Year 11 · Computer Science · Spring 2 – Summer 1
Final revision and the exams · Lesson 1 of 5
Red and amber points, pupil by pupil
Exam practice1.3 Data storage and compression3.1 Computer architecture7.7 Trace tables
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
You will learn to
- Identify your red and amber syllabus points from your tracker.
- Use the revision cycle on one point: study page, key terms, quiz, then a past-paper question.
- Evaluate your progress: change a point's colour only after a full-mark answer.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Success criteria
You have got it when you can say:
- I can list my three reddest points before the lesson starts.
- I can write the key terms for a point from memory, then check them.
- I can show a marked past-paper question for each point I worked on.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last unit
A local variable…
A Can be used anywhereB Never changesC Exists only inside its subroutine
C. Exists only inside its subroutine
- Last unit
XOR gives 1 when…
A Exactly one input is 1B Both inputs are 1C Neither input is 1
A. Exactly one input is 1
- Earlier this year
Which of these is system software?
A A word processorB The operating systemC A game
B. The operating system
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (1 of 9)
- Bit
- A single binary digit, 0 or 1.
- Nibble
- A group of 4 bits, which matches one hexadecimal digit.
- Byte
- A group of 8 bits.
- Binary
- The base 2 number system computers use, with digits 0 and 1.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (2 of 9)
- Character set
- A list of characters a computer can use, each with its own binary code.
- Sample rate
- The number of sound samples taken each second, measured in hertz (Hz).
- Sample resolution
- The number of bits used to store each sound sample.
- Pixel
- The smallest single-coloured square in a bitmap image.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (3 of 9)
- Image resolution
- The number of pixels in an image, often written as width × height.
- Colour depth
- The number of bits used to store the colour of each pixel.
- Lossy compression
- Making a file smaller by permanently removing some data.
- Lossless compression
- Making a file smaller without losing any data, so the original can be rebuilt exactly.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (4 of 9)
- Run-length encoding (RLE)
- A lossless method that stores each run of repeated values as a count and the value.
- CPU
- The central processing unit, which processes instructions and data by fetching, decoding and executing them.
- Microprocessor
- A CPU made as an integrated circuit on a single chip.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (5 of 9)
- Von Neumann architecture
- A computer design where instructions and data are stored together in the same memory.
- ALU
- Arithmetic logic unit: does calculations and logical comparisons.
- Control unit
- Manages the fetch–decode–execute cycle and sends control signals.
- Register
- A tiny, very fast store inside the CPU that holds one item.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (6 of 9)
- Bus
- A set of parallel wires that carries addresses, data or control signals.
- Cache
- Small, very fast memory in or near the CPU that holds frequently used data and instructions.
- Clock speed
- The number of cycles the CPU does each second, measured in hertz.
- Core
- A complete processing unit; a multi-core CPU can process several instructions at once.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (7 of 9)
- Instruction set
- The list of all the instructions a particular CPU can carry out.
- Embedded system
- A computer system inside a larger device that does one dedicated job.
- Sensor
- An input device that measures a physical property and sends the reading to a computer.
- NIC
- Network interface card: the hardware that connects a device to a network.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (8 of 9)
- MAC address
- A unique 48-bit address given to a NIC at manufacture, written in hex.
- IP address
- An address given to a device on a network so data can be sent to it.
- Algorithm
- A precise, ordered set of steps that solves a problem.
- Program development life cycle
- The four stages of making a program: analysis, design, coding and testing.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Key words (9 of 9)
- Pseudocode
- A structured, code-like way of writing an algorithm that is not tied to one language.
- Counting
- Adding 1 to a counter each time something happens.
- Check digit
- An extra digit calculated from the other digits, used to catch typing errors.
- Trace table
- A table used in a dry run to record the value of each variable as an algorithm runs.
The explanation · step 1 of 21
Start from your tracker
Your tracker lists every syllabus point with a colour. Work on red points first, then amber.
Green points only need a short quiz now and then.
Why not revise what I already know?
It feels good, but it gains few marks. Marks come from closing gaps.
The explanation · step 2 of 21 · part 1 of 3
The revision cycle
- Read the study page for one point.
- Close it. Write the key terms and their meanings from memory.
- Open it again. Check and correct your terms in another colour.
- Do the quiz for that point.
- Do one past-paper question on it, timed.
- Mark it with the mark scheme.
The explanation · step 2 of 21 · part 2 of 3
The revision cycle (continued)
- Change the colour on your tracker only if you got full marks.
Red point 1.3, file sizes. Read the step. Write the image and sound formulas from memory. Do two calculations, then one past-paper question.
Testing yourself works better than reading again. Rereading feels easy because the page is in front of you.
