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How is an instruction executed? This instruction set can be handled in a five step cycle (H&P pp.127):

IF

instruction fetch

Program counter sent to main memory

ID

instruction decode

Instruction received from main memory. Registers used in instruction are accessed.

EX

execution Arithmetic and logical operations are performed, including calculation of effective addresses for loads and stores.

MEM

memory reference

The memory access for load and store instructions takes place.

WB

write back

Write the result to the register specified (either from the ALU or from the memory)