The CPU, the fetch-execute cycle and performance
Everything a computer does is one cycle repeated billions of times a second. Learn the cycle and the performance questions answer themselves.
Lesson overview
What you'll learn in this lesson
Explain systems architecture, memory and storage, and data representation.
Key learning points
- • Von Neumann architecture
- • The fetch-execute cycle
- • Performance factors
- • Memory and storage
This lesson at a glance
- 30 minutes
- 21 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 21
Visual introductionPicture this
The CPU, the fetch-execute cycle and performance
Everything a computer does is one cycle repeated billions of times a second. Learn the cycle and the performance questions answer themselves.
In a nutshell
Explain systems architecture, memory and storage, and data representation.
Learning cycle
Part 2 of 21
Learning cycle 1 of 2
Part 1 · Von Neumann architecture
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 21
Learn
Von Neumann architecture
Instructions and data share the same memory. The CPU contains the control unit, the arithmetic logic unit and registers: the program counter holds the address of the next instruction, the memory address register holds the address being accessed, the memory data register holds the value fetched, and the accumulator holds results of calculations.
Reset break
Part 4 of 21
Pause
That's sitting 1 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 5 of 21
Learn
The fetch-execute cycle
Fetch: the address in the program counter is copied to the MAR, the instruction is fetched into the MDR, and the counter increments. Decode: the control unit works out what the instruction means. Execute: the ALU performs the operation and the result is stored, often in the accumulator. Then it repeats.
Quick check
Part 6 of 21
Quick check
Part 7 of 21
Reset break
Part 8 of 21
Pause
That's sitting 2 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 9 of 21
Learning cycle 2 of 2
Part 2 · Performance factors
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 21
Learn
Performance factors
Clock speed sets cycles per second; more cores allow genuinely parallel execution, though only if software is written to use them; cache is small, very fast memory holding frequently used instructions, so a larger cache reduces slow trips to RAM. Answer performance questions by naming the factor and its mechanism.
Explore the idea
Part 11 of 21
Learn
Memory and storage
RAM is volatile working memory; ROM is non-volatile and holds the boot instructions; virtual memory uses secondary storage when RAM is full, which slows the system because disk access is far slower. Secondary storage is chosen on capacity, speed, portability, durability and cost.
Reset break
Part 12 of 21
Pause
That's sitting 3 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 13 of 21
Quick check
Part 14 of 21
Explore the idea
Part 15 of 21
Worked example
Model answer: 'Explain how increasing cache size can improve performance' (4 marks)
Cache is a small amount of very fast memory located close to the CPU that stores frequently used instructions and data. When the CPU needs an instruction it checks the cache first, and a cache hit is served far faster than fetching from RAM. Increasing the cache size means more instructions and data can be held there, so the hit rate rises and the CPU spends less time idle waiting for slower main memory. Performance therefore improves, although the gain diminishes once the working set already fits in cache.
Reset break
Part 16 of 21
Pause
That's sitting 4 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 17 of 21
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 18 of 21
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Instructions and data share the same ____.
RAM is volatile working memory; ROM is non-volatile and holds the boot instructions; virtual memory uses secondary ____ when RAM is full, which slows the system because disk access is far slower.
Challenge round
Part 19 of 21
Game · Recall cards
What does the program counter hold?
Card 1 of 4
Mastery quiz
Part 20 of 21
Marked quiz
End of lesson quiz: The CPU, the fetch-execute cycle and performance
4 questions, marked with the reasoning shown. No timer.
1. What does the program counter hold?
2. Why does virtual memory slow a system?
3. Which is volatile?
4. Extra cores only improve performance when…
Lesson round-up
Part 21 of 21
Lesson round-up
Ready when you are
Quiz score
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Points this lesson
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Best run
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Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?
Ask LunaPart 1 of 21 · Watch & discover