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Edexcel GCSE Computer Science (1CP2) Exam Questions

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4 questions · 12 marks

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1Explain2 marksImages Swap Remove

A bitmap image uses a colour depth of 8 bits. Each pixel is stored as a binary value.

Explain how the colour of a pixel is represented in a bitmap image.

[2 marks]
Mark scheme

Award up to two marks for a linked explanation such as:

  • Each pixel is stored as a binary value/code (1) which represents/maps to a colour (1)
  • The binary value is used to look up a colour in a palette (1) so the pixel can be displayed in that colour (1)
  • A bit pattern is assigned to each colour (1) and each pixel stores the bit pattern for its colour (1)
  • The number of bits per pixel sets how many different colour codes exist (1) so each code can represent a different colour (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'pixels are binary' or 'the image is made of 1s and 0s' without saying a pixel's colour is represented by a binary value.

Do not accept RGB values stored as denary or hexadecimal.

Example answers and how they were marked
Full marks2/2

Each pixel stores a binary value (1); this value is used to look up the pixel's colour in a palette (1).

Why: Linked point (1) and reason (1): a binary value is stored for each pixel and maps to a colour.

Some marks1/2

A bit pattern is assigned to each colour.

Why: One mark for stating that colours have bit patterns; it does not say that each pixel stores the pattern for its colour.

No marks0/2

Each pixel stores the RGB colour as denary numbers between 0 and 255.

Why: Bitmap pixels are stored in binary, not as denary values; this common misconception earns no mark.

2Explain2 marksImages Swap Remove

An image uses a colour depth of 2 bits per pixel. The original image contains 12 different colours.

Explain why the image cannot be represented accurately at this colour depth.

[2 marks]
Mark scheme

Award up to two marks for a linked explanation such as:

  • Only 4 colours can be represented (1) because 2 bits give 2^2 = 4 possible values (1)
  • Some of the 12 colours cannot be stored (1) so they are lost/replaced with a colour that can be represented (1)
  • The image will not match the original exactly (1) because there are fewer colour codes than the number of colours needed (1)
  • Many colours will share the same binary value (1) so the image will lose colour detail (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'four colours' or '2^2'.

Do not accept 'only two colours' (confusing the number of bits with the number of values) or 'resolution is too low'.

Example answers and how they were marked
Full marks2/2

Only four colours can be represented (1) because two bits give four possible binary values (1).

Why: Linked point (1) and reason (1): 2 bits give 2^2 = 4 colour codes.

Some marks1/2

Only four colours can be represented.

Why: One mark for identifying that four colours can be represented; the linked reason about two bits giving four values is missing.

No marks0/2

Only two colours can be represented because each pixel has two bits.

Why: A common misconception: two bits give four possible values, not two, so no creditworthy point is given.

3Explain2 marksSound Swap Remove

Each sample of a sound wave is stored as a binary number.

Explain how increasing the bit depth affects the representation of a sound wave.

[2 marks]
Mark scheme

Award up to two marks for a linked explanation such as:

  • Each sample is stored using more bits (1) so the amplitude can be recorded more precisely (1)
  • More different amplitude values can be represented (1) because each extra bit doubles the number of possible values (1)
  • The height of the wave at each sample point is measured more accurately (1) so the digital wave is closer to the analogue wave (1)
  • The gap between the true amplitude and the stored value is smaller (1) because there are more values for the sample to be rounded to (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'the sound is sampled more often' or 'more samples are taken per second', as these describe the sample rate, not the bit depth.

Do not accept 'the file size increases' on its own as an effect on the representation of the sound.

Example answers and how they were marked
Full marks2/2

Each sample is stored using more bits, so the amplitude at each sample point can be recorded more precisely.

Why: Point (more bits per sample) (1) with linked reason (amplitude recorded more precisely) (1).

Some marks1/2

Each sample is stored using more bits.

Why: Creditworthy point (1) but no linked effect on the accuracy of the representation, so only one mark.

No marks0/2

It takes more samples every second, so the wave is stored in more detail.

Why: Confuses bit depth with sample rate, so no mark is awarded.

4Discuss6 marksCharacter Sets Swap Remove

A library is digitising its catalogue. The titles of books and the names of authors are stored as text.

Discuss how 7-bit ASCII is used to encode the text in the library catalogue.

