This topic summary covers Knowledge Organiser: CPU Architecture within CPU Architecture for GCSE Computer Science. Revise CPU Architecture in 3.4 Computer Systems for GCSE Computer Science with 16 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: CPU Architecture
Key Terms
- CPU: Central Processing Unit — the component that processes all instructions
- Control Unit (CU): Part of CPU that fetches, decodes and coordinates instructions
- ALU: Arithmetic Logic Unit — performs all arithmetic, logic and comparison operations
- Register: Tiny ultra-fast temporary storage location inside the CPU
- Address Bus: One-way bus carrying the memory address (WHERE to look)
- Data Bus: Two-way bus carrying the actual data being transferred (WHAT)
- Control Bus: Two-way bus carrying command signals (read, write, clock)
- Von Neumann architecture: Design where programs and data are stored in the same memory
Must-Know Facts
- The CPU has three main components: Control Unit, ALU, and Registers
- The Control Unit fetches instructions, decodes them, and sends control signals
- The ALU performs arithmetic (+ - × ÷), logic (AND OR NOT), and comparison operations
- Registers are faster than RAM but very small — hold data being currently processed
- The Address Bus is one-way (CPU to memory only); the Data Bus is two-way
- ABC memory trick: Address = WHERE, Data = WHAT, Control = HOW
- Von Neumann bottleneck: CPU must wait while data travels along buses
Key Concepts
- Von Neumann architecture stores both instructions AND data in the same RAM
- The CU acts like an orchestra conductor — directs other components without doing the computing itself
- All ALU results are stored in the Accumulator (ACC) register
- Key registers: MAR (address), MDR (data), PC (next instruction), ACC (result)
- Wider address bus = more memory locations addressable; wider data bus = more data per transfer
Common Mistakes
- Confusing CU and ALU: The CU controls/coordinates — it does NOT do calculations; the ALU performs arithmetic and logic
- Saying the CPU "stores" data: The CPU processes data — storage is done by RAM (temporary) and secondary storage (permanent)
- Mixing up buses: The address bus carries memory locations, the data bus carries actual data, the control bus carries signals — they are separate
- Forgetting Von Neumann bottleneck: Programs and data share the same bus, which slows down the CPU — this is a weakness, not a feature to praise
Practice questions for CPU Architecture
Which component of the CPU carries out arithmetic and logical operations?
Name three main components of the CPU and state the purpose of each.
Quick recall flashcards
What does ALU stand for?
Arithmetic Logic Unit
What does CPU stand for?
Central Processing Unit