A pipelined CPU with bypassing, and an assembler for it.
See doc.pdf after running make doc, or the above doc.svg. Some details are
omitted for clarity:
- connections to the clock tree and the reset signal
- bus sizes
- occasionnal bus resize operations
Run make asb to compile the assembler, then run ./asb <filename> to assemble
a program. test.asb is provided for testing purposes.
- RF: 2^4 = 16 registers of 16 bits
- ROM: 2^12 instructions of 26 bits
- RAM: 2^12 memory cells of 16 bits
While most instructions need several small operands (src1 and src2/short immediate), a few of them need just one but full-size operand. To shorten the general instruction length, opcodes are encoded in two parts: the op field, common to all formats, and a funct field, for more operations granularity when working with small operands but inexistant when space is needed for big operands.
FMT FIELDS[LENGTH]
00 fmt[2] op[4] dest[4] src1[4] funct[2] [6] src2[4]
01 fmt[2] op[4] dest[4] src1[4] funct[2] imm[10]
10 fmt[2] op[4] dest[4] imm[16]
That gives several instructions types:
TYPE COMMENT
A need funct, fmt = 00 or 01
B need full-size immediate, fmt = 10
C others, fmt = 00 or 10
OP TYPE FUNCT MNEMONIC COMMENT
0000 A bitwise operations
00 not
01 and
10 or
11 xor
0001 A shift operations
00 rls
01 lls
0010 A arithmetic operations
00 add
01 sub
10 mul
0011 A stack manipulation
00 pop
01 read
10 push
0100 B jmp
0101 C mov
0110 B call
0111 C ret
Pseudo-instructions understood by the assembler:
- inc, dec
- beq, bne, bgt, bge, blt, ble
For compactness, conditionnal and unconditionnal jumps are encoded with the same instruction (op = 0100). More precisely, all jumps are conditionnal, with the condition being trivially true for "unconditionnal" jumps.
Conditions are encoded as a bit mask in the dest field. It can be seen as an array of recquirements flags. For example, 0000 corresponds to the absence of recquirements (unconditionnal jump), while 0110 recquires both "non equal" and "greater than or equal" comparisons, meaning "stricly greater than" and used with the pseudo-instruction "bgt" (branch greater than).
This thinking process is to be handled by the assembler, not the user, thanks to the set of conditionnal jump pseudo-instructions (beq, bne, bgt, bge, blt, ble) and the unconditionnal jump instruction jmp.
The read instruction (op = 0011, funct = 01) read a value in the stack at an arbitrary depth, specified in the immediate field or stored in the src2 register, without modifying the stack. It is helpful for function calls: push parameters on the stack and read them when needed (the compiler know the stack state and thus can compute at compile time the depth of each parameter at any moment).