (export "unix_ftruncate_64") (export "caml_unix_ftruncate_64")
+      (param $fd (ref eq)) (param $len (ref eq)) (result (ref eq))
+      (return_call $ftruncate (local.get $fd)
+         (call $Int64_val (local.get $len))))
+)
+(@else
    (func (export "unix_ftruncate") (export "caml_unix_ftruncate")
       (param $fd (ref eq)) (param $vlen (ref eq))
       (result (ref eq))
@@ -711,7 +1680,35 @@
              (struct.set $fd_offset $offset (local.get $fd_offset)
                 (local.get $len))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $access "access")
 
+   ;; We can only check that the file exists
+   (func (export "unix_access") (export "caml_unix_access")
+      (param $path (ref eq)) (param $flags (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $access) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $access) (local.get $path))))
+      (return (ref.i31 (i32.const 0))))
+)
+(@else
    (global $access_flags (ref $flags)
       (array.new_fixed $flags 4
          (i32.const 1) (i32.const 2) (i32.const 4) (i32.const 8)))
@@ -730,8 +1727,69 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+   (type $flags (array i16))
+
+(@if wasi
+(@then
+   ;;    0x1 O_RDONLY
+   ;;    0x2 O_WRONLY
+   ;;    0x3 O_RDWR
+   ;;  0x400 O_NONBLOCK
+   ;;  0x100 O_APPEND
+   ;;   0x10 O_CREAT
+   ;;   0x80 O_TRUNC
+   ;;   0x40 O_EXCL
+   ;;      0 O_NOCTTY
+   ;;  0x200 O_DSYNC
+   ;; 0x1000 O_SYNC
+   ;;  0x800 O_RSYNC
+   (global $unix_open_flags (ref $flags)
+      (array.new_fixed $flags 15
+         (i32.const 1) (i32.const 2) (i32.const 3) (i32.const 0x400)
+         (i32.const 0x100) (i32.const 0x10) (i32.const 0x80) (i32.const 0x40)
+         (i32.const 0) (i32.const 0x200) (i32.const 0x1000) (i32.const 0x800)
+         (i32.const 0) (i32.const 0) (i32.const 0)))
+
+   (@string $open "open")
 
-   (type $flags (array i8))
+   (func (export "unix_open") (export "caml_unix_open")
+      (param $vpath (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
+      (result (ref eq))
+      (local $flags i32) (local $offset i64)
+      (local $path (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $path
+         (call $unix_resolve_path (global.get $open) (local.get $vpath)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $flags
+         (call $convert_flag_list
+            (global.get $unix_open_flags) (local.get $vflags)))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $path))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $path))
+            (tuple.extract 3 2 (local.get $path))
+            (i32.and (i32.shr_u (local.get $flags) (i32.const 4))
+              (i32.const 0xF))
+            (select
+               (i64.const 0x860007e)
+               (select (i64.const 0x860007c) (i64.const 0x820003e)
+                  (i32.and (local.get $flags) (i32.const 2)))
+               (i32.eq (i32.and (local.get $flags) (i32.const 3)) (i32.const 3)))
+            (i64.const 0)
+            (i32.shr_u (local.get $flags) (i32.const 8))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $path)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $open) (local.get $vpath))))
+      (ref.i31 (i32.load (local.get $buffer))))
+)
+(@else
    ;;    1 O_RDONLY
    ;;    2 O_WRONLY
    ;;    4 O_RDWR
@@ -771,6 +1829,7 @@
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (call $initialize_fd_offset (local.get $fd) (local.get $offset))
       (ref.i31 (local.get $fd)))
+))
 
    (global $io_buffer (mut externref) (ref.null extern))
 
@@ -786,6 +1845,217 @@
             (br_on_null $null (call $get_fd_offset_unchecked (local.get $fd)))))
       (struct.new $fd_offset (i64.const 0) (i32.const 0)))
 
