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Year 8 · Autumn 1 · Computer Systems

RAM, ROM and secondary storage

TheoryPresent ▶

What this lesson covers

Topic 2 Bits and bytes

KS3 Computing · A bit is one switch: 0 or 1. A byte is 8 bits. Every photo, song and game is measured by counting these.

Topic 8 Inside the computer

KS3 Computing · The CPU does the work, RAM holds what it's using now, and storage keeps everything long-term. The CPU repeats one loop: fetch, decode, execute.

Computer SystemsCambridge Lower Secondary Computing 0860

You will learn to

  • Explain why RAM is volatile: it loses everything when the power goes off.
  • Explain why ROM is needed to start the computer.
  • Compare magnetic, solid state and optical storage on speed, size, cost and portability.
  • Use the units ladder to compare sizes, like how many 4 MB photos fit on 64 GB.

Success criteria

  • I can explain the desk and filing cabinet picture of RAM and storage.
  • I can find how much RAM is in use in Task Manager.
  • I can choose storage for five scenarios and give a reason for each.

Key words

Say each word, then read what it means. Every one is in the glossary too.

Volatile
Loses its contents when the power goes off.
ROM
Read Only Memory: non-volatile, holds start-up instructions.
Virtual memory
Storage used as extra RAM when RAM is full. Slow.
Bit
One binary digit, 0 or 1. One switch.
Byte
8 bits. 256 possible patterns.
Nibble
4 bits. One hex digit.
CPU
Central Processing Unit: the processor that runs instructions.
Register
Tiny storage inside the CPU, like the PC or ACC.
Bus
A set of wires that carries addresses, data or control signals between parts of the computer.
RAM
Random Access Memory: fast, volatile working memory.
Cache
Small, very fast memory on the CPU chip.
Clock speed
How many cycles a CPU does per second, in GHz.
Embedded system
A small computer built into a device for one job.

The explanation

Read it here, or open the study page below to go one step at a time with things to try.

Converting between units

★Remember

Going to a bigger unit: divide. Going to a smaller unit: multiply.

?Why?

Why? Think of money. 3 euros = 300 cents. Cents are smaller, so you need more of them. More = bigger number = multiply.

e.g.Example

3 MB → bytes: 3 × 1000 = 3,000 KB. Then 3,000 × 1000 = 3,000,000 bytes. Then × 8 = 24,000,000 bits.

RAM and ROM

RAMROM
HoldsPrograms and data in use nowStart-up instructions
Power offVolatile: everything lostNon-volatile: kept
Changeable?Yes, all the timeNormally no
Size8–32 GB in a typical PCTiny
?Why?

Why is ROM needed? When you switch on, RAM is empty. Something must already be there to start the computer. That lives in ROM.

Virtual memory: when RAM is full, the computer borrows part of the storage drive. It works, but it's slow.

Storage

TypeHow a bit is storedExamplesGoodBad
MagneticTiny magnetised areas on a spinning diskHard disk (HDD)Cheap, hugeMoving parts, slower
Solid stateCharge trapped in flash memorySSD, USB stick, phonesFast, silent, toughCosts more per GB
OpticalPits read by a laserCD, DVD, Blu-rayCheap to postSmall, slow, scratches
★Remember

Choose by: capacity, speed, portability, durability, reliability, cost.

Study it

Open the study page and go one step at a time.

Topic 8 · Year 8

Inside the computer

The CPU does the work, RAM holds what it's using now, and storage keeps everything long-term. The CPU repeats one loop: fetch, decode, execute.

Topic 2 · Year 7

Bits and bytes

A bit is one switch: 0 or 1. A byte is 8 bits. Every file is measured by counting these.

The steps for this lesson

Check yourself

One question at a time. The reason shows after each answer.

Saved on this device.