Year 11 · Computer Science · Autumn 2 – mid-December

Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Paper 1 practice: topics 1 to 3

Exam practice1.1 Number systems2.2 Methods of error detection3.1 Computer architecture

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

You will learn to

  • Answer a timed Paper 1 section on topics 1 to 3, without a calculator.
  • Use the marks in brackets to decide how many separate points to write.
  • Use the mark scheme to mark your answers and find the syllabus words that earn marks.
  • Rate each syllabus point red, amber or green on your tracker.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Success criteria

You have got it when you can say:

  • I can finish the section in the time given.
  • I can write as many separate, correct points as there are marks.
  • I can mark my own paper with the scheme and update my tracker.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Retrieval starter

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

  1. Last lesson

    Which of these is lossy?

    A JPEGB PNGC ZIP

    A. JPEG

  2. Last unit

    Pharming is…

    A A fake email asking for a passwordB Being sent to a fake website even when you type the right addressC Guessing every password

    B. Being sent to a fake website even when you type the right address

  3. Earlier this year

    A compiler…

    A Translates the whole program once into machine codeB Translates one line at a time, every runC Only checks spelling

    A. Translates the whole program once into machine code

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (1 of 10)

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.
Denary
The base 10 number system we use every day, with digits 0 to 9.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (2 of 10)

Binary
The base 2 number system computers use, with digits 0 and 1.
Hexadecimal
The base 16 number system, with digits 0 to 9 and A to F, used to write binary in a short form.
Overflow
An error when a result is too big to fit in the number of bits available.
Logical shift
Moving all the bits left or right, losing bits off one end and filling the other end with 0s.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (3 of 10)

Two's complement
A way to store negative numbers, where the leftmost bit has a negative place value (−128 in 8 bits).
Colour depth
The number of bits used to store the colour of each pixel.
Packet
A small piece of data sent across a network, with a header, payload and trailer.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (4 of 10)

Packet switching
Sending each packet separately, possibly by different routes, then reordering them at the destination.
Interference
Unwanted outside signals that can corrupt data on the way.
Parity bit
An extra bit that makes the number of 1s in a byte odd or even.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (5 of 10)

Checksum
A value calculated from a block of data, recalculated by the receiver to check for errors.
Echo check
The receiver sends the data back so the sender can compare it with the original.
Check digit
A digit calculated from the other digits of a number, used to catch typing errors.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (6 of 10)

ARQ
Automatic repeat query: acknowledgements and timeouts make the sender re-send data that is wrong or lost.
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 · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (7 of 10)

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 · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (8 of 10)

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 · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (9 of 10)

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 · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Key words (10 of 10)

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.

The explanation · step 1 of 33

What Paper 1 looks like

Computer Science has two written papers. Both count the same.

PaperWhat it testsTimeMarksShare of the grade
Paper 1 Computer SystemsTopics 1 to 61 hour 45 minutes7550%
Paper 2 Algorithms, Programming and LogicTopics 7 to 101 hour 45 minutes7550%

Each question shows its marks in brackets, like [3].

!Watch out

You cannot use a calculator in either paper. Show every step of a calculation, with units.

The explanation · step 2 of 33 · part 1 of 2

One mark, one point

A 3-mark question needs three separate, correct points. One long sentence often earns only one mark.

  • Write each point on a new line, or as a short separate sentence.
  • Use the syllabus words. The mark scheme is written in them.
  • Do not repeat the question back. It earns nothing.
  • Vague words like 'faster' or 'safer' need a reason: faster than what, and why?

The explanation · step 2 of 33 · part 2 of 2

One mark, one point (continued)

e.g.Example

Describe how a firewall protects a network. [3] • It monitors the traffic coming in and going out. • It checks the traffic against a set of rules. • It blocks any traffic that does not meet the rules.

★Remember

Look at the marks before you write. They tell you how many points the examiner wants.

The explanation · step 3 of 33 · part 1 of 2

Mark it, then rate it

  1. Mark your answers with the mark scheme, in a different colour pen.
  2. Tick a point only if it matches a mark-scheme point.
  3. For each lost mark, write the missing point in full under your answer.
  4. Colour each syllabus point on your tracker: red, amber or green.

The explanation · step 3 of 33 · part 2 of 2

Mark it, then rate it (continued)

ColourIt meansWhat to do next
RedI lost most of the marks, or I do not understand it yet.Study page, key terms, quiz, then one past-paper question.
AmberI got some marks, but there are gaps.Past-paper questions on that point, marked with the scheme.
GreenI got full marks.A short quiz every few weeks so you keep it.
?Why?

