Good Learning Co.
SavannahYear 8
← Computing
ComputingGCSE computer science: systems and data30 min★★★ difficulty

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
  • 17 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

systemsarchitecturememorystoragerepresentation

Scroll down — the lesson carries on below

Part 1 of 17 · Discover6%
1

Watch & discover

Part 1 of 17

Watch first

The CPU, the fetch-execute cycle and performance

An English teaching video on this topic from Mr Moore | Computer Science. Watch it, then scroll down for the lesson.

In a nutshell

Explain systems architecture, memory and storage, and data representation.

2

Learning cycle

Part 2 of 17

Learning cycle 1 of 2

Part 1 · Von Neumann architecture

A short piece of teaching, then a check to make sure it has landed.

3

Explore the idea

Part 3 of 17

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.

4

Explore the idea

Part 4 of 17

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.

5

Quick check

Part 5 of 17

Quick check

What does the program counter hold?

6

Quick check

Part 6 of 17

Quick check

Why does virtual memory slow a system?

7

Learning cycle

Part 7 of 17

Learning cycle 2 of 2

Part 2 · Performance factors

A short piece of teaching, then a check to make sure it has landed.

8

Reset break

Part 8 of 17

Pause

That's sitting 1 of 2 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.

Stop here for now
9

Explore the idea

Part 9 of 17

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.

10

Explore the idea

Part 10 of 17

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.

11

Quick check

Part 11 of 17

Quick check

Which is volatile?

12

Quick check

Part 12 of 17

Quick check

Extra cores only improve performance when…

13

Explore the idea

Part 13 of 17

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.

14

Challenge round

Part 14 of 17

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

15

Challenge round

Part 15 of 17

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.

16

Mastery quiz

Part 16 of 17

Marked quiz

End of lesson quiz: The CPU, the fetch-execute cycle and performance

4 questions, marked with the reasoning shown. No timer.

  1. 1. What does the program counter hold?

  2. 2. Why does virtual memory slow a system?

  3. 3. Which is volatile?

  4. 4. Extra cores only improve performance when…

17

Lesson round-up

Part 17 of 17

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Article

How a supermarket checkout knows the price

When a barcode is scanned, the checkout does not store the price itself. It sends the barcode number to a database and reads back a record: product name, price, whether it needs an age check. The record is the single place the price lives, so changing one row changes every till in the shop at once.

Problems come from bad data, not bad scanners. If two products share a barcode, or a price update reaches the shop overnight but the till holds an old copy in its cache, customers are charged the wrong amount. Shops check this with test transactions: scan a known item, expect a known price, record what actually happened.

Good systems also log every scan. If a customer complains, the log shows exactly which record was used and when.

Written for Good Learning Co.

Reading questions

  1. Write in your own words what the checkout sends and what it receives back.
  2. Explain one way bad data could cause a wrong price, and how logging helps find it.
  3. Design a test table with four tests for a new checkout, with expected and actual results.

  • Reading and notes

    Reading

    Read the lesson text again and note the key terms and how each one is used, in your own words — as you would for revision.

    Half a page of notes

  • Extended writing

    Writing

    Explain a real system that uses this idea — how it works, where it can go wrong, and how you would make it safer or better.

    250–350 words

  • Algorithm on paper

    Diagram

    Write your steps out as pseudocode or a flowchart before you type anything, then photograph it.

    1 flowchart or 10 lines

  • Code write-up

    Writing

    Copy your code in, then explain underneath what each part does and what you had to fix.

    Code + 5 sentences

  • Test table

    Working out

    Make a test table: what you tried, what you expected, what actually happened.

    4–6 tests

Saved as you type.

Part 1 of 17 · Watch & discover

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