• 14 parts
  • Checked October 2026

Parts of a Processor

The inside of a CPU as a block diagram: five blocks, five registers and three buses.

Parts of a Processor

Name Date

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1 2 3 4 5 6 7 8 9 10 11 12 13 14
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Write the name of each numbered part.Answer key

  1. Control unit
  2. Arithmetic logic unit
  3. Registers
  4. Cache
  5. Clock
  6. Address bus
  7. Data bus
  8. Control bus
  9. Program counter
  10. Memory address register
  11. Current instruction register
  12. Memory data register
  13. Accumulator
  14. Main memory

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

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All 14 parts on the diagram

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Blocks and buses 8

  1. Control unit It fetches, decodes and carries out instructions and sends control signals. It also moves data around the system.
  2. Arithmetic logic unit Where the processor does its calculations and makes logical decisions.
  3. Registers Small amounts of very fast memory inside the processor. They hold what is needed during processing.
  4. Cache A small amount of fast memory built into the processor. It keeps data and instructions the processor will likely reuse.
  5. Clock It sends out a regular pulse that keeps every part in time.
  6. Address bus It takes memory addresses from the processor to memory and other parts.
  7. Data bus It carries the data itself between the processor and other parts.
  8. Control bus It takes control signals from the processor and status signals back. It also carries the pulses of the clock.

The five registers 5

  1. Program counter It holds the next instruction's memory address to fetch.
  2. Memory address register It holds the address in memory being read or written.
  3. Current instruction register It holds the instruction being decoded and carried out.
  4. Memory data register It holds data just read from memory or about to be written. It is also called the memory buffer register.
  5. Accumulator It holds the data in process and the results. The arithmetic logic unit stores its answers here.

Other 1

  1. Main memory The instructions and data wait here; the three buses reach it.

How this list was compiled: Read 2026-10-01. (1) BBC Bitesize, GCSE Computer Science, Systems architecture: Common CPU components, https://www.bbc.co.uk/bitesize/guides/z7qqmsg/revision/1 (the CPU consists of six main components: control unit, arithmetic logic unit, registers, cache, buses and clock; what each does; the address bus, data bus and control bus; the clock's regular pulse), Von Neumann architecture, https://www.bbc.co.uk/bitesize/guides/z7qqmsg/revision/3 (program counter, memory address register, memory buffer register also called memory data register, current instruction register, accumulator, and what each holds) and The fetch-execute cycle, https://www.bbc.co.uk/bitesize/guides/z7qqmsg/revision/4 (the address goes from the program counter to the memory address register and out on the address bus; the data comes back on the data bus to the memory data register and is copied to the current instruction register; results go to the accumulator). (2) Wikipedia: Central processing unit, https://en.wikipedia.org/wiki/Central_processing_unit (control unit, arithmetic logic unit, program counter, cache, clock signal); Processor register, https://en.wikipedia.org/wiki/Processor_register (instruction register, memory buffer register also known as memory data register, memory address register, accumulator); Bus (computing), https://en.wikipedia.org/wiki/Bus_(computing) (address bus, data bus, control bus); Control bus, https://en.wikipedia.org/wiki/Control_bus (it takes the CPU's commands out and brings the devices' status signals back). Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

The parts of a processor on one block diagram. Inside the CPU are the control unit, the arithmetic logic unit, the registers, the cache and the clock. Three buses join it to main memory.

Read the full guide

The blocks inside

The control unit fetches, decodes and carries out instructions. The arithmetic logic unit, or ALU, does the calculations and makes the decisions. The registers and the cache are fast memory inside the processor, and the clock sends the pulse that keeps every part in time.

Buses and registers

The address bus carries memory addresses out of the processor, the data bus carries the data itself in both directions, and the control bus carries control signals. Each register is told apart by what it joins: the memory address register feeds the address bus, the memory data register meets the data bus, and the accumulator is joined to the ALU.

  • The address bus is the only wide arrow with one head: addresses only ever leave the processor.

Questions and answers

What are the main parts of a processor?

The control unit, the arithmetic logic unit and the registers. Most school courses add the cache, the clock and the buses that join the processor to memory.

What does the control unit do?

It fetches each instruction, decodes it and carries it out, and it sends the control signals that tell the other parts of the computer what to do.

What is the difference between the address bus and the data bus?

The address bus carries the memory address the processor wants, and only away from the processor. The data bus carries the data stored at that address, and it works in both directions.

What is the program counter?

A register that holds the memory address of the instruction to be fetched next. After each fetch it is increased by one.

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