GCSE Computer Science: Systems Architecture and Memory Guide

The CPU

The Central Processing Unit (CPU) carries out instructions from programs. Its performance is affected by clock speed (how many instructions it can process per second), the number of cores, and cache size.

The fetch-execute cycle

The CPU repeatedly fetches an instruction from memory, decodes what it means, and executes it, using registers to store data temporarily during this process.

Primary storage

RAM (Random Access Memory) temporarily stores data and programs currently in use, and is volatile (its contents are lost when power is off). ROM (Read-Only Memory) stores permanent, unchangeable instructions like startup instructions, and is non-volatile.

Secondary storage

Secondary storage holds data permanently even when powered off, such as hard disk drives (HDD), solid state drives (SSD) and optical media, each with different speed, cost and capacity trade-offs.

Common mistakes

• Mixing up RAM (volatile, temporary) and ROM (non-volatile, permanent).
• Forgetting the correct order of the fetch-execute cycle.
• Assuming a faster clock speed alone always means better overall performance, ignoring cores and cache.