Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• Only 16 different characters can be represented (1) so there are not enough characters codes for every letter of the alphabet (1)
• 4 bits give only 16 different combinations (1) which is not enough for the uppercase letters, lowercase letters, digits and symbols on a keyboard (1)
• More than 16 characters are needed on a keyboard (1) so some letters, digits or symbols cannot be represented (1)
• The number of character codes available is limited to 16 (1) because 4 bits give 2 to the power of 4 combinations (1)

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

Calculating that 4 bits give 2 to the power of 4 = 16 combinations can earn either the point or the linked reason, provided the other half of the answer says what this means (too few characters/codes).

Allow an expansion that names only the letters of the alphabet, e.g. there are not enough character codes for the 26 letters (1).

Do not accept '4 bits can only store 4 characters' (the number of bits is not the number of characters).

Do not accept 'it can only store 16 bits' (16 is the number of character codes/combinations, not the number of bits).

Do not accept 'not enough memory/storage' (the limitation is the number of character codes, not the amount of storage).

Do not accept 'it is limited/smaller' without saying how many characters/codes are available or what cannot be represented.

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

• Fewer samples are taken every second (1) so less detail of the sound wave is captured (1)
• Fewer samples are taken every second (1) so the file is smaller/uses less storage space (1)
• The wave is measured less often (1) so parts/changes in the wave between samples are missed (1)

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

The 'fewer samples are taken/measured less often' point can only be credited once; the second mark is for a valid expansion linked to it.

Do not accept restating the question, e.g. 'the time between measurements is longer' or 'the sample interval is longer', as either the point or the expansion.

Do not accept 'the sample rate is lower' as the expansion, as this says the same thing as a longer sample interval.

Do not accept 'the sound is lower quality/less accurate' on its own without saying what is lost, e.g. less detail of the wave captured/parts of the wave missed.

Do not accept answers about using fewer bits per sample or about compression, as these do not follow from the sample interval.

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

• 7 bits give 128 different codes/combinations (1) which is enough to represent all the characters on a keyboard (1)
• 7 bits allow 128 unique characters (1) so uppercase and lowercase letters, digits and punctuation can all be represented (1)
• 6 bits give only 64 combinations (1) which is not enough for the number of characters on a keyboard (1)
• A keyboard needs to represent more than 64 different characters (1) so 7 bits are needed to give the 128 combinations required (1)
• A fixed number of 7 bits gives 128 possible codes (1) which is enough for the characters ASCII represents, so more bits are not needed (1)

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

Do not accept 'because ASCII is 7-bit' or 'because 7 bits is enough' as these repeat the question without giving the number of combinations or saying what they allow.

Do not accept '7 bits give 256 characters'; 256 combinations come from 8 bits (extended ASCII).

Allow 'ASCII has 128 characters/codes' for the 128 point.

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.