The purpose of the CPU

The Central Processing Unit (CPU) is the brain of the computer. Its job is to fetch, decode and execute the instructions of a program stored in memory, one after another.

• It processes data and instructions to make the computer work.
• It carries out the fetch–decode–execute cycle continuously.
• Its main parts are the Control Unit (CU), the Arithmetic Logic Unit (ALU), the cache and the registers.

Key parts: the CU directs the operation of the processor and controls the flow of data; the ALU does all the arithmetic (add, subtract) and logic (AND, OR, comparisons); the cache is fast memory close to the CPU.

What does the ALU do?

Von Neumann architecture & registers

Most computers use the Von Neumann architecture: data and instructions are stored together in the same memory, and the CPU fetches them one at a time. Special-purpose, very fast stores inside the CPU are called registers.

• MAR (Memory Address Register) — holds the address in memory to be read from or written to.
• MDR (Memory Data Register) — holds the data or instruction just fetched from, or about to be written to, memory.
• PC (Program Counter) — holds the address of the next instruction to be fetched.
• ACC (Accumulator) — stores the result of calculations carried out by the ALU.

Which register holds an address?

The fetch–decode–execute cycle

The CPU repeats the fetch–decode–execute cycle for every instruction:

• Fetch — the instruction at the address in the PC is copied from memory into the CPU (via MAR & MDR); the PC is increased.
• Decode — the CU works out what the instruction means.
• Execute — the instruction is carried out (e.g. the ALU does a calculation; the result may go to the ACC).

CPU performance

Three factors change how fast a CPU runs:

• Clock speed — how many cycles per second, measured in hertz. A higher clock speed = more instructions per second. (e.g. 3.5 GHz = 3.5 × 10⁹ cycles/second.)
• Number of cores — each core can fetch–decode–execute independently, so more cores can process more instructions at once. (Not always double the speed — some tasks can't be split.)
• Cache size — cache is very fast memory inside/near the CPU. A bigger cache stores more frequently used data, so the CPU waits less for RAM.

Units: 1 kHz = 1,000 Hz, 1 MHz = 1,000,000 Hz, 1 GHz = 1,000,000,000 Hz. So a 2 GHz clock ticks 2 billion times a second.

Your turn — clock cycles

Why add more cores?

Embedded systems

An embedded system is a computer built inside a larger device to do one specific job. It is dedicated to that task rather than being a general-purpose computer.

• Examples: a washing machine controller, dishwasher, traffic lights, a car's engine management, a digital watch, a microwave.
• They are usually cheaper, smaller, use less power and are more reliable than a general-purpose PC because they run fixed programs.

Contrast: a laptop or phone is general-purpose — it runs many different programs. An embedded system runs one dedicated control program.

Spot the embedded system

Which register does what?

Tap a job, then tap the register (or part) it belongs to.

Match term to meaning

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The big ideas to know

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