Why write out the missing point?

You learn the exact words the examiner wanted. Next time you can use them.

1.1 Number systems · The explanation · step 4 of 33 · part 1 of 2

Why computers use binary

A computer is made of billions of tiny switches called transistors. Each switch is either off or on.

We write off as 0 and on as 1. A number system with only two digits is called binary.

One binary digit is a bit. Eight bits together make a byte.

Every extra bit doubles the number of different patterns you can make.

1.1 Number systems · The explanation · step 4 of 33 · part 2 of 2

Why computers use binary (continued)

e.g.Example
1 bit: 0, 1 → 2 patterns 2 bits: 00, 01, 10, 11 → 4 patterns 3 bits: 000, 001, 010, 011, 100, 101, 110, 111 → 8 patterns 8 bits: 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 = 256 patterns
?Why?

Two states are very reliable. A switch is clearly on or clearly off, so the computer rarely misreads a value.

1.1 Number systems · The explanation · step 4 of 33

The maths: How many patterns can n bits make?

1.1 Number systems · The explanation · step 5 of 33 · part 1 of 3

Place value: denary and binary

Denary is our everyday number system. It has ten digits, 0 to 9, and each column is worth 10 times the column to its right.

Binary works the same way, but each column is worth 2 times the column to its right.

For 8 bits the column values, from left to right, are 128, 64, 32, 16, 8, 4, 2, 1.

To turn binary into denary, add up the column values that have a 1 under them.

1.1 Number systems · The explanation · step 5 of 33 · part 2 of 3

Place value: denary and binary (continued)

Try the toy: switch on the 128 bit. What happens to the denary value? Now make 255.

1286432168421
01011010
e.g.Example
Binary 01011010 to denary. The 1s sit under 64, 16, 8 and 2. 64 + 16 + 8 + 2 = 90

1.1 Number systems · The explanation · step 5 of 33 · part 3 of 3

Place value: denary and binary (continued)

!Watch out

Always write the column values above the bits first. Most mistakes come from adding the wrong column.

There is something to try for this step in the IGCSE library.

2.2 Methods of error detection · The explanation · step 23 of 33

Echo check

In an echo check, the receiver sends a copy of the data straight back to the sender.

The sender compares the copy with the original. If they are the same, the data arrived correctly. If not, the sender sends it again.

!Watch out

An echo check doubles the traffic. Also, if the copies differ, you cannot tell if the error happened on the way there or on the way back.

The explanation · 27 more steps

More on the lesson page

The lesson page has the rest of the explanation, step by step:

  • Denary to binary
  • The biggest number n bits can hold
  • Hexadecimal: base 16
  • Hex to denary and denary to hex
  • Binary and hex using nibbles
  • Why hexadecimal is used
  • Adding binary: the four rules
  • A full 8-bit addition
  • Overflow
  • Logical left shift
  • Logical right shift
  • Two's complement: negative numbers
  • Denary to two's complement
  • Why errors happen
  • Parity check: odd and even
  • Parity byte and parity block
  • Checksum
  • Check digits
  • Automatic repeat query (ARQ)
  • 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

Open the lesson page

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Quick quiz · 1 of 3

A walkie-talkie, where only one side can talk at a time, is…

  1. A Full-duplex
  2. B Half-duplex
  3. C Simplex

B. Half-duplex

Both ways, but not at once. Full-duplex is both at once, like a phone.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Quick quiz · 2 of 3

With even parity, the 7 data bits 0110100 need the parity bit…

  1. A It cannot be worked out
  2. B 0
  3. C 1

C. 1

0110100 has three 1s, which is odd. A 1 makes the count even.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Quick quiz · 3 of 3

Parity can…

  1. A Detect and fix any error
  2. B Detect one flipped bit but not fix it
  3. C Detect two flipped bits

B. Detect one flipped bit but not fix it

Two flips keep the count even, so they slip through, and parity never says which bit flipped.

Year 11 · Computer Science · Automated and emerging technologies; Paper 1 practice · Lesson 4 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. In asymmetric encryption the message is locked with the…

    A Private keyB Same key that unlocks itC Public key

    C. Public key Anyone can lock with the public key. Only the private key unlocks.

  2. ARQ stands for…

    A Address resolution questionB Average rate queryC Automatic repeat request

    C. Automatic repeat request The receiver acknowledges; if no acknowledgement arrives before the timeout, the sender resends.