How does a CPU work?
A CPU repeats a simple loop: fetch an instruction, decode it, do it, save the result, billions of times each second.
1. Four steps, over and over
Like a cook with a recipe, the CPU fetches an instruction, decodes it, executes it and writes back the result. Unlike a cook, it does not think: the program decides every step.
2. Several at once
To go faster, the CPU works on several instructions at once, each at a different step. This is a pipeline: each tick, every instruction moves forward one step, like dishes moving from station to station in one kitchen.
3. A shelf beside the stove
A cache is a small, fast memory beside the core, like a shelf by the stove. It holds copies of recent data, so the core looks there first, before the slower main memory (RAM).
4. Resting when idle
When a core has no work, it stops its clock or switches off to save power. It wakes up quickly when work arrives.
Words to know
- Pipeline
- Steps lined up so several instructions are in progress at once.
- Clock
- A steady beat that keeps every step in time. 4 GHz (gigahertz) means 4 billion ticks a second.
- Cache
- A small, fast memory next to the core that keeps copies of recently used data.
- RAM (random access memory)
- The main memory: big, but slower than a cache.
Where you meet it at work
Chips manage power on top of this loop: busy cores tick faster, and idle cores stop their clocks or switch off. Testers check that both happen at the right time.
Check yourself
What four steps does a CPU repeat for every instruction?Think first, then tap to see the answer.
Fetch, decode, execute and write back.