To model and implemented a CPU(Central Processing Unit) inside an FPGA, the CPU should be able to perform the task that of a normal CPU. The main processes are instruction fetch, instruction decode, memory read, execute and memory write back.
Hence,CPU’s main function is to Fetch instructions, decode them and execute the given instruction set. Instruction set is based on MIPS.
Instruction set:
| Instruction | Function | Encoding | op | funct |
| add r1,r2,r3 | addition: r1 <- r2 + r3 | Register | 000000 | 100000 |
| sub r1,r2,r3 | subtraction: r1 <- r2 – r3 | Register | 000000 | 100010 |
| mult r1,r2,r3 | multiply: r1 <- r2 * r3 | Register | 000000 | 100100 |
| div r1,r2,r3 | division: r1 <- r2 / r3 | Register | 000000 | 100101 |
| and r1,r2,r3 | bitwise and: r1 <- r2 and r3 | Register | 000000 | 101000 |
| or r1,r2,r3 | bitwise or: r1 <- r2 or r3 | Register | 000000 | 101001 |
| slt r1,r2,r3 | set less than: r1 <- 1 if r2 < r3, r1 <- 0 otherwise | Register | 000000 | 110100 |
| srl r1,r2,r3 | shift from right to left: r1 <- r2 amount of r3 | Register | 000000 | 111000 |
| slr r1,r2,r3 | shift from left to right: r1 <- r2 amount of r3 | Register | 000000 | 111001 |
| addi r1,r2,imm | add immediate: r1 <- r2 + imm | Immediate | 100001 | |
| beq r1,r2,imm (destination) | branch if equal r1 = r2: PC <- PC + imm × 4 | Immediate | 000010 | |
| jmp destination | jump: PC <- PC + destination × 4 | Jump | 000001 | |
| lb r1,r2,imm | load byte: r1 <- mem[r2 + imm] | Immediate | 100000 | |
| sb r1,r2,imm | store byte: mem[r2 + imm] <- r1 | Immediate | 110000 | |
Instruction format (32 bit):
Immediate:
| 6 bit | 5 bit | 5 bit | 16 bit |
| op | r1 | r2 | imm |
Jump:
| 6 bit | 26 bit |
| op | destination |
Register:
| 6 bit | 5 bit | 5 bit | 5 bit | 5 bit | 6 bit |
| op | r1 | r2 | r3 | shift amount | funct |