Clock Speed
The clock speed (clock rate) is the number of clock cycles the CPU can perform per second, measured in Hz (hertz). Modern CPUs operate in the GHz range (billions of cycles per second). Each fetch-execute cycle takes one or more clock cycles.
- Higher clock speed → more cycles per second → faster instruction processing
- Measured in GHz (gigahertz); e.g. 3.5 GHz = 3,500,000,000 cycles per second
- Overclocking: running the CPU faster than its rated speed (increases heat and power usage)
Number of Cores
A core is an independent processing unit within the CPU that can fetch and execute instructions. A multi-core processor has multiple cores on one chip.
- Dual-core: 2 cores; Quad-core: 4 cores; Octa-core: 8 cores
- Multiple cores allow parallel processing — different cores can execute different tasks simultaneously
- Benefit: running multiple applications at the same time without one blocking another
- Limitation: not all software is designed to use multiple cores efficiently
Cache Size
Cache is very fast memory built into or near the CPU. It stores copies of frequently used instructions and data so the CPU doesn't have to fetch them from slower RAM. There are multiple levels:
| Cache Level | Speed | Size (typical) | Location |
|---|
| L1 cache | Fastest | 32–256 KB | Inside each core |
| L2 cache | Fast | 256 KB – 8 MB | Inside/near each core |
| L3 cache | Fast | 4–32 MB | Shared across all cores |
- Larger cache → more data held near the CPU → fewer slow RAM accesses → faster performance
- A cache hit = data found in cache (fast); a cache miss = data not in cache, must fetch from RAM (slow)
Summary of CPU Performance Factors
| Factor | How it improves performance |
|---|
| Higher clock speed | More fetch-execute cycles per second → faster instruction processing |
| More cores | Multiple tasks processed simultaneously → better multitasking |
| Larger cache | More data held near CPU → fewer slow RAM accesses → less waiting |
📝 Exam Tip: When asked "how would doubling the clock speed affect performance?", state that the CPU would perform more fetch-execute cycles per second, meaning more instructions are processed in the same time. Also mention it generates more heat and uses more power.
⚠️ Common Mistakes
- Saying more cores always means faster — only if the software can use multiple cores (parallel processing)
- Confusing clock speed with number of cores — a 2 GHz quad-core is not the same as an 8 GHz single-core
- Saying cache "stores programs permanently" — cache is volatile and stores temporary, frequently-accessed data only