Topic 3 Data

Question 1

What is Binary?

Answer 1

A system of data representation which uses only 1’s and 0’s. 1 represents on/yes/true and 0 represents off/no/false. This relates directly to transistors in the CPU which can be on or off.

Question 2

What is a bit?

Answer 2

A single 1 or 0 in Binary.

Question 3

What is a byte?

Answer 3

8 bits in Binary (8 zeros or ones)

Question 4

What is a nibble?

Answer 4

4 bits in Binary (half a byte)

Question 5

List the column headings for a full byte.

Answer 5

128, 64, 32, 16, 8, 4, 2, 1

Question 6

What is the highest number that can be represented in one full byte?

Answer 6

255

Question 7

What is Denary?

Answer 7

Also called Decimal number system. Standard system we use using digits 0 - 9.

Question 8

What is an overflow error?

Answer 8

An error in a calculation which requires more bits than are available.

Question 9

In Binary, what are the answers to the following: 0 + 0 1 + 0 1 + 1 1 + 1 + 1

Answer 9

0 + 0 = 0 1 + 0 = 1 1 + 1 = 10 (the binary code for 2) 1 + 1 + 1 = 11 (the binary code for 3)

Question 10

What effect does a logical shift left have on a Binary number?

Answer 10

Multiplies a number by powers of 2. One left shift x 2^1 Two left shifts x 2^2 Three left shifts x 2^3 etc…

Question 11

What effect does a logical shift right have on a Binary number?

Answer 11

Divides a number by powers of 2. One right shift / 2^1 Two right shifts / 2^2 Three right shifts /2^3 etc…

Question 12

Describe the sign and magnitude method of representing negative numbers in Binary.

Answer 12

The Most Significant Bit (MSB - furthest left) is not used to represent a number, but is used for the sign. 1 means negative. 0 means positive.

Question 13

Why is sign and magnitude not used by computers?

Answer 13

• Zero could be represented as both positive or negative. - Causes errors with binary addition.

Question 14

Describe the twos complement method of representing negative numbers in Binary.

Answer 14

The Most Significant Bit (MSB - furthest left) is used to represent the negative equivalent of that number. For example in a single byte the columns headings would be: -128, 64, 32, 16, 8, 4, 2, 1

Question 15

When are arithmetic shifts used?

Answer 15

Arithmetic shifts are used instead of logical shifts for signed numbers. Left to multiply, right to divide.

Question 16

What is different about carrying out an arithmetic shift as opposed to a logical shift.

Answer 16

Arithmetic shifts are used for signed numbers (+ / -) so the MSB must remain the same as the original to

Question 17

Describe the Hexidecimal system or data representation.

Question 18

Explain how to convert an 8 bit Binary number directly into Hexidecimal (without first converting to Denary).

Question 19

List four uses of Hexidecimal.

Answer 19

Used to help humans cope with long strings of binary digits, as Hex numbers are shorter.

Errors codes are usually given as Hex values.

Used to represent numerical values in assembly language.

Question 20

In most computers, how are characters and text represented?

Question 21

Describe ASCII code.

Question 22

What is meant by a character set?

Question 23

How many characters can be represented using standard ASCII code?

Question 24

If the ASCII code for the character ‘C’ is 67, how could you work out the ASCII code for the character ‘M’.

Answer 24

As the codes are consecutive you can work out the number of characters between C and M, and then add this number to 67 to work out the code for M.

Answer = 77

Question 25

How can you calcate the file size of an image?

Answer 25

This depends on the image size (WxH) and the colour depth (D):

The calculation is W x H x D

The better the image quality, the higher the file size.

Question 26

A 24 bit image has the dimensions 200 x 300. Construct an expression to calculate the size of the image in MB.

Answer 26

(200 x 300 x 24 ) / 8 / 1024 / 1024

Dividing by 8 converts bits to bytes.

Dividing by 1024 once converts bytes to KB

Dividing by 1024 again converts KB to MB

Question 27

What is sound sampling?

Answer 27

Sound is an analogue form of data. To convert it to digital you need to take a sample of the sound at particular intervals called the sample frequency. The higher the sample frequency, the higher the quality of the digital sound.

Question 28

Construct an express to calculate the size of an audio file.

Answer 28

Sample frequency x bit depth x recording length in seconds

Question 29

Construct an expression to calculate the size in MB of a digital audio file of 4 minutes with a sample frequency of 44.1 KHz and a resolution of 24 bits.

Answer 29

Sample frequency x resolution x recording length in seconds

( 44100 x 24 x 240 ) / 8 / 1024 / 1024

Question 30

How are binary digits represented inside the computer?

Answer 30

A computer processor uses billions of transistors acting as switches. These can either be ON (1) or OFF(0).

Question 31

Describe lossless compression and give examples of its use.

Answer 31

• Reduces file size without deleting any data
• Nothing is lost
• Works by looking for redundancy where same data is stored many times
• Used for text files, or smaller graphic files
• Not so good for audio files or larger image files

Question 32

Describe lossy compression and give examples of its use.

Answer 32

• Reduces the file size by deleting some data
• The original can never be reconstructed
• It works in image files by analysing the image to find areas with slight differences which are given the same value to reduce the number of bits
• It works in audio files by removing small differences in frequency, tone and volume
• Not so good for text files

Question 33

Convert the string below using run-length encoding. What is the file size before and after?

ggggfffoooooyyeeee

Answer 33

After RLE the string is:

4g3f5o2y4e

Before RLE file size is 18 bytes

After RLE file size is 10 bytes

Question 34

What is the purpose of encryption?

Answer 34

Encryption scrambles sensitive data to ensure only authorised people can read it.

Question 35

What is symmetric encryption?

Answer 35

Encryption methods which use the same key to both encrypt and decryt the data.

Question 36

Describe asymmetric encryption.

Answer 36

Asymmetric encryption used two different keys to encrypt and decrypt the message.

A message encrypted with a public key, can only be decypted by the corresponding private key (and vice versa).

Question 37

Describe the Caeser Cipher method of encryption.

Answer 37

Each character in a message is replaced by another character by shifting each by a number known as the key.

Question 38

How is data encrypted on the internet?

Answer 38

By using the HTTPS protocol.

This is the HTTP protocol with the SSL/TLS protocol added to it.

Question 39

Describe the following terms used in databases:

1. Attribute
2. Table
3. Primary Key
4. Entity
5. Foreign Key

Answer 39

1. Attribute - a property / fieldname of an entity (eg Surname of STUDENT)
2. Table - Representation of an entity. (eg STUDENT table contains fieldname and records about students)
3. Primary Key - Unique identified for each record in a table
4. Entity - An object/thing represented in a database (eg STUDENT, CLASSROOM, CAR, REG-NO etc)
5. Foreign Key - A primary key from another table inserted into another table as a link in a relational database