The explanation · step 2 of 21 · part 3 of 3
The revision cycle (continued)
Copying out notes or making a neat poster is not revision. If you are not recalling, you are not revising.
1.3 Data storage and compression · The explanation · step 3 of 21 · part 1 of 3
Units of data
A bit is one 0 or 1. A nibble is 4 bits. A byte is 8 bits.
After the byte, each unit is 1024 times bigger than the one before.
1024 is used because it is a power of 2: 2¹⁰ = 1024. Computers count in powers of 2.
Try the toy: type 1 and choose GiB. Read the MiB and KiB rows in the table. How many times did you multiply by 1024 to reach KiB?
1.3 Data storage and compression · The explanation · step 3 of 21 · part 2 of 3
Units of data (continued)
| Unit | Equal to | In bytes |
|---|---|---|
| 1 nibble | 4 bits | half a byte |
| 1 byte (B) | 8 bits | 1 |
| 1 kibibyte (KiB) | 1024 bytes | 1,024 |
| 1 mebibyte (MiB) | 1024 KiB | 1,048,576 |
| 1 gibibyte (GiB) | 1024 MiB | 1,073,741,824 |
| 1 tebibyte (TiB) | 1024 GiB | about 1.1 million million |
| 1 pebibyte (PiB) | 1024 TiB | 2⁵⁰ |
| 1 exbibyte (EiB) | 1024 PiB | 2⁶⁰ |
1.3 Data storage and compression · The explanation · step 3 of 21 · part 3 of 3
Units of data (continued)
Cambridge uses 1024, not 1000. Shops often say KB, MB and GB meaning 1000. In the exam, always use 1024 and the KiB, MiB, GiB names.
There is something to try for this step in the IGCSE library.
1.3 Data storage and compression · The explanation · step 4 of 21 · part 1 of 2
Converting between units
Going to a smaller unit: multiply (× 1024, or × 8 from bytes to bits).
Going to a bigger unit: divide (÷ 1024, or ÷ 8 from bits to bytes).
Move one unit at a time and write each step. It is slower but it avoids mistakes.
1.3 Data storage and compression · The explanation · step 4 of 21 · part 2 of 2
Converting between units (continued)
Paper 1 has no calculator. Exam numbers are usually chosen so that dividing by 8 and 1024 comes out neatly.
1.3 Data storage and compression · The explanation · step 4 of 21
The maths: Changing units
1.3 Data storage and compression · The explanation · step 7 of 21 · part 1 of 2
Why we compress files
Compression makes a file smaller, so it uses fewer bits.
Smaller files use less storage space on a device or server.
Smaller files need less bandwidth. Bandwidth is how much data a connection can carry each second.
So smaller files are sent faster: quicker downloads, quicker uploads, and less buffering when streaming.
Many services set a size limit. An email attachment limit of 25 MB is a real example.
1.3 Data storage and compression · The explanation · step 7 of 21 · part 2 of 2
Why we compress files (continued)
Storage and network time both cost money and battery. Sending fewer bits saves both.
The explanation · 16 more steps
More on the lesson page
The lesson page has the rest of the explanation, step by step:
- Calculating image file size
- Calculating sound file size
- Lossy compression
- Lossless compression
- Run-length encoding (RLE)
- RLE: working out the saving
- The CPU and the microprocessor
- Von Neumann architecture
- The five registers
- The three buses
- The fetch–decode–execute cycle
- The instruction set
- Cores, cache and clock speed
- Embedded systems
- What a trace table is
- Worked dry run
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Quick quiz · 1 of 3
How many bits are in a byte?
- A 8
- B 4
- C 16
A. 8
A byte is 8 bits. 4 bits is a nibble.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Quick quiz · 2 of 3
Hex B2 in binary is…
- A 1100 0010
- B 1011 0100
- C 1011 0010
C. 1011 0010
B = 11 = 1011 and 2 = 0010. One hex digit is 4 bits.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Quick quiz · 3 of 3
Adding 11001100 and 01011010 in an 8-bit register gives…
- A An overflow error
- B 11111111
- C 00100110 with no problem
A. An overflow error
The true answer needs 9 bits (100100110). The ninth bit does not fit.
Year 11 · Computer Science · Final revision and the exams · Lesson 1 of 5
Exit ticket
Answer on paper before you go. Then check.
Shifting 00001010 (10) one place left gives…
A 00010100 (20)B 00000101 (5)C 00001011 (11)
A. 00010100 (20) A left shift doubles. 10 becomes 20.
In 8-bit two's complement, 11111111 is…
A 255B −1C −127
B. −1 The left bit is worth −128. −128 + 127 = −1.