Computer Systems

Question 1

What happens in the fetch-execute cycle?

Answer 1

The instruction is fetched from the main memory into the CPU and is then executed by the CPU.

Question 2

Which are the four steps of the fetch-execute cycle?

Answer 2

1. The processor sets up the address bus with the required address.
2. The processor activates the read line on the control bus.
3. An instruction is fetched from the memory location using the data bus and stored in the instruction register.
4. The instruction in the instruction register is then interpreted by the decoder and carried out.

Question 3

Which are the four factors affecting system performance?

Answer 3

Number of processors (cores)
Width of data bus
Cache memory
Clock speed

Question 4

How does the number of processors (cores) affect system performance?

Answer 4

The more cores the CPU has, the more CPU’s are found on one chip.
The more cores, the more sets of instructions the processor can receive and process at the same time.

Question 5

How does the width of the data bus affect system performance?

Answer 5

The wider the data bus, the more parallel wires it has available to send data.
The more wires, the more data can be passed at the same time.
For example, between 32 bits and 64 bits, the amount of data that can be transferred is doubled.

Question 6

How does the cache memory affect system performance?

Answer 6

The processor checks cache memory for data or instructions first before accessing main memory.
If the data/instruction is found then it can be accessed quicker as the processor would not have to check the main memory for it.
Many computers have multi-level caches in which case the computer checks level 1 first, which is the fastest cache, followed by level 2, etc.. The less levels the processor has to check for the data the faster the data is accessed.

Question 7

How does clock speed affect system performance?

Answer 7

Clock pulses are what trigger components to take their next step.
The higher the clock rate, the faster the components move on to the next step so the faster a computer may complete a set of instructions.

Question 8

How do you change a decimal number to its floating point representation?

Answer 8

Step 1: Move the decimal to the front of the number (i.e. just in front of the first 1)
Step 2: Multiply the new number by 2 to the power of the exponent
Step 3: The exponent is the number of places you have moved the decimal point to bring it to its new position. If you moved the decimal point to the left, the exponent is positive, if you moved the decimal point to the right, the exponent is negative.

Question 9

How do you write the floating point representation correctly?

Answer 9

Step 1: Is the whole number negative? If there is a “-“ it is negative. In this case the sign bit is 1. If there is no “-“ then the number is positive and the sign bit is 0.

Step 2: Write the mantissa, the whole part of the number, in the mantissa section. Remember if you were allocated 16 bits, for example, you need to deduct 1 space as that was taken up by the sign bit. If the number is too large remove digits from the right hand side, if the number is too small add 0’s to the right hand side.

Step 3: Store the exponent using two’s complement.

Question 10

What is the floating point representation of 1000010.101?

Answer 10

-0.1000010101 x 2 ^ 0111

Question 11

What is the floating point representation of -101.00011?

Answer 11

-0.10100011 x 2 ^ 011

Question 12

What is the floating point representation of 0.0010101?

Answer 12

0.10101 x 2 ^ 110

Question 13

What is the floating point representation of -0.00001101?

Answer 13

-0.1101 x 2 ^ 1100

Question 14

How do you store a number as two’s complement?

Answer 14

Remember the first bit (MSB - most significant bit) is negative.
Change the positive version of the number to binary, if it is not already, making sure the first digit is a 0.
Change all 0s to 1s and all 1s to 0s.
Add 1.

Question 15

How many bits does Unicode use to represent characters?

Answer 15

16 or more

Question 16

How many different characters can Unicode store?

Answer 16

2^16 or 65,536 characters

Question 17

How are bit-mapped images stored on a computer device?

Answer 17

They paint the picture by changing the colour of the pixels which make up the screen display.
They store the colour of each pixel in the image using a fixed number of bits, which can range between 1 and 24 bits.

Question 18

What are the advantages of bit-mapped graphics (versus vector graphics)?

Answer 18

• It can represent more colours than vector graphics (up to 16, 777, 216) which means it can represent more realistic images such as photographs.
• You zoom in on the image to edit individual pixels.
• Adding detail to the image will not change the file size as you are only changing the colours of the pixels.

Question 19

What are the disadvantages of bit-mapped graphics (versus vector graphics)?

Answer 19

• Overlapping images will override each other so that only the top image is saved.
• Larger file size than vector as all bits are stored even if it is blank.
• Resolution is dependent on the screen you are using. As a result, if zoomed in or scaled up the image can become pixelated and/or blurry, losing quality.

Question 20

How are vector graphics stored on a computer device?

Answer 20

Vector graphics draw objects on the screen. These objects produced by a vector graphics package have their descriptions stored as a file of data called object attributes. These attributes are used by the computer to create the graphic using a mathematical equation.

