CPU Performance

This lesson covers the three main factors that affect CPU performance: clock speed, number of cores, and cache size. These are commonly examined in AQA and OCR GCSE Computer Science.

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Why Does CPU Performance Matter?

The speed at which a CPU can process instructions determines how quickly programs run, how smoothly games play, and how fast files are processed. Three key hardware features influence this performance:

1. Clock speed
2. Number of cores
3. Cache size

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Performance Factors at a Glance

graph TD
    A["CPU Performance"] --> B["Clock Speed (GHz)"]
    A --> C["Number of Cores"]
    A --> D["Cache Size"]
    B --> E["Cycles per second"]
    C --> F["Parallel processing"]
    D --> G["L1 Cache"]
    D --> H["L2 Cache"]
    D --> I["L3 Cache"]

1. Clock Speed

What Is Clock Speed?

The clock speed (also called clock rate or clock frequency) measures how many instructions a CPU can process per second. It is measured in hertz (Hz).

• 1 MHz (megahertz) = 1 million cycles per second
• 1 GHz (gigahertz) = 1 billion cycles per second

A typical modern desktop CPU runs at between 3 GHz and 5 GHz — that means it can perform 3 to 5 billion cycles per second.

How Does It Affect Performance?

• A higher clock speed means the CPU can carry out more fetch-decode-execute cycles per second, so programs run faster.
• However, increasing clock speed too much generates excess heat, which can damage the processor or reduce its lifespan.

Clock Speed	Cycles Per Second	Relative Speed
1 GHz	1,000,000,000	Baseline
2 GHz	2,000,000,000	2x faster
4 GHz	4,000,000,000	4x faster

Overclocking

Overclocking is the practice of 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 make the system unstable or reduce the CPU's lifespan.
• May void the warranty.

Exam Tip: When asked "How does clock speed affect performance?", always explain the link to the FDE cycle: a higher clock speed means more cycles per second, therefore more instructions are processed per second.

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2. Number of Cores

What Is a Core?

A core is an independent processing unit within the CPU. Each core has its own ALU, CU, and registers, meaning each core can carry out its own fetch-decode-execute cycle.

Term	Meaning
Single-core	CPU with one processing unit
Dual-core	CPU with two processing units
Quad-core	CPU with four processing units
Octa-core	CPU with eight processing units

How Do Cores Affect Performance?

• Multiple cores allow the CPU to carry out multiple instructions simultaneously (this is called parallel processing).
• A dual-core processor can theoretically process two instructions at the same time, a quad-core can process four, and so on.

Important Limitations

• Having more cores does not automatically double performance.
• Software must be written to take advantage of multiple cores (this is called multi-threaded programming).
• Some tasks are inherently sequential and cannot be split across cores.
• Not all programs are optimised for multi-core processors.

Exam Tip: A common 4-mark question asks you to explain why a quad-core CPU is not four times faster than a single-core. The answer is that software must be designed to split tasks across cores, and some tasks cannot be parallelised.

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3. Cache Size

What Is Cache?

Cache is a very small amount of extremely fast memory located inside or very close to the CPU. It stores frequently used data and instructions so the CPU can access them quickly without going to the slower main memory (RAM).

Levels of Cache

Modern CPUs have multiple levels of cache:

Level	Size	Speed	Location
L1 cache	Very small (16–64 KB per core)	Fastest	Built into each core
L2 cache	Small (256 KB–1 MB per core)	Fast	Close to each core
L3 cache	Larger (4–32 MB shared)	Slower than L1/L2 but faster than RAM	Shared between all cores

How Does Cache Affect Performance?

• More cache = more data can be stored close to the CPU = fewer trips to slower RAM = faster performance.
• Cache operates at CPU speed rather than RAM speed, so accessing data from cache is significantly quicker.
• If the data the CPU needs is in the cache, this is called a cache hit. If it is not, this is called a cache miss, and the CPU must access the slower main memory.

Exam Tip: When explaining cache, always compare its speed to RAM. Cache is faster because it is physically closer to the CPU and uses faster memory technology (SRAM vs DRAM).

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Comparing the Three Factors

Factor	What It Does	Trade-off
Clock speed	Determines how many FDE cycles per second	Higher speed = more heat
Number of cores	Allows parallel processing	Not all software uses multiple cores
Cache size	Stores frequently used data close to the CPU	Cache is expensive to manufacture; diminishing returns above a certain size

Which Factor Matters Most?

There is no single answer — it depends on the task:

• Gaming / single-threaded tasks: Clock speed often matters most.
• Video editing / multi-tasking: Number of cores matters most.
• General responsiveness: Cache size helps reduce delays.

In practice, manufacturers balance all three factors to produce a well-rounded CPU.

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Worked Example

Question: A student is choosing between two CPUs. CPU A has a clock speed of 4.0 GHz and 4 cores. CPU B has a clock speed of 3.0 GHz and 8 cores. Explain which CPU would be better for (a) gaming and (b) video editing.