A1.1.1–1.1.4 — CPU, Registers, Buses & Processors
The CPU
The CPU is the primary computational engine of the computer. It executes instructions and coordinates data movement. Three main elements: components, registers, and buses.
Components
distinct functional unit inside the CPU with a specific role.
| Component | Function |
|---|---|
| ALU (Arithmetic Logic Unit) | Performs arithmetic (add, subtract, multiply, divide) and logical operations (AND, OR, NOT). |
| CU (Control Unit) | Manages the fetch-decode-execute cycle. Decodes instructions, generates control signals, and directs data flow between components. |
Registers
Tiny, ultra-fast storage locations inside the CPU. Hold data the CPU is actively using. Volatile.
| Register | What it holds |
|---|---|
| IR (Instruction Register) | The current instruction being executed. |
| PC (Program Counter) | Address of the next instruction. Auto-increments after each instruction. |
| MAR (Memory Address Register) | Address in memory where data/instruction will be read from or written to. |
| MDR (Memory Data Register) | The actual data being transferred to or from memory. |
| AC (Accumulator) | Stores intermediate results of arithmetic/logic operations. |
MAR = address. MDR = data. PC auto-increments. These three are tested constantly.
Buses
Communication pathways that transfer data between CPU and other components. The three buses together form the system bus.
| Bus | Carries | Direction |
|---|---|---|
| Address Bus | Memory addresses from CPU to memory controller. | Unidirectional |
| Data Bus | Actual data and instructions between CPU and memory. | Bidirectional |
| Control Bus | Control signals: read, write, interrupt, clock, acknowledge. | Bidirectiona |
Types of CPU Processors
| Type | Description | Limitation |
|---|---|---|
| Single-core | One core. Executes instructions sequentially, one at a time. | Bottlenecks under multitasking or heavy workloads. |
| Multi-core | Two or more cores on one chip. Each runs independently — enables parallel execution. | Software must be written to exploit multiple cores. |
| Co-processors | Specialised processors that offload specific tasks from the CPU (e.g. GPU for graphics, DSP for signals). | Only useful for their designated task type. |
Parallel processing: Multiple processors or cores execute different parts of a task simultaneously to increase overall speed and efficiency.