Hardware · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
The fetch-decode-execute cycle and von Neumann architecture are tested almost every year.
Three characteristics of a CPU set how quickly it can process work: clock speed, cache size, and number of cores.
Clock speed is the number of fetch-decode-execute cycles the CPU can complete in one second, measured in hertz (Hz).
A higher clock speed means the CPU completes more cycles per second, so it processes more instructions per second. Doubling the clock speed roughly doubles the rate of instructions (assuming nothing else becomes a bottleneck).
Why not just push the clock speed to 100 GHz? Higher clock speeds:
So modern CPUs balance clock speed against power and heat, often around 3 to 5 GHz.
Cache is a small, high-speed memory built into (or sitting beside) the CPU. It holds copies of frequently-used instructions and data so the CPU can grab them without making a slow trip out to main memory (RAM).
A larger cache size means better CPU performance, because the CPU spends less time waiting for data to arrive from main memory.
A core is a complete processing unit within a CPU chip, with its own Control Unit, ALU, registers and accumulator. A modern chip usually has multiple cores that can each fetch, decode and execute instructions at the same time.
For tasks that can be split across cores (video encoding, scientific computation, running many apps at once), more cores means more work done per second.
Example — A quad-core CPU runs at 3 GHz. How many instructions per second can the CPU theoretically execute in total?
In real life, not every program can be split perfectly across cores, and there are overheads in coordinating the cores. So the actual speedup is usually less than the theoretical multiplier, but more cores still helps in most situations.
| Characteristic | If increased | Why performance improves |
|---|---|---|
| Clock speed | More cycles per second | More instructions executed per second |
| Cache size | More frequently-used data stays close to CPU | Fewer slow trips to main memory |
| Number of cores | More instruction streams run at once | Parallel work finishes in less wall-clock time |
Manufacturers usually advertise the clock speed first, but cache and cores often matter just as much. A 4 GHz quad-core with a large cache will usually outperform a 5 GHz single-core with a tiny cache on real-world workloads. When comparing two CPUs, you need to look at all three characteristics.