Instruction execution steps

1. IF (Instruction Fetch)

address of instruction to be executed is in PC (program counter, a hidden register)

instruction is copied from memory to IR (instruction register, another hidden register)

2. D (Decode the instruction, Fetch Operands)

determine what operation to perform (opcode, function)

get operand values

add

get 2 operands from registers

addi

get 1 operand from register, 1 from instruction itself (sign-extended immediate)

3. ALU (Perform the operation)

arithmetic, logical, etc.

action performed by ALU circuits

4. MEM (Memory access)

MIPS: only load or store instructions

5. WB (Write Back)

result of third step is written to the appropriate register

6. PC Update (Program Counter Update)

normally, PC <- PC + 4

branch or jump: some other address