Your answer should include:

  • how characters are mapped to codes
  • why 7 bits are used for each character
  • the benefits of using ASCII for the catalogue.
[6 marks]
Mark scheme

Indicative content

How characters are mapped to codes

  • Each character in a book title or author name is given a unique 7-bit binary code from the ASCII character set.
  • The letter A, for example, is stored as the code 1000001 and the letter B as 1000010, so the catalogue can store letters as binary.
  • Uppercase letters, lowercase letters, digits, punctuation and the space character each have their own ASCII code.
  • The same character always uses the same code, so the text can be stored and read back correctly by the library system.

Why 7 bits are used for each character

  • Seven bits give 2 to the power of 7, which is 128 different bit patterns, and this is enough to represent the letters, digits and punctuation needed in the catalogue.
  • A standard keyboard has around 98 characters, so 6 bits would give only 64 codes and would not be enough.
  • Using a fixed 7 bits per character means every character takes the same amount of storage, which makes the catalogue file simple to process.

Benefits of using ASCII for the catalogue

  • ASCII is a standard character set, so the catalogue data can be exchanged between different computers and software.
  • Author names and titles entered on one system will display correctly on another system that uses ASCII.
  • ASCII uses only 7 bits per character, so the catalogue takes less storage than a character set that uses more bits.
  • It is simple to encode and decode because each character has a single fixed code.

Levels

  • Level 0 (0 marks): No rewardable content.
  • Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
  • Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
  • Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Do not accept 8 bits or 256 characters as features of 7-bit ASCII; these are extended ASCII.

Example answers and how they were marked
Full marks6/6

Each character in a catalogue title or author name is given its own 7-bit ASCII code. For example, the letter A is 1000001 and the letter B is 1000010, so the library can store the text as binary. Seven bits give 128 different codes, which is enough for uppercase and lowercase letters, digits, punctuation and spaces. Six bits would give only 64 codes, which would not be enough for all the characters in the catalogue. ASCII is also a standard character set, so the catalogue data can be exchanged between different computers and will display correctly. Because each character uses only 7 bits, the catalogue also takes less storage than it would with a character set that uses more bits per character.

Why: Covers all three bullets with sustained application: mapping and example codes; 7 bits give 128 codes and why 6 bits is insufficient; standardisation and storage benefits. Level 3.

Some marks4/6

ASCII gives each character a unique code. For example, A is stored as 1000001 and B is stored as 1000010. Seven bits are used for each character because seven bits give 128 codes, which is enough for the letters and punctuation in the catalogue. The library can store titles and author names as binary.

Why: Covers mapping and the 7-bit reason with some application, but does not discuss the benefits of ASCII for the catalogue. Level 2.

Some marks1/6

ASCII is a programming language that gives each letter a binary code. It uses 8 bits for each character, so it can store 256 characters. This means it can store every book title in the library.

Why: A basic point that letters are given binary codes is credited, but the answer treats ASCII as a programming language and uses 8 bits/256 characters, which are misconceptions. Level 1.

Questions by topic

2.1 Binary

Build a paper on this topic 58
Adding Binary Numbers 6 questions Binary and Hexadecimal Basics 10 questions Binary Shifts 6 questions Binary to Denary Conversion 6 questions Binary to Hexadecimal Conversion 6 questions Denary to Binary Conversion 6 questions Denary to Hexadecimal Conversion 6 questions Denary to Negative Binary Conversion 6 questions Hexadecimal to Binary Conversion 6 questions

2.2 Data representation

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Character Sets 18 questions Images 19 questions Sound 19 questions

2.3 Data storage and compression

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Compression 18 questions Units & Conversion 6 questions

3.1 Hardware

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Embedded Systems 17 questions Fetch Decode Execute Cycle 15 questions Primary Memory 15 questions Secondary Memory 15 questions Stored Program Computers 15 questions

3.2 Software

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File Management 18 questions Memory Management 16 questions Peripheral Management 11 questions Process Management 12 questions Purpose & Function of Operating Systems 17 questions Software Basics 13 questions User Management 18 questions Utility Software 19 questions

3.3 Programming languages

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Low and High Level Languages 19 questions Translators 18 questions

4.1 Networks

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Addressing 16 questions LANs & WANs 16 questions Network Speed 5 questions Packet Switching 16 questions Protocols & Layers 17 questions The Internet 16 questions Topologies 17 questions Wired Connections 17 questions Wireless Connections 18 questions

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