Systems architecture: inside the machine
Computer Science · GCSE · The School
Fetch, decode, execute
The CPU fetches the next instruction from memory, decodes what it means, then executes it — billions of times a second, forever. Everything a computer does is that loop repeated. Registers hold the values being worked on right now; the program counter remembers which instruction is next.
What makes a CPU fast
Clock speed (cycles per second), number of CORES (independent workers), and CACHE size (a small very fast memory that avoids waiting for RAM). More cores only help software written to use them, which is why doubling cores rarely doubles speed — a useful corrective to the marketing.
The memory hierarchy
Registers (tiny, instant), cache (small, very fast), RAM (larger, fast, forgets when powered off), secondary storage (huge, slow, permanent). ROM holds start-up instructions and is read-only. The whole hierarchy exists because fast memory is expensive — so computers keep what is needed NOW near the CPU and everything else further away.