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Edexcel GCSE Computer Science (1CP2) Exam Questions

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3 questions · 9 marks

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1Short recall1 markImages Swap Remove

A bitmap image uses one bit per pixel.

State the limitation this places on the number of colours in the image.

[1 mark]
Mark scheme

Award one mark for any of the following:

  • Only two different colours can be represented/stored (1)
  • Each pixel can only be one of two values (1)
  • The image can only be shown in black and white (1)

Allow answers that give the number of colours, e.g. two colours, 2 colours

Allow any pair of two colours, e.g. black and white, black and grey

Do not accept low resolution/poor quality/lower quality

Do not accept only one colour/can only show black

Do not accept 2 bits/2^1 without a statement about the number of colours that can be represented

Example answers and how they were marked
Full marks1/1

Only two colours can be represented, such as black and white.

Why: States that only two different colours can be represented (1)

No marks0/1

The image will have a low resolution.

Why: Confuses colour depth with resolution; low resolution is not accepted

No marks0/1

The image can only show black.

Why: One colour is all a single value can give; the limitation is that only two colours can be represented, so this is not accepted

2Explain2 marksCharacter Sets Swap Remove

The ASCII code for the uppercase letter A is 65, which is stored as the 7-bit pattern 1000001.

Explain why the lowercase letter a is stored using a different 7-bit pattern.

[2 marks]
Mark scheme

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

  • The computer needs to tell/distinguish the uppercase A from the lowercase a (1) because each different character has its own unique 7-bit code/pattern (1)
  • A and a are different characters (1) so each needs a different 7-bit pattern to be stored and displayed correctly (1)
  • If A and a had the same 7-bit pattern (1) the computer could not tell which character to display (1)

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

Do not accept 'because they are different letters/cases' without saying they need different/unique codes or that the computer must tell them apart.

Do not accept 'because A is a capital letter' on its own.

Do not accept 'because the codes are different numbers' or '65 is not 97' without linking to the need to tell the characters apart or to unique codes.

Example answers and how they were marked
Full marks2/2

Each character needs its own unique code, so the computer can tell that A and a are different characters and display the right one.

Why: Gives the point that the computer must tell A and a apart (1) and links it to the reason that each character needs its own unique code (1).

Some marks1/2

The computer must be able to tell the difference between uppercase and lowercase letters.

Why: Gives the point that the computer must tell the two characters apart (1) but gives no linked reason about unique codes or different 7-bit patterns.

No marks0/2

Because A is a capital letter and a is lowercase.

Why: Only states that the letters are different cases, which the mark scheme says is not accepted without saying they need unique codes or that the computer must tell them apart.

3Discuss6 marksImages Swap Remove

A school is creating a small logo to display on its website. The logo is stored as a bitmap image. Each pixel in the logo is stored as a binary value.

Discuss how the bitmap image of the logo is represented in binary.

Your answer should include:

  • pixels
  • binary values
  • the grid of pixels.
[6 marks]
Mark scheme

Indicative content

Pixels

  • A bitmap image is made up of picture elements, called pixels.
  • Each pixel is the smallest identifiable area of the image and has a single colour.
  • The logo is stored as a grid of pixels, and its width and height in pixels give its dimensions.
  • Each pixel is stored separately, so a larger grid needs more pixel values to be stored.

Binary values

  • Each pixel's colour is represented by a binary value.
  • The number of bits used for each pixel is the colour depth.
  • With n bits per pixel there are 2^n different binary values, so 2^n possible colours.
  • A black-and-white image can use one bit per pixel, with 0 for white and 1 for black.
  • More bits per pixel allow more colours to be represented in the logo.

Grid

  • The binary values are stored in order, row by row, so the computer can rebuild the grid.
  • The resolution is the number of pixels in the image, for example width × height.
  • A higher-resolution grid stores more pixels, so the logo can show more detail.
  • The image file stores the pixel values and data such as the image dimensions.

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)

Level 3 responses must cover both pixels and binary values.

Do not accept resolution as the number of colours.

Example answers and how they were marked
Full marks6/6

A bitmap is a grid of pixels. Each pixel is one small area of the logo and has a single colour. The width and height of the grid are measured in pixels, so the logo's dimensions tell us how many pixels it contains. Each pixel's colour is stored as a binary value. The number of bits used for each pixel is the colour depth, and with n bits there are 2^n possible values, so 2^n possible colours. For example, 3 bits per pixel would allow 8 colours. The binary values are stored row by row so that the computer can rebuild the grid. A larger grid stores more pixel values, but it can show more detail. More bits per pixel let the logo use more colours, such as the school's colours.

Why: Covers pixels as the smallest areas of a grid (AO1), binary values with colour depth and 2^n colours (AO1), and applies these to the logo's grid and colours (AO2). Sustained and structured, so Level 3.

Some marks4/6

A bitmap is a grid of pixels. Each pixel is a small square with one colour. Each pixel is stored as a binary value. The logo needs a binary value for every pixel, so a bigger logo has more values. More bits per pixel means more colours.

Why: Gives pixels and binary values, with a simple link to the logo's size. Points are mostly independent and not developed into a full discussion, so Level 2.

Some marks1/6

A bitmap image is a grid of pixels. Resolution is the number of colours in the image. Each pixel is stored as a letter, so the logo can use any colour.

Why: One basic point: a bitmap is a grid of pixels. The rest is incorrect: resolution is not the number of colours, and pixel values are stored as binary, not letters.

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