The parts of a CPU
ALU does the maths; control unit conducts; registers hold the working values.
The Central Processing Unit (CPU) is the brain of the computer. It has three main parts.
1. ALU (Arithmetic and Logic Unit). Does the actual computation:
- Arithmetic: ADD, SUBTRACT, MULTIPLY, DIVIDE.
- Logical: AND, OR, NOT, comparisons (>, <, =).
2. Control Unit (CU). Conducts the orchestra:
- Decodes instructions.
- Sends control signals along the control bus to coordinate the other components.
- Manages the FDE cycle.
3. Registers. Tiny, super-fast storage locations inside the CPU. Each has a specific role:
| Register | Role |
|---|---|
| PC (Program Counter) | Address of the NEXT instruction. |
| MAR (Memory Address Register) | Holds the address being read/written. |
| MDR (Memory Data Register) | Holds the data/instruction transferred. |
| CIR (Current Instruction Register) | Holds the instruction being decoded/executed. |
| ACC (Accumulator) | Holds the result of arithmetic/logic. |
Cambridge tip. Examiner reports flag candidates who write 'memory' when they should write 'register'. They are different: registers are INSIDE the CPU and orders of magnitude faster than RAM.
- ALU = arithmetic + logic.
- CU decodes and signals.
- Registers are CPU-local fast storage.
- PC, MAR, MDR, CIR, ACC β know them.