+(@if wasi
+(@then
+   (func (export "unix_write") (export "caml_unix_write")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $fd i32) (local $s (ref $bytes))
+      (local $pos i32) (local $len i32) (local $numbytes i32)
+      (local $written i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vbuf)))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (loop $loop
+         (if (i32.gt_u (local.get $len) (i32.const 0))
+            (then
+               (local.set $numbytes
+                  (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+                     (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+               (call $blit_substring_to_memory
+                  (local.get $buffer) (local.get $s) (local.get $pos)
+                  (local.get $numbytes))
+               (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+               (local.set $res
+                  (call $fd_write
+                      (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+                      (local.get $nwritten)))
+               (if (local.get $res)
+                  (then
+                     (call $caml_unix_error (local.get $res) (@string "write")
+                        (global.get $no_arg))))
+               (local.set $n (i32.load (local.get $nwritten)))
+               (local.set $written (i32.add (local.get $written) (local.get $n)))
+               (local.set $pos (i32.add (local.get $pos) (local.get $n)))
+               (local.set $len (i32.sub (local.get $len) (local.get $n)))
+               (br $loop))))
+      (ref.i31 (local.get $n)))
+
+   (func (export "unix_single_write") (export "caml_unix_single_write")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $fd i32) (local $s (ref $bytes))
+      (local $pos i32) (local $len i32) (local $numbytes i32)
+      (local $written i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vbuf)))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.eqz (local.get $len))
+         (then (return (ref.i31 (i32.const 0)))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $numbytes
+         (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+            (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+      (call $blit_substring_to_memory
+         (local.get $buffer) (local.get $s) (local.get $pos)
+         (local.get $numbytes))
+      (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+      (local.set $res
+         (call $fd_write
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nwritten)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "write")
+               (global.get $no_arg))))
+      (ref.i31 (i32.load (local.get $nwritten))))
+
+   (func (export "unix_read") (export "caml_unix_read")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nread i32)
+      (local $fd i32) (local $pos i32) (local $len i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))
+         (then
+            (local.set $len (global.get $IO_BUFFER_SIZE))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $len))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "read")
+               (global.get $no_arg))))
+      (local.set $n (i32.load (local.get $nread)))
+      (call $blit_memory_to_substring (local.get $buffer)
+         (ref.cast (ref $bytes) (local.get $vbuf))
+         (local.get $pos) (local.get $n))
+      (ref.i31 (local.get $n)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func (export "unix_write_bigarray") (export "caml_unix_write_bigarray")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (param $vsingle (ref eq)) (result (ref eq))
+      (local $fd i32) (local $data (ref $data)) (local $buf (ref $bytes))
+      (local $pos i32) (local $len i32) (local $n i32) (local $written i32)
+      (local $buffer i32) (local $nwritten i32) (local $iovs i32)
+      (local $iovs_len i32) (local $numbytes i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $data
+         (ref.cast (ref $data)
+            (any.convert_extern (call $caml_ba_get_data (local.get $vbuf)))))
+      (local.set $buf
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $pos
+          (i32.add (i31.get_u (ref.cast (ref i31) (local.get $vpos)))
+             (struct.get $data $offset (local.get $data))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (loop $loop
+         (if (i32.gt_u (local.get $len) (i32.const 0))
+            (then
+               (local.set $numbytes
+                  (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+                     (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+               (call $blit_substring_to_memory
+                  (local.get $buffer) (local.get $buf) (local.get $pos)
+                  (local.get $numbytes))
+               (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+               (local.set $res
+                  (call $fd_write
+                      (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+                      (local.get $nwritten)))
+               (if (local.get $res)
+                  (then
+                     (call $caml_unix_error (local.get $res) (@string "write")
+                        (global.get $no_arg))))
+               (local.set $n (i32.load (local.get $nwritten)))
+               (local.set $written (i32.add (local.get $written) (local.get $n)))
+               (local.set $pos (i32.add (local.get $pos) (local.get $n)))
+               (local.set $len (i32.sub (local.get $len) (local.get $n)))
+               (br_if $loop
+                  (ref.eq (local.get $vsingle) (ref.i31 (i32.const 0)))))))
+      (ref.i31 (local.get $written)))
+
+   (func (export "unix_read_bigarray") (export "caml_unix_read_bigarray")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $fd i32) (local $data (ref $data)) (local $buf (ref $bytes))
+      (local $pos i32) (local $len i32) (local $n i32)
+      (local $buffer i32) (local $nread i32) (local $iovs i32)
+      (local $iovs_len i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $data
+         (ref.cast (ref $data)
+            (any.convert_extern (call $caml_ba_get_data (local.get $vbuf)))))
+      (local.set $buf
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $pos
+          (i32.add (i31.get_u (ref.cast (ref i31) (local.get $vpos)))
+             (struct.get $data $offset (local.get $data))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))
+         (then
+            (local.set $len (global.get $IO_BUFFER_SIZE))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $len))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "read")
+               (global.get $no_arg))))
+      (local.set $n (i32.load (local.get $nread)))
+      (call $blit_memory_to_substring (local.get $buffer)
+         (local.get $buf) (local.get $pos) (local.get $n))
+      (ref.i31 (local.get $n)))
+)
+(@else
    (func (export "unix_write") (export "caml_unix_write")
       (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
@@ -985,7 +2255,28 @@
       (struct.set $fd_offset $offset (local.get $fd_offset)
          (i64.add (local.get $offset) (i64.extend_i32_s (local.get $n))))
       (ref.i31 (local.get $n)))
+))
 
