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MskProd Year 9 unit exam 25 minutes 25 marks Original paper

Data Representation: Unit Exam A

Binary, denary and hexadecimal conversions, binary addition and overflow, units, ASCII and Unicode, image file sizes and sound.

Instructions

Time allowed: 25 minutes
Total marks: 25
Calculator: No calculator is permitted.

Answer all the questions.
The number of marks for each question is shown in brackets. Use it as a guide to how much to write.
The clock at the top counts down from the real time allowed. When it reaches zero the paper is locked and marked, as it would be collected in the exam hall.
Read every question to the end before answering. Where a question says tick two boxes, tick exactly two: ticking more than asked scores nothing. Type one word and number answers carefully, because they are marked exactly.

Questions

Answer every question, then finish the paper to have it marked. Marking as you go teaches you far less.

25 marks

1Convert each 8-bit binary number to denary.3 marks

0010 1101 =1000 0001 =1111 0000 =

Mark scheme0 / 3

L45
L129
L240

Model answer

0010 1101 = 32 + 8 + 4 + 1 = 45. 1000 0001 = 128 + 1 = 129. 1111 0000 = 128 + 64 + 32 + 16 = 240.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

2Convert each denary number to an 8-bit binary number.2 marks

77 =200 =

Mark scheme0 / 2

L01001101
L11001000

Model answer

77 = 64 + 8 + 4 + 1 = 0100 1101. 200 = 128 + 64 + 8 = 1100 1000.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

3Convert each binary number to hexadecimal.2 marks

1010 1111 =0011 1100 =

Mark scheme0 / 2

Model answer

Split into nibbles: 1010 is A and 1111 is F, giving AF. 0011 is 3 and 1100 is C, giving 3C.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

4Convert the hexadecimal number 2B to denary.1 mark

Mark scheme0 / 1

Model answer

2 x 16 + 11 = 43.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

5Why do programmers use hexadecimal?1 mark

AIt uses less memoryBIt is more accurate than binaryCIt is shorter and easier for people to read than binaryDComputers store data in hexadecimal

Tick one box.

Mark scheme0 / 1

LC It is shorter and easier for people to read than binary

Model answer

Each hex digit stands for four bits, so values are shorter and fewer mistakes are made. The computer still stores binary.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

6Add these two 8-bit binary numbers. Give your answer in binary.0101 1010 + 0011 01101 mark

Mark scheme0 / 1

L10010000

Model answer

90 + 54 = 144, which is 1001 0000.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

7What is overflow?1 mark

AWhen a file is too big for memoryBWhen the result of a calculation is too large to fit in the bits availableCWhen a number is negativeDWhen the CPU overheats

Tick one box.

Mark scheme0 / 1

LB When the result of a calculation is too large to fit in the bits available

Model answer

An 8-bit result above 255 cannot be stored: that is overflow.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

81100 0000 + 1000 0000 overflows when stored in 8 bits. What is the true answer in denary?1 mark

Mark scheme0 / 1

L320

Model answer

192 + 128 = 320, which is more than 255.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

9Tick one box in each row.4 marks

	Bit	Nibble	Byte	Kilobyte
4 bits	Bit	Nibble	Byte	Kilobyte
8 bits	Bit	Nibble	Byte	Kilobyte
A single 0 or 1	Bit	Nibble	Byte	Kilobyte
1000 bytes	Bit	Nibble	Byte	Kilobyte

Tick one box in each row.

Mark scheme0 / 4

L4 bits: Nibble
L8 bits: Byte
LA single 0 or 1: Bit
L1000 bytes: Kilobyte

Model answer

A bit is one binary digit, a nibble is 4 bits, a byte is 8 bits and a kilobyte is 1000 bytes.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

10Why was Unicode created when ASCII already existed?1 mark

AASCII is too slowBASCII only stores numbersCUnicode uses fewer bitsDASCII cannot represent the characters of every language, or emoji

Tick one box.

Mark scheme0 / 1

LD ASCII cannot represent the characters of every language, or emoji

Model answer

ASCII has room for 128 characters. Unicode has room for every writing system.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

11In ASCII, A is 65. What is the code for E?1 mark

Mark scheme0 / 1

Model answer

E is four letters after A: 65 + 4 = 69.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

12An image is 200 pixels wide and 100 pixels high, with a colour depth of 8 bits. Calculate the file size in bytes. Show your working.3 marks

Show your working

Answerbytes

Mark scheme0 / 3

LCorrect answer, full marks: 20000 bytes
LMethod: Multiplies the width by the height
LMethod: Multiplies by the colour depth, then divides by 8 to turn bits into bytes

Model answer

200 x 100 = 20,000 pixels. 20,000 x 8 bits = 160,000 bits. 160,000 / 8 = 20,000 bytes.

Examiner overrideThe marker looks for key phrases. If you said it another way and it is still right, give yourself the mark.

13What happens to an image's file size if its colour depth is increased?1 mark

AIt increases, because more bits are stored for every pixelBIt decreasesCThe image loses pixelsDIt stays the same

Tick one box.

Mark scheme0 / 1

LA It increases, because more bits are stored for every pixel

Model answer

More bits per pixel means more bits overall.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

14Explain how increasing the sample rate affects a sound file.2 marks

Explain

Mark scheme0 / 2

LMore samples are taken each second, so the recording is closer to the original sound
LThe file size increases

Model answer

A higher sample rate takes more measurements every second, so the recording follows the original sound more closely and sounds better. Each sample has to be stored, so the file gets bigger.

Examiner overrideThe marker looks for key phrases. If you said it another way and it is still right, give yourself the mark.

15What is metadata in an image file?1 mark

AThe colours of the pixelsBA copy of the imageCData about the image, such as its width, height and colour depthDA smaller version of the image

Tick one box.

Mark scheme0 / 1

LC Data about the image, such as its width, height and colour depth

Model answer

Metadata describes the file so it can be displayed correctly.

Examiner overrideMarked exactly against the mark scheme. Change it only if an examiner would accept your answer.

Machine mark 0 / 25

About this paper

This is an original Key Stage 3 assessment. Most questions are closed, so the marking is exact, and the written answers are marked against a mark scheme in the same style as a GCSE paper, so the format is familiar long before it counts.