CPU Components
Overview
The Central Processing Unit (CPU) is the component that executes program instructions. It is built from several sub-components, each with a specific role.
Core Components
Control Unit (CU)
- Decodes instructions fetched from memory
- Sends control signals to coordinate all other components
- Does not perform calculations itself, it directs traffic
Arithmetic Logic Unit (ALU)
- Performs arithmetic operations: addition, subtraction, multiplication, division
- Performs logical operations: AND, OR, NOT, comparisons
- Results are stored in the Accumulator (ACC)
Cache
- Small, extremely fast memory located inside or very close to the CPU
- Stores frequently accessed instructions and data
- Reduces the time the CPU spends waiting for data from slower main memory (RAM)
- See also: Von Neumann Architecture (Von Neumann bottleneck)
Clock
- Generates a constant electrical pulse that synchronises all CPU operations
- Every operation takes a fixed number of clock cycles
- Clock speed = number of cycles per second, measured in Hz (GHz for modern CPUs)
- A faster clock = more instructions executed per second
Registers
- High-speed storage locations inside the CPU
- Hold data that is being actively worked on
- Much faster than RAM but extremely small capacity
- See CPU Registers for detail on each register
Component Summary Table
| Component | Role |
|---|---|
| Control Unit (CU) | Decodes instructions; sends control signals to all components |
| ALU | Performs arithmetic and logical calculations |
| Cache | Fast temporary storage for frequently used data/instructions |
| Clock | Synchronises operations with a regular pulse |
| Registers | Ultra-fast storage inside the CPU (PC, MAR, MDR, ACC) |
How They Work Together
During the Fetch-Decode-Execute Cycle:
- The CU uses the PC to fetch an instruction via System Buses
- The CU decodes the instruction
- If a calculation is needed, the ALU performs it, storing the result in the ACC
- The Clock keeps all of this in step
Exam Questions
Q: What is the role of the Control Unit?
It decodes instructions and sends control signals to coordinate the operation of all other CPU components.
Q: Where are the results of ALU operations stored?
In the Accumulator (ACC) register.
Q: How does cache memory improve performance?
By storing frequently accessed data and instructions in fast memory close to the CPU, reducing the time spent waiting for slower main memory (RAM).
Exam Tip
The CU does not do calculations, that is the ALU’s job. The CU controls; the ALU computes.