AQA 8525 · Section 3.4 · 3.4.1

Hardware
Computer Systems

CPU · RAM · Storage · Input/Output · Von Neumann Architecture

CSZoneAQA GCSE Computer Science 8525

The CPU — Central Processing Unit

The Brain of the Computer

ALU (Arithmetic Logic Unit) — performs calculations and logical operations (AND, OR, comparisons)

Control Unit (CU) — directs and coordinates all components; manages the fetch-decode-execute cycle

Registers — tiny, ultra-fast storage inside the CPU (PC, ACC, MAR, MDR, CIR)

Cache — fast memory between CPU and RAM; stores frequently used data

Performance factors:Clock speed (GHz) · Number of cores · Cache size · Word length

Memory

RAM vs ROM vs Secondary Storage

Type	Volatile?	Purpose
RAM	Yes (loses on power off)	Stores currently running programs and data
ROM	No (permanent)	Stores BIOS/firmware — read only
HDD/SSD	No (permanent)	Long-term storage of files and programs
Cache	Yes	Fastest memory — stores recently used CPU data

Storage Devices

HDD vs SSD vs Optical vs Flash

HDD (Hard Disk Drive) — magnetic, spinning disks. Cheap, high capacity, slower, moving parts

SSD (Solid State Drive) — flash memory, no moving parts. Faster, more durable, more expensive per GB

Optical (CD/DVD/Blu-ray) — laser reads pits/lands on disc. Portable, slower, lower capacity

Flash/USB — NAND flash memory, portable, convenient

Exam Practice

Have a go at this question

AQA-style question

A laptop has 16 GB RAM and a 512 GB SSD. Explain the role of RAM and why its contents are lost when the laptop is switched off.

3 marks

RAM stores the currently running programs and data being actively used by the CPU [1]. It is volatile memory [1], meaning it requires a constant power supply to retain data — when power is removed, the stored charges are lost and the data is erased [1].

Key Takeaways

What to Remember

CPU contains ALU, Control Unit, Registers, Cache

RAM = volatile, fast, current programs · ROM = non-volatile, read-only, BIOS

SSD: faster, durable · HDD: cheaper, higher capacity

CPU speed depends on: clock speed, cores, cache size