CPU Performance Factors

This lesson covers the three main factors that affect CPU performance: clock speed, cache size, and number of cores. These are specified in OCR J277 Section 1.1.1 and are frequently tested in the exam.

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Factor 1: Clock Speed

What Is Clock Speed?

The clock speed of a CPU is the number of fetch-decode-execute (FDE) cycles the CPU can complete per second. It is determined by the system clock — an electronic component that generates regular pulses to synchronise the CPU's operations.

How Is It Measured?

Unit	Cycles per second	Example
1 Hz	1 cycle	Original clocks
1 MHz	1 million cycles	Early PCs (1980s)
1 GHz	1 billion cycles	Modern CPUs

A modern desktop CPU might have a clock speed of 3.5 GHz to 5.0 GHz, meaning it can perform 3.5 to 5 billion cycles every second.

How Does Clock Speed Affect Performance?

• A higher clock speed means more FDE cycles per second, so instructions are processed more quickly.
• However, increasing clock speed generates more heat, which requires better cooling.
• There is a practical limit — pushing clock speed too high can cause instability and thermal throttling (the CPU automatically slowing down to prevent damage).

Overclocking

Overclocking means increasing the clock speed beyond the manufacturer's recommended setting. This can improve performance but:

• Generates more heat
• May require additional cooling (e.g. liquid cooling)
• Can reduce the lifespan of the CPU
• May void the warranty

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Factor 2: Cache Size

What Is Cache?

Cache memory is a small amount of very fast memory located inside or very close to the CPU. It stores copies of frequently accessed data and instructions so the CPU does not have to fetch them from the slower main memory (RAM) every time.

Levels of Cache

Level	Speed	Size	Location
L1 cache	Fastest	Smallest (typically 32-64 KB per core)	Built into each CPU core
L2 cache	Fast	Medium (typically 256 KB - 1 MB per core)	Close to each CPU core
L3 cache	Slower than L1/L2	Largest (typically 4-32 MB)	Shared between all cores

How Does Cache Affect Performance?

• When the CPU needs data, it checks the cache first (cache hit = data found, cache miss = data not found).
• A cache hit is much faster than fetching from RAM because cache is closer to the CPU and uses faster memory technology (SRAM).
• More cache means more data can be stored close to the CPU, increasing the chance of cache hits and reducing wait times.