Question 21

What are the advantages of vector graphics (versus bit-mapped graphics)?

Answer 21

• Smaller file size as only the information about the objects is stored.
• Resolution independent, so scaling or zooming in on the image will not lose quality as it is redrawn.
• Overlapping images do not override each other, the information for all objects is still stored, making it easier to edit individual objects.
• Objects can be moved around easily.

Question 22

What are the disadvantages of vector graphics (versus bit-mapped graphics)?

Answer 22

• Colours are less realistic so images often have a cartoon feel to them.
• Adding objects to the image will increase the file size as you need to store information about extra objects.

Question 23

Define tracking cookies.

Answer 23

Text files that store information about your browsing habits such as usernames, passwords, time spent on a web page, media that you have viewed, etc.

Question 24

What are the security risks of tracking cookies?

Answer 24

Tracking cookies can be used by companies to send (sell) as much data about you to a third party as they can and these details can then be used for identity theft.

Question 25

What is DoS?

Answer 25

Denial of Service Attack.

Question 26

What are the symptoms of DoS attacks?

Answer 26

• Slow performance
• Inability to access the website

Question 27

What are the effects of DoS attacks?

Answer 27

Disruption to users and businesses

Question 28

What are the costs of DoS attacks?

Answer 28

• Lost revenue
• Labour in rectifying fault

Question 29

What types of faults can DoS attacks result in?

Answer 29

• Bandwidth consumption
• Resource starvation
• Domain Name Service (DNS)

Question 30

What could the reasons for a DoS attack be?

Answer 30

• Financial
• Political
• Personal

Question 31

What is encryption?

Answer 31

Encryption is the act of encoding (making unreadable) readable data into encoded (unreadable) data that cannot be read.

Question 32

How is data decoded?

Answer 32

Data can be decoded (made readable) by using a specific unique key.

Question 33

Describe public-key encryption.

Answer 33

• Everyone has a public key
• Each individual has a private key which is kept secret
• Data is encrypted using sender’s private key to prove the sender
• Data is then encrypted again using the receiver’s public key
• When received, the data is decrypted by the receiver’s private key
• Data is then decrypted using the sender’s public key

Question 34

What is a digital signature?

Answer 34

It is created using private keys to prove that the data was sent by a particular person.

It ensures that a message, software or digital document is valid and has not been tampered with.

Question 35

What is a digital certificate?

Answer 35

It verifies that data is being sent securely. They include the identity of the sender and information that identifies the issuing authority.

Question 36

What is a criminal offence according to the Computer Misuse Act of 1990?

Answer 36

• To gain unauthorised access to computer material
• To gain unauthorised access with intent to commit or facilitate commission of further offences
• To carry out unauthorised acts with intent to impair the operation of a computer or causing or creating risk of serious damage or making, supplying or obtaining articles for use in the commission of these offences.

Question 37

What are the implications of the Computer Misuse Act of 1990 for individuals?

Answer 37

• Passwords that are difficult for others to guess
• Biometrics as an alternative use to passwords
• Anti-virus software
• Firewalls
• Web proxies

Question 38

What are the implications of the Computer Misuse Act of 1990 for businesses?

Answer 38

• Risk of the computer system being put out of action resulting in a loss of income for the company until repairs have been carried out.
• Cost of repairing the computer system, restoring the data and improving the security of the system to reduce the change of unauthorised access in the future.
• Losing customers to alternative businesses or suppliers whose computer systems are unaffected by hacking.
• Losing trade secrets which may be of value to a competitor’s business.
• Customers being put at risk if their personal details (including passwords) are revealed.

Question 39

How can smart heating systems be used for a better environmental impact?

Answer 39

• Using activity sensors, some smart systems learn the temperatures that you prefer in certain rooms and at what times.
• Monitoring the activity in rooms can mean that the smart system adjusts the heating up or down depending on whether there is unusual activity in the house.
• The thermostat is connected to wi-fi and can be manually controlled by using an app on your phone. This allows you to turn the heating on/off so that it is at the optimum temperature when you arrive home.

Question 40

How can traffic control be used for a better environmental impact?

Answer 40

Intelligent transport systems use software, hardware and communications technology, to try to keep traffic moving, reducing the amount of harmful emissions. This is done by guiding drivers away from traffic congestions to more free-flowing routes.

Question 41

How can car management systems be used for a better environmental impact?

Answer 41

• Start/stop systems automatically shut down the engine when the car is not moving to reduce the amount of time the engine spends idling.
• Engine control units use sensors to ensure the engine’s air/fuel ratio can be controlled very accurately, ensuring optimum fuel consumption and reduction of carbon dioxide emissions.