+(@if wasi
+(@then
+   (func $lseek
+      (param $fd (ref eq)) (param $offset i64) (param $cmd (ref eq))
+      (result i64)
+      (local $res i32) (local $buffer i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $offset)
+            (i31.get_u (ref.cast (ref i31) (local.get $cmd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "lseek") (global.get $no_arg))))
+      (i64.load (local.get $buffer)))
+)
+(@else
    (func $lseek_exn (param $errno i32) (result (ref eq))
       (array.new_fixed $block 5
          (ref.i31 (i32.const 0))
@@ -1021,6 +2312,7 @@
       (struct.set $fd_offset $offset (local.get $fd_offset) (local.get $offset))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
       (local.get $offset))
+))
 
    (func (export "unix_lseek") (export "caml_unix_lseek")
       (param $fd (ref eq)) (param $ofs (ref eq)) (param $cmd (ref eq))
@@ -1042,6 +2334,20 @@
             (call $Int64_val (local.get $ofs))
             (local.get $cmd))))
 
+(@if wasi
+(@then
+   (func (export "unix_fsync") (export "caml_unix_fsync")
+      (param $fd (ref eq)) (result (ref eq))
+      (local $res i32)
+      (local.set $res
+         (call $fd_sync (i31.get_u (ref.cast (ref i31) (local.get $fd)))))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "fsync") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_fsync") (export "caml_unix_fsync")
       (param $fd (ref eq)) (result (ref eq))
       (try
@@ -1050,6 +2356,7 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
    (@string $out_channel_of_descr "out_channel_of_descr")
    (@string $in_channel_of_descr "in_channel_of_descr")
@@ -1060,6 +2367,32 @@
          (global.get $in_channel_of_descr)
          (local.get $out)))
 
+(@if wasi
+(@then
+   (func $caml_unix_check_stream_semantics (param $fd (ref eq)) (param $out i32)
+      (local $s (ref $block)) (local $kind i32)
+      (local $buffer i32) (local $res i32) (local $file_type i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_filestat_get
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res)
+               (call $channel_of_descr_name (local.get $out))
+               (global.get $no_arg))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (block $ok
+         (block $bad
+            (br_table $bad $bad $ok $bad $ok $bad $ok $bad (local.get $kind)))
+         (call $caml_unix_error
+            (i32.const 28) (; EINVAL ;)
+            (call $channel_of_descr_name (local.get $out))
+            (global.get $no_arg))))
+)
+(@else
    (func $caml_unix_check_stream_semantics (param $fd (ref eq)) (param $out i32)
       (local $s (ref $block)) (local $kind i32)
       (local.set $s
@@ -1085,6 +2418,7 @@
                (ref.i31 (i32.const 12)) ;; EINVAL
                (call $channel_of_descr_name (local.get $out))
                (global.get $no_arg)))))
+))
 
