MIPS on FPGA

MIPS architecture processor on Intel FPGA for learning.

*runs on modelsim.

Logic circuit

Implemeneted instructions

name	desc
addiu	rt = rs + imm
addu	rd = rs + rt;
bne	if (rs != rt) pc += offset * 4
j	pc = pc_upper | (target << 2)
jal	r31 = pc; pc = target << 2
or	rd = rs | rt
jr	pc = rs
lw	rt = *(int*)(offset + rs)
sw	*(int*)(offset + rs) = rt
lui	rt = imm << 16

Test code

code below calculates a fibonacci number.

int fib(int n);

int main()
{
    int a = fib(10);
}

int fib(int n)
{
    if (n == 0) return 0;
    if (n == 1) return 1;
    return fib(n - 1) + fib(n - 2);
}

into assembly

addiu       sp,sp,-32
sw          ra,28(sp)
sw          s8,24(sp)
move (addu) s8,sp
li (addiu)  a0,10
jal         0x44
move (or)   at,at
move (addu) v1,v0
lui         v0,0x1
sw          v1,-32768(v0)
move (addu) v0,zero
move (addu) sp,s8
lw          ra,28(sp)
lw          s8,24(sp)
addiu       sp,sp,32
jr          ra
move (or)   at,at

addiu       sp,sp,-40
sw          ra,36(sp)
sw          s8,32(sp)
sw          s0,28(sp)
move (addu) s8,sp
sw          a0,40(s8)
lw          v0,40(s8)
move (or)   at,at
bnez (bne)  v0,0x78
move        at,at
move        v0,zero
j           0xcc
move        at,at
lw          v1,40(s8)
li (addiu)  v0,1
bne         v1,v0,0x94
move        at,at
li(addiu)   v0,1
j           0xcc
move        at,at
lw          v0,40(s8)
move        at,at
addiu       v0,v0,-1
move        a0,v0
jal         0x44
move        at,at
move        s0,v0
lw          v0,40(s8)
move        at,at
addiu       v0,v0,-2
move        a0,v0
jal         0x44
move        at,at
addu        v0,s0,v0
move        sp,s8
lw          ra,36(sp)
lw          s8,32(sp)
lw          s0,28(sp)
addiu       sp,sp,40
jr          ra
move        at,at
move        at,at
move        at,at

Run a simulation via modelsim

• prerequisites
  • CLI command shell (for Windows)
  • command vlog introduced on your PATH
  • command vsim (for Linux) or modelsim.exe (for Windows) introduced on your PATH

$ cd modelsim/mips
$ make simx  # for Linux
$ make simw  # for Windows

Refs

• registers
• opcodes
• instruction format
  • type R
  • type I
  • type J