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The Fetch–Decode–Execute Cycle

Step through how the CPU fetches an instruction, works out what it means and carries it out — billions of times a second.

The Cycle

The CPU does one thing again and again: it fetches an instruction from memory, decodes it to find out what to do, and executes it. This is the fetch-decode-execute cycle (also called the instruction cycle or machine cycle).

1 The address in the PC is copied into the MAR . 2 The address travels along the address bus; the control unit sends a READ signal on the control bus . 3 The instruction at that address travels along the data bus into the MDR . 4 The instruction is copied from the MDR into the CIR . 5 The PC is increased by 1, so it points to the next instruction.

The control unit reads the instruction in the CIR and splits it into two parts: the opcode — what to do (for example LOAD, ADD, STORE), the operand — what to do it to (often a memory address).

The instruction is carried out. For example, ADD 6 fetches the value at address 6 and the ALU adds it to the accumulator . Then the cycle starts again with the next fetch.

Step through the fetch–decode–execute cycle

CPU registers

PC — program counter

MAR — memory address

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MDR — memory data

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CIR — current instruction

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ACC — accumulator

Address busData busControl bus

Main memory (RAM)

The program in memory adds two numbers (at addresses 5 and 6) and stores the answer at address 7. Press Next to start.

Try it:

Press Next to follow every step. Watch which register changes (highlighted) and which bus is in use.

A tiny program: LOAD 5 (put the value at address 5 in the accumulator), ADD 6, STORE 7, HALT.

Practice

Which of these happen during the FETCH stage?

Choose all that apply.

1. Fetch

1 The address in the PC is copied into the MAR . 2 The address travels along the address bus; the control unit sends a READ signal on the control bus . 3 The instruction at that address travels along the data bus into the MDR . 4 The instruction is copied from the MDR into the CIR . 5 The PC is increased by 1, so it points to the next instruction.

3. Execute

The instruction is carried out. For example, ADD 6 fetches the value at address 6 and the ALU adds it to the accumulator . Then the cycle starts again with the next fetch.