Two laptops can have very different speeds even with the same amount of RAM. Inside the processor, three things make the difference: how many cores it has, how much cache it has and how fast its clock ticks.
A core is a complete processing unit that can fetch, decode and execute instructions; more cores mean more instructions can be processed at the same time. Cache is a small amount of very fast memory inside or next to the CPU that stores frequently used instructions and data; a bigger cache means fewer slow trips to RAM. The clock sends regular pulses that time each FDE cycle; a higher clock speed (measured in hertz, for example 3.5 GHz) means more cycles per second.
Fig. 1: cores, cache and clock speed each change how many instructions a CPU can process per second
Section 01
The clock.
In the lab, change the clock speed, the cache size and the number of cores, and watch how long the same set of tasks takes.
Clock: an electronic clock inside the CPU sends out pulses at a steady rate. Each pulse keeps the components in step and starts the next step of the cycle.
Clock speed: the number of cycles per second, measured in hertz (Hz). 3.5 GHz is 3.5 billion cycles every second.
Effect: a faster clock means more fetch-decode-execute cycles per second, so more instructions are processed per second.
Limit: a faster clock makes the CPU hotter. Too high a speed (overclocking) can make it overheat or become unstable.
Examiner trap
Clock speed is not a measure of memory. 3.5 GHz describes how often the CPU's clock ticks, not how much it can store.
Section 02
Cache memory.
RAM is fast compared with a hard disk, but slow compared with the CPU. Cache is a small, very fast store that the CPU checks first.
What it stores: the instructions and data that are used most often, or used most recently.
Why it helps: the CPU can read from cache much faster than from RAM, so less time is spent waiting for data.
Bigger cache: more instructions and data can be kept close to the CPU, so they are found in the cache more often and fewer slow fetches from RAM are needed.
Your turn
Why is cache not simply made big enough to replace RAM?
Cache memory is very expensive to make and takes space on the chip, so only a small amount is used for the most frequently needed data.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the lab, what happens to the time for the same tasks when the clock speed goes up?
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What is cache?
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A CPU has a clock speed of 2.4 GHz. What does this mean?
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Why might doubling the number of cores not double the performance?
Section 03
More cores.
A core is a processing unit with its own control unit, ALU and registers. A quad-core CPU has four of them on one chip.
Effect: each core can run its own FDE cycle, so several instructions are processed at the same time.
Not always double: two cores do not always mean twice the speed. Some tasks cannot be split up, and the cores spend time communicating and sharing data.
Software: a program only benefits fully if it has been written to use several cores.
Worked example Compare two CPUs
CPU A: 4 cores, 3.0 GHz, 8 MiB cache. CPU B: 2 cores, 3.0 GHz, 4 MiB cache. Which is likely to perform better, and why?
The clock speeds are the same, so compare the other two factors.
A has more cores, so it can process more instructions at the same time.
A has a bigger cache, so it needs fewer slower fetches from RAM.
Answer: CPU A, because of its extra cores and larger cache
Examiner trap
Always say how the factor improves performance: "more cores, so more instructions can be processed at the same time", not just "more cores make it faster".
Stage 2 · Exam
Exam-style questions
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Which change is most likely to reduce how often the CPU must fetch data from RAM?
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What is a core?
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Why can increasing the clock speed cause problems?
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Which CPU is likely to be fastest, if all else is equal?
Skill unlocked
Cores, cache and clock speed, mastered.
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