The CPU and Fetch–Execute Cycle

Purpose of the CPU

The Central Processing Unit processes all data and instructions — the "brain" of the computer.

Von Neumann architecture components

• Control Unit (CU) – directs operations, decodes instructions, manages flow.
• Arithmetic Logic Unit (ALU) – calculations and logical comparisons.
• Registers – tiny, ultra-fast storage inside the CPU:
• PC (Program Counter) – address of the next instruction
• MAR (Memory Address Register) – the address being read/written
• MDR (Memory Data Register) – the data read/written
• Accumulator – stores results of calculations
• Cache – small, fast memory holding frequently used data.

The fetch–execute cycle

FETCH    – get the next instruction from memory (using PC, MAR, MDR)
DECODE   – the CU works out what it means
EXECUTE  – carry it out (e.g. ALU does a calculation)

…then repeat, billions of times per second.

Performance factors

• Clock speed (GHz) – cycles per second; higher = more instructions/sec.
• Number of cores – each core runs instructions in parallel.
• Cache size – more cache = fewer slow trips to RAM.

Exam tip

Order: Fetch → Decode → Execute. Link each performance factor to why it speeds the CPU up.