    (func (export "unix_inchannel_of_filedescr")
       (export "win_inchannel_of_filedescr")
@@ -1100,6 +2434,20 @@
       (call $caml_unix_check_stream_semantics (local.get $fd) (i32.const 1))
       (return_call $caml_ml_open_descriptor_out (local.get $fd)))
 
+(@if wasi
+(@then
+   (func (export "unix_close") (export "caml_unix_close")
+      (param $fd (ref eq)) (result (ref eq))
+      (local $res i32)
+      (local.set $res
+         (call $fd_close (i31.get_u (ref.cast (ref i31) (local.get $fd)))))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "close") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_close") (export "caml_unix_close")
       (param $fd (ref eq)) (result (ref eq))
       (call $release_fd_offset (i31.get_u (ref.cast (ref i31) (local.get $fd))))
@@ -1109,9 +2457,18 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_isatty") (export "caml_unix_isatty")
+      (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (export "unix_isatty" (func $isatty))
    (export "caml_unix_isatty" (func $isatty))
+))
 
    (func (export "unix_getuid") (export "caml_unix_getuid")
       (export "unix_geteuid") (export "caml_unix_geteuid")
diff --git a/runtime/wasm/wasi_errors.wat b/runtime/wasm/wasi_errors.wat
new file mode 100644
index 0000000000..577fb410fa
--- /dev/null
+++ b/runtime/wasm/wasi_errors.wat
@@ -0,0 +1,86 @@
+(module
+(@if wasi
+(@then
+   (type $block (array (mut (ref eq))))
+   (type $bytes (array (mut i8)))
+   (global (export "error_messages") (ref $block)
+      (array.new_fixed $block 77
+         (@string "Success")
+         (@string "Argument list too long")
+         (@string "Permission denied")
+         (@string "Address in use")
+         (@string "Address not available")
+         (@string "Address family not supported")
+         (@string "Resource unavailable, or operation would block")
+         (@string "Connection already in progress")
+         (@string "Bad file descriptor")
+         (@string "Bad message")
+         (@string "Device or resource busy")
+         (@string "Operation canceled")
+         (@string "No child processes")
+         (@string "Connection aborted")
+         (@string "Connection refused")
+         (@string "Connection reset")
+         (@string "Resource deadlock would occur")
+         (@string "Destination address required")
+         (@string "Mathematics argument out of domain of function")
+         (@string "Reserved")
+         (@string "File exists")
+         (@string "Bad address")
+         (@string "File too large")
+         (@string "Host is unreachable")
+         (@string "Identifier removed")
+         (@string "Illegal byte sequence")
+         (@string "Operation in progress")
+         (@string "Interrupted function")
+         (@string "Invalid argument")
+         (@string "I/O error")
+         (@string "Socket is connected")
+         (@string "Is a directory")
+         (@string "Too many levels of symbolic links")
+         (@string "File descriptor value too large")
+         (@string "Too many links")
+         (@string "Message too large")
+         (@string "Reserved")
+         (@string "Filename too long")
+         (@string "Network is down")
+         (@string "Connection aborted by network")
+         (@string "Network unreachable")
+         (@string "Too many files open in system")
+         (@string "No buffer space available")
+         (@string "No such device")
+         (@string "No such file or directory")
+         (@string "Executable file format error")
+         (@string "No locks available")
+         (@string "Reserved")
+         (@string "Not enough space")
+         (@string "No message of the desired type")
+         (@string "Protocol not available")
+         (@string "No space left on device")
+         (@string "Function not supported")
+         (@string "The socket is not connected")
+         (@string "Not a directory or a symbolic link to a directory")
+         (@string "Directory not empty")
+         (@string "State not recoverable")
+         (@string "Not a socket")
+         (@string "Not supported, or operation not supported on socket")
+         (@string "Inappropriate I/O control operation")
+         (@string "No such device or address")
+         (@string "Value too large to be stored in data type")
+         (@string "Previous owner died")
+         (@string "Operation not permitted")
+         (@string "Broken pipe")
+         (@string "Protocol error")
+         (@string "Protocol not supported")
+         (@string "Protocol wrong type for socket")
+         (@string "Result too large")
+         (@string "Read-only file system")
+         (@string "Invalid seek")
+         (@string "No such process")
+         (@string "Reserved")
+         (@string "Connection timed out")
+         (@string "Text file busy")
+         (@string "Cross-device link")
+         (@string "Capabilities insufficient")))
+))
+)
diff --git a/runtime/wasm/wasi_memory.wat b/runtime/wasm/wasi_memory.wat
new file mode 100644
index 0000000000..0e737a46db
--- /dev/null
+++ b/runtime/wasm/wasi_memory.wat
@@ -0,0 +1,98 @@
+(module
+(@if wasi
+(@then
+   (import "libc" "memory" (memory 2))
+   (import "libc" "malloc" (func $malloc (param i32) (result i32)))
+   (import "libc" "free" (func $free (param i32)))
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
+
+   (type $bytes (array (mut i8)))
+
+   (func (export "checked_malloc") (param $size i32) (result i32)
+      (local $p i32)
+      (local.set $p (call $malloc (local.get $size)))
+      (if (i32.eqz (local.get $p))
+         (then (call $caml_raise_out_of_memory)))
+      (local.get $p))
+
+   (func (export "blit_substring_to_memory")
+      (param $buf i32) (param $s (ref $bytes)) (param $ofs i32) (param $len i32)
+      (local $i i32)
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (i32.store8 (i32.add (local.get $buf) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $ofs) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $blit_string_to_memory (export "blit_string_to_memory")
+      (param $buf i32) (param $s (ref $bytes))
+      (local $i i32) (local $len i32)
+      (local.set $len (array.len (local.get $s)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (i32.store8 (i32.add (local.get $buf) (local.get $i))
