Year 8 · Autumn 1

Computer Systems · Lesson 3 of 5

RAM, ROM and secondary storage

TheoryTopic 2 Bits and bytesTopic 8 Inside the computer

Year 8 · Computer Systems · Lesson 3 of 5

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.

Year 8 · Computer Systems · Lesson 3 of 5

Success criteria

You have got it when you can say:

  • 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.

Year 8 · Computer Systems · Lesson 3 of 5

Retrieval starter

Questions from before. Write your answers, then show them.

  1. Last lesson

    During fetch, what goes into the MDR?

    A The result of the calculationB The instruction stored at that addressC The address of the instruction

    B. The instruction stored at that address

  2. Earlier this unit

    Two transistors in a line (series) light a lamp only when…

    A Both gates are onB Neither gate is onC Either gate is on

    A. Both gates are on

  3. Last year

    In def area(w, h): … and a = area(4, 3), which are the arguments?

    A area and aB 4 and 3C w and h

    B. 4 and 3

Year 8 · Computer Systems · Lesson 3 of 5

Key words (1 of 4)

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.

Year 8 · Computer Systems · Lesson 3 of 5

Key words (2 of 4)

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.

Year 8 · Computer Systems · Lesson 3 of 5

Key words (3 of 4)

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.

Year 8 · Computer Systems · Lesson 3 of 5

Key words (4 of 4)

Embedded system
A small computer built into a device for one job.

The explanation · step 1 of 3 · part 1 of 2

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.

The explanation · step 1 of 3 · part 2 of 2

Converting between units (continued)

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.

The explanation · step 1 of 3

The maths: Changing units

The explanation · step 2 of 3 · part 1 of 2

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.

The explanation · step 2 of 3 · part 2 of 2

RAM and ROM (continued)

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

The explanation · step 3 of 3

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.

Year 8 · Computer Systems · Lesson 3 of 5

Quick quiz · 1 of 3

A 100 Mbps connection downloads at most…

  1. A 12.5 MB per second
  2. B 100 MB per second
  3. C 800 MB per second

A. 12.5 MB per second

Speeds are in bits. Divide by 8 to get bytes.

Year 8 · Computer Systems · Lesson 3 of 5

Quick quiz · 2 of 3

5 KB into bytes (kilo = 1000). You should…

  1. A Multiply by 1000
  2. B Multiply by 8
  3. C Divide by 1000

A. Multiply by 1000

Bytes are smaller, so you need more of them: multiply. 5 KB = 5,000 bytes.

Year 8 · Computer Systems · Lesson 3 of 5

Quick quiz · 3 of 3

Why is a byte 8 bits?

  1. A Physics says so
  2. B 8 is the biggest number a switch can hold
  3. C It was chosen: it fits one character and splits into two 4-bit halves

C. It was chosen: it fits one character and splits into two 4-bit halves

People chose 8. It fits a character, is a power of 2, splits into nibbles, and IBM made it standard.

Year 8 · Computer Systems · Lesson 3 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. A '1 TB' drive shows 931 GB in Windows because…

    A Windows divides by 1,024 but still writes GBB The drive is fakeC Some space is broken

    A. Windows divides by 1,024 but still writes GB Same number of bytes, measured with a different ruler.

  2. 5 KB into bytes. You should…

    A Divide by 1000B Multiply by 1000

    B. Multiply by 1000 Bytes are smaller, so you need more of them: multiply. 5 KB = 5,000 bytes.