Computer systems

A computer system combines hardware and software to accept input, process data and produce output. Understanding how the components inside a computer work — and how they are organised — is a core part of GCSE Computer Science.

Hardware and software (CS4.1)

Hardware is the physical components of a computer system — the parts you can touch: CPU, RAM, keyboard, monitor, hard drive.

Software is a set of instructions that tells the hardware what to do. Without software, hardware is useless. Software includes:

• System software — the OS, utilities, language translators
• Application software — programs users run to accomplish tasks (word processors, browsers, games)

Embedded systems are purpose-built systems with fixed hardware and software, designed for a single function (washing machine controller, car engine management unit).

Boolean logic (CS4.2)

All computation ultimately reduces to Boolean (true/false) logic. Three fundamental gates:

• AND — output is 1 only if both inputs are 1
• OR — output is 1 if at least one input is 1
• NOT — inverts the input (1 → 0, 0 → 1)

Logic circuits are drawn with gate symbols; their behaviour is described in truth tables.

Software classification (CS4.3 and CS4.4)

The operating system (OS) is the most important piece of system software. Its jobs:

• Process management — runs multiple programs simultaneously, allocates CPU time
• Memory management — decides what is in RAM at any moment
• File management — organises data on storage devices
• Peripheral management — communicates with devices via device drivers
• Security — user authentication, access control

Utility programs perform maintenance tasks: antivirus, defragmenter, backup software, file compression/encryption tools (CS4.5).

Von Neumann architecture (CS4.6)

Modern computers follow the stored program concept: both data and instructions are held in memory and fetched as needed. Key components:

• CPU — processes instructions
• Main memory (RAM) — holds current program and data
• Address bus — carries memory addresses (CPU → memory)
• Data bus — carries data in both directions
• Control bus — carries control signals

The fetch–decode–execute cycle is the heartbeat of the CPU: fetch instruction from memory → decode what it means → execute it → repeat.

CPU components and performance (CS4.7)

Inside the CPU:

• ALU (Arithmetic Logic Unit) — performs calculations and comparisons
• Control Unit — manages the fetch–decode–execute cycle
• Registers — tiny, ultra-fast storage inside the CPU (PC, MAR, MDR, Accumulator)

CPU performance depends on:

• Clock speed (GHz) — how many cycles per second
• Cache size — faster memory between CPU and RAM
• Number of cores — more cores allow parallel processing

Memory (CS4.8)

Memory	Volatile?	Notes
RAM	Yes (lost on power off)	Holds current program and data
ROM	No (permanent)	Stores boot firmware
Cache	Yes	Very fast; small; between CPU and RAM
Virtual memory	(uses storage)	Uses part of disk as extra RAM when RAM full

Secondary storage (magnetic HDD, optical CD/DVD, solid-state SSD) is non-volatile and stores data long-term.