+                  (array.get $bytes (local.get $s) (local.get $i)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func (export "blit_memory_to_substring")
+      (param $buf i32) (param $s (ref $bytes)) (param $ofs i32) (param $len i32)
+      (local $i i32)
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $s)
+                  (i32.add (local.get $ofs) (local.get $i))
+                  (i32.load8_u (i32.add (local.get $buf) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $blit_memory_to_string (export "blit_memory_to_string")
+      (param $buf i32) (param $len i32) (result (ref $bytes))
+      (local $s (ref $bytes))
+      (local $i i32)
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $s) (local.get $i)
+                  (i32.load8_u (i32.add (local.get $buf) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $s))
+
+   (func (export "write_string_to_memory")
+      (param $buf i32) (param $avail i32) (param $v (ref eq))
+      (result i32)
+      (local $s (ref $bytes)) (local $i i32) (local $len i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get $v)))
+      (local.set $len (array.len (local.get $s)))
+      (if (i32.lt_u (local.get $avail) (i32.add (local.get $len) (i32.const 1)))
+         (then
+            (local.set $buf
+               (call $checked_malloc (i32.add (local.get $len) (i32.const 1))))))
+      (call $blit_string_to_memory (local.get $buf) (local.get $s))
+      (i32.store8 (i32.add (local.get $buf) (local.get $len)) (i32.const 0))
+      (local.get $buf))
+
+   (func (export "release_memory") (param $initial_buffer i32) (param $buf i32)
+      (if (i32.ne (local.get $initial_buffer) (local.get $buf))
+         (then
+            (call $free (local.get $buf)))))
+
+   (global $buffer (mut i32) (i32.const 0))
+
+   (func $get_buffer (export "get_buffer") (result i32)
+      (if (i32.eqz (global.get $buffer))
+         (then
+            (global.set $buffer
+               (call $checked_malloc
+                  (i32.add (global.get $IO_BUFFER_SIZE) (i32.const 12))))))
+      (global.get $buffer))
+))
+)
diff --git a/runtime/wasm/weak.wat b/runtime/wasm/weak.wat
index 1f704b8071..68d5328c04 100644
--- a/runtime/wasm/weak.wat
+++ b/runtime/wasm/weak.wat
@@ -21,6 +21,19 @@
       (func $caml_obj_dup (param (ref eq)) (result (ref eq))))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param $arg (ref eq))))
+
+(@if wasi
+(@then
+   (func $wrap (param (ref eq)) (result (ref eq))
+      (local.get 0))
+   (func $unwrap (param (ref eq)) (result (ref eq))
+      (local.get 0))
+   (func $weak_new (param $v (ref eq)) (result (ref eq))
+      (local.get $v))
+   (func $weak_deref (param $r (ref eq)) (result (ref eq))
+      (local.get $r))
+)
+(@else
    (import "bindings" "weak_new"
       (func $weak_new (param (ref eq)) (result anyref)))
    (import "bindings" "weak_deref"
@@ -32,6 +45,8 @@
       (func $map_set (param (ref any)) (param (ref eq)) (param (ref any))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
+))
+
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $js (struct (field anyref)))
@@ -62,6 +77,8 @@
          (block $released
             (br_if $no_data
                (ref.eq (local.get $d) (global.get $caml_ephe_none)))
+(@if (not wasi)
+(@then
             (local.set $i (global.get $caml_ephe_key_offset))
             (local.set $len (array.len (local.get $x)))
             (local.set $m (ref.as_non_null (call $unwrap (local.get $d))))
@@ -81,9 +98,11 @@
                         (br_on_null $released
                            (call $map_get (local.get $m) (local.get $v))))
                      (br $loop))))
+            (local.set $d (ref.cast (ref eq) (local.get $m)))
+))
             (return
               (array.new_fixed $block 2 (ref.i31 (i32.const 0))
-                 (ref.cast (ref eq) (local.get $m)))))
+                 (local.get $d))))
          (array.set $block (local.get $x) (global.get $caml_ephe_data_offset)
             (global.get $caml_ephe_none)))
       (ref.i31 (i32.const 0)))
@@ -110,6 +129,8 @@
       (local $m (ref any)) (local $m' (ref any))
       (local $i i32)
       (local.set $x (ref.cast (ref $block) (local.get $vx)))
+(@if (not wasi)
+(@then
       (local.set $i (array.len (local.get $x)))
       (local.set $m (local.get $data))
       (loop $loop
@@ -133,8 +154,10 @@
                (array.set $block (local.get $x) (local.get $i)
                   (global.get $caml_ephe_none))
                (br $loop))))
+      (local.set $data (call $wrap (local.get $m)))
+))
       (array.set $block (local.get $x) (global.get $caml_ephe_data_offset)
-         (call $wrap (local.get $m)))
+         (local.get $data))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ephe_unset_data")
diff --git a/runtime/wasm/zstd.wat b/runtime/wasm/zstd.wat
index 54a984aaed..dc5bff2d92 100644
--- a/runtime/wasm/zstd.wat
+++ b/runtime/wasm/zstd.wat
@@ -16,7 +16,7 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-(@if (>= ocaml_version (5 1 0))
+(@if (and (>= ocaml_version (5 1 0)) (not wasi))
 (@then
    (import "bindings" "ta_new" (func $ta_new (param i32) (result (ref extern))))
    (import "bindings" "ta_blit_from_bytes"
@@ -58,5 +58,9 @@
    (func (export "caml_zstd_initialize") (param (ref eq)) (result (ref eq))
       (global.set $caml_intern_decompress_input (ref.func $decompress))
       (ref.i31 (i32.const 1)))
+)
+(@else
+   (func (export "caml_zstd_initialize") (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
 ))
 )
diff --git a/tools/ci_setup.ml b/tools/ci_setup.ml
index 4cb1867dea..94efd01e0c 100644
--- a/tools/ci_setup.ml
+++ b/tools/ci_setup.ml
@@ -64,6 +64,7 @@ let node_wrapper =
  (name node_wrapper)
        (libraries unix))|} )
   ; "node_wrapper/node_wrapper_per_profile.ml", {|let args = []|}
+  ; "node_wrapper/node_wrapper_per_engine.ml", {|let engine = "node"|}
   ; "node_wrapper/dune-project", "(lang dune 3.17)"
   ; "node_wrapper/node_wrapper.opam", ""
   ]
diff --git a/tools/dune b/tools/dune
index 5953c6bc08..e478bcbf98 100644
--- a/tools/dune
+++ b/tools/dune
@@ -1,8 +1,17 @@
 (executable
  (name node_wrapper)
- (modules node_wrapper)
+ (link_deps
+  (env_var WASM_ENGINE))
+ (modules node_wrapper node_wrapper_per_engine)
  (libraries unix))
 
