ALU, CU, Cache, Registers

Common CPU components and their function: ALU, CU, Cache, Registers

1.1.1 Systems Architecture

Systems Architecture — Note 2 of 3

What you need to know

The CPU contains several key components that work together to process instructions. The Arithmetic Logic Unit (ALU), Control Unit (CU), cache, and registers each have specific roles in ensuring the CPU operates correctly.

What to know

Cache is faster than RAM but smaller in capacity. It reduces the number of times the CPU needs to access slower main memory.

Key terms

ALUArithmetic Logic Unit: Performs mathematical calculations (e.g. addition, subtraction) and logical operations (e.g. comparisons).
CUControl Unit: Coordinates the operation of the CPU, controls the fetch-execute cycle, and sends signals/commands to other components.
Cache: A small, fast memory located ON the CPU that stores frequently used data and instructions to speed up processing.
Registers: Fast-to-access storage locations inside the CPU used to temporarily hold small amounts of data needed during processing.

Exam technique

Tip:When describing the ALU, be specific — it does BOTH arithmetic (maths) AND logic (comparisons). Do not just say 'does calculations'.
Mistake:Saying the CU 'performs' or 'executes' instructions. The CU CONTROLS and COORDINATES the fetch-execute cycle and DECODES instructions — it does not execute them itself.
Tip:Saying the CU 'sends signals to components' alone is not enough — you must say what the signals DO (e.g. 'sends signals to synchronise/control the flow of data').

Example:Table question — 'It stores a single address' = MAR and PC. 'It stores frequently used instructions' = Cache. 'It is a register' = MAR, MDR, PC. 'It stores all currently running data' = RAM.

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