CPU simulation at lower abstraction level

At the lower levels of abstraction, a central processing unit is simulated not as a queueing server but as a hardware machine, modelling its internal registers, buses and the way it executes instructions. The aim is to check a CPU design or to run programs on a machine that does not yet exist.

Levels (lower side)

1. Instruction-set (functional) level: models what each instruction does to registers and memory; timing is by instruction counts.
2. Register-transfer level (RTL): models data moving between registers through the ALU on every clock cycle, controlled by the control unit.
3. Gate / circuit level: models logic gates and electrical behaviour (usually done with hardware description languages).

Elements modelled

• Registers: program counter (PC), instruction register (IR), accumulator, general registers, status flags.
• ALU operations, memory array, buses.
• Control unit: decodes opcode and generates micro-operations.
• Optional: pipeline stages, cache, interrupts.

Instruction cycle

 +-------+   +--------+   +---------+
 | FETCH |-->| DECODE |-->| EXECUTE |--+
 +-------+   +--------+   +---------+  |
     ^                                  |
     +--------- update PC ---------------+

1. Fetch: IR = Memory[PC]; PC = PC + 1.
2. Decode: determine the opcode and operands.
3. Execute: perform the operation, update registers, flags and memory.
4. Add the clock cycles used to the simulated clock.

Example (simple simulator loop)

while (not halted):
    IR = MEM[PC]; PC += 1
    op, addr = decode(IR)
    if op == LOAD:  ACC = MEM[addr]
    if op == ADD:   ACC = ACC + MEM[addr]
    if op == STORE: MEM[addr] = ACC
    if op == JMP:   PC = addr
    cycles += cost[op]

Running a small program shows the final register contents and the total cycles, so the design can be verified.

Remarks

Lower-level simulation is very accurate but slow and needs detailed knowledge; higher levels are faster and are used for performance studies. The level is chosen according to the question being asked.