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.
FETCH: the program counter holds the address of the next instruction; the CPU copies that instruction from RAM into the instruction register and increments the counter. DECODE: the control unit works out what it means — say ADD. EXECUTE: the ALU adds two values and stores the result. Then it starts again. Billions of times a second, and every program you have ever run is this loop.
Von Neumann, and why it is a bottleneck
Instructions and data share one memory and one path to the CPU, which is what makes a stored-program computer general purpose — you change what it does by changing memory, not wiring. The cost is that instructions and data compete for the same route, which is precisely why cache exists.
Storage is chosen by its job
Solid state is fast, silent and has no moving parts; magnetic is cheaper per gigabyte and still used for bulk storage; optical is cheap to distribute and slow to read. Exam answers should justify a choice against the scenario — capacity, portability, durability, cost — rather than assert that one is simply best.