3.4.5 - CPU components and the Fetch-Execute cycle
Learn how the CPU's major components cooperate with main memory, then follow an instruction through fetching, decoding and execution. Specific register names are not required.
ALU, control unit, clock, registers and buses
The central processing unit carries out instructions by coordinating several components with different roles.
| Component | Role in carrying out an instruction |
|---|---|
| Arithmetic logic unit (ALU) | Performs arithmetic such as addition and logical operations such as comparisons. |
| Control unit | Decodes instructions and sends control signals to coordinate the required operations. |
| Clock | Produces regular timing pulses that synchronise processor operations. |
| Registers | Very small, fast storage locations in the CPU holding information currently needed, such as an instruction, a value or an intermediate result. |
| Bus | A collection of wires carrying data or signals between components. |
For an addition instruction, registers can hold the values currently being used. The control unit arranges the addition; the ALU performs it; the result is held temporarily or sent to another location as required. Clock pulses coordinate the steps. The bus provides communication paths rather than doing the arithmetic itself.
These components are not interchangeable. The ALU does not organise every device, and the clock does not decide the program's next operation. Registers store information; they are not a permanent file store. You need the general register role, without memorising particular register names, for this specification.
Main memory and CPU communication
Main memory holds instructions and data in locations that the CPU can access directly.
In this specification, main memory includes directly CPU-accessible memory such as RAM and ROM, but excludes cache and registers. RAM holds working programs and data while they are being used. When an application stored on a secondary drive is opened, needed instructions and data are loaded into RAM so the processor can use them.
The CPU requests a memory location and receives its stored contents through communication paths. To write, it supplies a location and the new value. A bus carries the necessary data and signals between components. A memory location may contain an instruction or data; the current operation determines how its contents are used.
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