Computer Architecture (Copy)
IGCSE / O Level Computer Science Cheat Sheet
Topic: 3.1 Computer Architecture
🧠 Central Processing Unit (CPU)
- CPU: Executes instructions and processes data
- Also called processor or microprocessor
📦 Von Neumann Architecture Components
| Component | Function |
|---|---|
| ALU (Arithmetic Logic Unit) | Performs calculations and logic comparisons |
| CU (Control Unit) | Directs the flow of instructions and controls components |
| Registers | Temporary high-speed memory used during processing |
| Buses | Pathways for data, address, and control signals |
🔁 Registers in CPU
| Register | Description |
|---|---|
| PC (Program Counter) | Holds address of next instruction |
| MAR (Memory Address Register) | Holds memory location to be accessed |
| MDR (Memory Data Register) | Holds data fetched from or to be written to memory |
| CIR (Current Instruction Register) | Holds current instruction |
| ACC (Accumulator) | Stores results of ALU operations |
🚍 Buses
| Bus | Function |
|---|---|
| Address Bus | Carries addresses only (one direction) |
| Data Bus | Carries actual data (both directions) |
| Control Bus | Carries control signals |
🔄 Fetch–Decode–Execute (FDE) Cycle
- Fetch:
- PC → MAR → memory location fetched → MDR → CIR
- Decode:
- Instruction in CIR is interpreted by CU
- Execute:
- CU sends signals to ALU or memory → result stored in ACC
⚙️ Factors Affecting CPU Performance
| Factor | Effect |
|---|---|
| Clock Speed | Higher = more instructions per second |
| Cache Size | Larger cache = faster access to frequently used data |
| Number of Cores | More cores = more tasks processed in parallel |
📘 Instruction Set
- Set of machine-code commands a CPU can understand
- Written in binary
- Each CPU model has its own instruction set
🤖 Embedded Systems
- Special-purpose systems built into a larger device
- Perform dedicated tasks
- Use microcontrollers with ROM, RAM, CPU
Examples:
- Washing machines
- Digital thermostats
- Car engine systems
- Smart TVs
| Embedded System | General-purpose Computer |
|---|---|
| Dedicated task | Multiple tasks |
| Limited resources | Expandable resources |
| Cost-effective | More expensive |