+(rule
+ (target node_wrapper_per_engine.ml)
+ (action
+  (with-stdout-to
+   %{target}
+   (run echo "let engine = \"%{env:WASM_ENGINE=node}\""))))
+
 (executable
  (name ci_setup)
  (modules ci_setup)
diff --git a/tools/node_wrapper.ml b/tools/node_wrapper.ml
index 9d58203591..a967c00d26 100644
--- a/tools/node_wrapper.ml
+++ b/tools/node_wrapper.ml
@@ -1,6 +1,21 @@
+let wizard_args =
+  [ "-ext:stack-switching"; "-ext:legacy-eh"; "-stack-size=2M"; "--dir=."; "--dir=/tmp" ]
+
+let wasmtime_args =
+  [ (* "-C"; "collector=null"; *) "-W=all-proposals=y"; "--dir=."; "--dir=/tmp" ]
+
+let wasmedge_args =
+  [ "--enable-gc"
+  ; "--enable-exception-handling"
+  ; "--enable-tail-call"
+  ; "--dir=."
+  ; "--dir=/tmp"
+  ]
+
 let extra_args_for_wasoo =
   [ "--experimental-wasm-imported-strings"
   ; "--experimental-wasm-stack-switching"
+  ; "--experimental-wasm-exnref"
   ; "--stack-size=10000"
   ]
 
