swapregs.uasm

.include /mit/6.004/bsim/beta.uasm
.include /mit/6.004/bsim/lab7macros.uasm

||| Handler for opcode 1 extension:
||| swapreg(RA,RC) swaps the contents of the two named registers.
||| UASM defn = .macro swapreg(RA,RC) betaopc(0x01,RA,0,RC)

regs:   RESERVE(32)                     | Array used to store register contents

UI: 
        save_all_regs(regs)
 
        LD(xp,-4,r0)                    | illegal instruction
        extract_field(r0, 31, 26, r1)   | extract opcode, bits 31:26
        CMPEQC(r1,0x1,r2)               | OPCODE=1?
        BT(r2, swapreg)                 | yes, handle the swapreg instruction.

        LD(r31,regs,r0)                 | Its something else.  Restore regs
        LD(r31,regs+4,r1)               | we've used, and go to the system's
        LD(r31,regs+8,r2)               | Illegal Instruction handler.
        BR(_IllegalInstruction)

swapreg:
        extract_field(r0, 25, 21, r1)   | extract rc field from trapped instruction
        MULC(r1, 4, r1)                 | convert to byte offset into regs array
        extract_field(r0, 20, 16, r2)   | extract ra field from trapped instruction
        MULC(r2, 4, r2)                 | convert to byte offset into regs array
        LD(r1, regs, r3)                | r3 <- regs[rc]
        LD(r2, regs, r4)                | r4 <- regs[ra]
        ST(r4, regs, r1)                | regs[rc] <- old regs[ra]
        ST(r3, regs, r2)                | regs[ra] <- old regs[rc]
        
        restore_all_regs(regs)
        JMP(xp)