Year 10 · Computer Science · Spring 1
Hardware · Lesson 4 of 5
Storage: primary, secondary, off-line; virtual memory; cloud
Theory3.3 Data storage
Year 10 · Computer Science · Hardware · Lesson 4 of 5
You will learn to
- Describe RAM and ROM, and explain why a computer needs both.
- Describe how magnetic, optical and solid-state storage store data.
- Explain what virtual memory is and why it is needed.
- Compare cloud storage with local storage, giving advantages and disadvantages.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Success criteria
You have got it when you can say:
- I can say which storage uses pits and lands, and which uses magnetised areas.
- I can choose storage for a laptop, a film archive and a camera, with a reason.
- I can explain what happens when RAM is full and virtual memory is used.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last lesson
A MAC address is…
A Given out by the router each timeB The name of a websiteC The fixed hardware address of a network card
C. The fixed hardware address of a network card
- Last unit
Parity can…
A Detect two flipped bitsB Detect and fix any errorC Detect one flipped bit but not fix it
C. Detect one flipped bit but not fix it
- Earlier this year
In 8-bit two's complement, 11111111 is…
A 255B −1C −127
B. −1
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Key words (1 of 2)
- CPU
- The central processing unit, which processes instructions and data by fetching, decoding and executing them.
- Control unit
- Manages the fetch–decode–execute cycle and sends control signals.
- Instruction set
- The list of all the instructions a particular CPU can carry out.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Key words (2 of 2)
- Sensor
- An input device that measures a physical property and sends the reading to a computer.
- Volatile
- Loses its contents when the power is switched off.
- Virtual memory
- Part of secondary storage used as extra RAM, with pages moved between them.
- IP address
- An address given to a device on a network so data can be sent to it.
The explanation · step 1 of 7 · part 1 of 3
Primary storage: RAM and ROM
Primary storage is memory the CPU can access directly. It includes RAM and ROM.
RAM (random access memory) holds the data and programs currently in use, including parts of the operating system. It can be read and written. It is volatile: everything is lost when the power goes off.
The explanation · step 1 of 7 · part 2 of 3
Primary storage: RAM and ROM (continued)
ROM (read-only memory) holds the start-up instructions (the bootloader firmware) that run when you switch on. It can only be read. It is non-volatile: it keeps its contents without power.
| RAM | ROM | |
|---|---|---|
| Volatile? | Yes: loses data without power | No: keeps data without power |
| Read / write? | Read and write | Read only |
| What it holds | Programs and data in use now | Start-up instructions (bootloader) |
| Size in a laptop | Large, e.g. 16 GiB | Small, e.g. a few MiB |
The explanation · step 1 of 7 · part 3 of 3
Primary storage: RAM and ROM (continued)
Both are needed. RAM is fast and changeable, so programs can run and data can change. But RAM is empty at switch-on, so ROM must hold the instructions that start the computer and load the operating system.
The explanation · step 2 of 7
Secondary storage
Secondary storage is non-volatile storage that the CPU does not access directly. Data is copied into RAM first.
It is needed to keep files, programs and the operating system when the power is off. It also holds far more than RAM, for less money per gigabyte.
There are three types: magnetic, optical and solid-state.
The explanation · step 3 of 7 · part 1 of 2
Magnetic storage: the hard disk drive
A hard disk drive (HDD) has metal or glass platters coated with a magnetic material. They spin very fast, for example 7,200 times a minute.
A read/write head on a moving arm floats a tiny distance above each surface. To write, it magnetises tiny areas in one direction for 1 and the other direction for 0. To read, it senses the direction.
The explanation · step 3 of 7 · part 2 of 2
Magnetic storage: the hard disk drive (continued)
Each platter is divided into tracks (circles) and sectors (slices of each track). Data is found by moving the head to the right track, then waiting for the right sector to spin past.
| Advantages | Disadvantages |
|---|---|
| Large capacity, cheap per gigabyte | Moving parts can break if dropped |
| Can be rewritten many times without wearing out | Slower to access data than solid-state (head must move, disk must spin) |
| Good for servers and backup | Heavier, noisier and uses more power |
The explanation · step 4 of 7 · part 1 of 3
Optical storage: CD, DVD and Blu-ray
An optical disc has a single spiral track with tiny dips called pits and flat areas called lands.
A laser shines on the spinning disc. Lands reflect the light back strongly; pits scatter it. A sensor reads these changes as 1s and 0s.
To write a recordable disc, a stronger laser changes small areas of a dye layer, so they reflect differently.
The explanation · step 4 of 7 · part 2 of 3
Optical storage: CD, DVD and Blu-ray (continued)
| Disc | Laser | Typical capacity (one layer) |
|---|---|---|
| CD | Red | 700 MB |
| DVD | Red | 4.7 GB |
| Blu-ray | Blue (shorter wavelength) | 25 GB |
Blue light has a shorter wavelength, so it can focus on smaller pits. Smaller pits packed closer together means more data on the same size disc.
The explanation · step 4 of 7 · part 3 of 3
Optical storage: CD, DVD and Blu-ray (continued)
Optical discs are cheap and portable, but slow, easily scratched and small compared with modern drives. They are used for films, games and software distribution.
The explanation · step 5 of 7 · part 1 of 2
Solid-state (flash) storage
Solid-state storage has no moving parts. It stores data in flash memory chips using transistors.
Each cell has a control gate and a floating gate. Electric charge (electrons) can be trapped in the floating gate. Charge trapped, or not, stands for a 0 or a 1, and stays without power.
Flash memory is built using NAND or NOR technology. NAND is denser and cheaper, so it is used in SSDs, SD cards and USB flash drives.
The explanation · step 5 of 7 · part 2 of 2
Solid-state (flash) storage (continued)
| Advantages | Disadvantages |
|---|---|
| Very fast access: no head to move | More expensive per gigabyte than HDD |
| No moving parts: survives knocks and drops | Each cell can only be written a limited number of times |
| Light, small, silent, low power: good for phones and laptops | Usually smaller capacity for the same price |
The explanation · 2 more steps
More on the lesson page
The lesson page has the rest of the explanation, step by step:
- Virtual memory
- Cloud storage
Year 10 · Computer Science · Hardware · Lesson 4 of 5
The maths: Changing units
Symbols first, then the words, then the equation. Change the numbers.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Quick quiz · 1 of 3
Which bus carries the actual data?
- A Data bus
- B Address bus
- C Control bus
A. Data bus
Address bus: where. Data bus: what. Control bus: when and which way.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Quick quiz · 2 of 3
The ALU does…
- A Fetching instructions
- B Storing files
- C Arithmetic and logic
C. Arithmetic and logic
Add, subtract, compare. The control unit does the fetching and decoding.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Quick quiz · 3 of 3
Cache is…
- A Small, fast memory beside the CPU for recently used data
- B The program counter
- C The hard disk
A. Small, fast memory beside the CPU for recently used data
Bigger cache means fewer slow trips to RAM.
Year 10 · Computer Science · Hardware · Lesson 4 of 5
Exit ticket
Answer on paper before you go. Then check.
Which memory is volatile?
A RAMB An SSDC ROM
A. RAM Volatile means wiped when the power goes. RAM is; ROM and SSDs are not.
A solid-state drive…
A Uses a laserB Uses spinning plattersC Has no moving parts
C. Has no moving parts Lasers are optical (DVD). Platters are magnetic (hard disk).