@@ -23,16 +38,31 @@ let env =
       else e)
     env
 
-let args =
+let environment_args () =
+  List.filter
+    (fun e -> not (String.contains e ','))
+    (Array.to_list (Array.map (fun e -> "--env=" ^ e) env))
+
+let wasm_file file =
+  Filename.concat (Filename.chop_extension file ^ ".assets") "code.wasm"
+
+let common_args file argv = environment_args () @ (wasm_file file :: List.tl argv)
+
+let exe, args =
   match Array.to_list Sys.argv with
   | exe :: argv ->
-      let argv =
+      let exe', argv =
         match argv with
-        | file :: _ when Filename.check_suffix file ".wasm.js" ->
-            extra_args_for_wasoo @ argv
-        | _ -> extra_args_for_jsoo @ argv
+        | file :: _ when Filename.check_suffix file ".wasm.js" -> (
+            match Node_wrapper_per_engine.engine with
+            | "wizard" -> "wizeng.x86-linux", wizard_args @ common_args file argv
+            | "wizard-fast" -> "wizeng.x86-64-linux", wizard_args @ common_args file argv
+            | "wasmtime" -> "wasmtime", wasmtime_args @ common_args file argv
+            | "wasmedge" -> "wasmedge", wasmedge_args @ common_args file argv
+            | _ -> "node", extra_args_for_wasoo @ argv)
+        | _ -> "node", extra_args_for_jsoo @ argv
       in
-      Array.of_list (exe :: argv)
+      exe', Array.of_list (exe :: argv)
   | [] -> assert false
 
 let () =
@@ -45,4 +75,4 @@ let () =
     | _, WEXITED n -> exit n
     | _, WSIGNALED _ -> exit 9
     | _, WSTOPPED _ -> exit 9
-  else Unix.execvpe "node" args env
+  else Unix.execvpe exe args env