Computer Systems — AQA GCSE Computer Science
This unit covers hardware and software, the CPU, the fetch–execute cycle, memory and storage, and logic gates.
Hardware and software
- Hardware — the physical parts of a computer.
- Software — the programs that run on it: system software (operating system, utilities) and application software.
The operating system manages hardware, memory, processes, files, peripherals and user accounts.
The CPU and the fetch–execute cycle
The CPU (central processing unit) processes instructions. Its main parts (von Neumann architecture):
- Control unit (CU) — manages the flow of data and decodes instructions.
- Arithmetic logic unit (ALU) — performs calculations and logical operations.
- Registers (including the program counter, MAR, MDR, accumulator).
The CPU runs the fetch–decode–execute cycle: it fetches an instruction from memory, decodes it, then executes it, repeating continuously.
CPU performance is affected by:
- clock speed (instructions per second),
- number of cores, and
- cache size.
Memory and storage
- RAM — volatile main memory holding currently running programs and data.
- ROM — non-volatile, holds the start-up instructions.
- Virtual memory — using secondary storage as extra RAM when RAM is full.
- Secondary storage — non-volatile storage: magnetic (hard disk), optical (CD/DVD) and solid state (SSD/flash). Choose by capacity, speed, portability, durability and cost.
Boolean logic
Logic gates process binary signals:
- AND — output 1 only if both inputs are 1.
- OR — output 1 if either input is 1.
- NOT — inverts the input.
You should be able to build and interpret logic circuits and truth tables.
Exam tips
- Learn the CPU components and the fetch–decode–execute cycle.
- Know factors affecting CPU performance (clock speed, cores, cache).
- Distinguish RAM (volatile) from ROM (non-volatile) and types of secondary storage.
- Complete truth tables for AND, OR and NOT gates.