Set 2

Question 1

Describe what an interrupt is and explain the purpose of interrupts.

Answer 1

A signal is sent to the processor (from a hardware device or program)
So that a device or source that needs the immediate attention of the processor can be dealt with.

Question 2

Describe, in detail, the steps taken by the processor to service an interrupt.

Answer 2

1. The processor receives the interrupt
2. The processor completes the fetch-decode-execute cycle of the instruction that it was running when it received the interrupt
3. The current contents of the processor registers (including the program counter) are saved to memory. This is called the volatile environment
4. The origin of the interrupt is identified so that the appropriate ISR (Interrupt Service Routine) is called
5. All other lower-priority interrupts are put on hold to allow the ISR to finish running
6. The program counter is updated with the address of the first instruction of the ISR
7. The ISR completes its execution
8. The volatile environment is reloaded with the values that were saved to memory
9. The lower-priority interrupts that were put on hold are re-enabled
10. The program counter is set to point to the address of the next instruction that needs to be executed in the program that the processor was running when it received the interrupt

Question 3

Describe how the hardware of a computer could be improved so that programs can be
executed more quickly.

Answer 3

• Replace the processor with one which has more cores A. Increase number of cores
• Replace the processor with one which has more cache memory // increase the amount of cache memory // add cache memory
• Increase clock speed of processor // replace the processor with one which runs at a faster clock speed NE. faster processor
• Use a parallel processor architecture // use more processors which can work in parallel
• Use a processor with a bigger word size
• Use a processor that makes (better) use of pipelining
• Install more RAM // main memory // primary memory
• Use RAM // main memory // primary memory with a faster access time
• Replace the motherboard with one which has buses which run at a faster clock speed A. increase bus clock speed
• Replace the motherboard with one which has more lines in data bus A. increase number of lines in data bus
• A. Replace HDDs with SSDs // replace HDDS with HDDs that can read data at a faster rate // replace SSDS with SSDs that can read data at a faster rate
• A. Use the Harvard architecture instead of the von Neumann architecture

Question 4

How many bytes are in a kilo/mega/giga byte?

Answer 4

1 thousand/1 million/1 billion or 1000/1,000,000/1,000,000,000

Question 5

How many bytes are in a kibibtye?

Answer 5

1024 bytes

Question 6

How many kibibytes are in a mebibyte?

Answer 6

1024 bibibytes

Question 7

List the prefixes starting kibi…

Answer 7

• kibi
• mebi
• gibi
• tebi

Question 8

How many bits does ASCII use? How many characters can it represent?

Answer 8

• ASCII uses 7 bits to represent characters (hence max 128 characters)
• However, 8 bits are used to store each character (preceding 0)
• The leading 0 sometimes used as parity bit instead

Question 9

How does ASCII work?

Answer 9

• Each of the characters is assigned a numeric character code
• Which is represented and stored in the computer as binary

Question 10

What is the downside of ASCII?

Answer 10

ASCII is not sufficient to represent all of the languages and scripts used around the world

Question 11

Why is unicode needed?

Answer 11

1. ASCII can’t represent all languages and scripts
2. ASCII can’t represent all possible symbols
3. The World Wide Web made it important to have a universal international coding system

Question 12

Which is the most common unicode encoding standard used?

Answer 12

UTF-8

Question 13

What are the first 128 codes in unicode the same as?

Answer 13

The first 128 codes in ASCII

Question 14

What is clock speed?

Answer 14

The frequency at which the system clock ticks

Question 15

What is word length?

Answer 15

Word Length is the number of bits in a word

(A word is the fixed size unit of data. It affects register size, address size and other aspects of the computer)

Question 16

What is the address bus width?

Answer 16

• The number of bits that can be used to form an address of a memory location
• Or the number of parallel wires allocated to the address bus

Question 17

What is the data bus width?

Answer 17

• The number of bits that can be transferred to or from memory in one operation
• Or the number of parallel wires allocated to the data bus

Question 18

How is fetch time affected if the data bus isn’t big enough for a piece of data?

Answer 18

• the piece of data will be split into multiple parts
• meaning fetch time will be multiplied

Question 19

What is cache memory? What does it contain?

Answer 19

Cache memory is a relatively small capacity of very fast memory that sits on or close to the processor.

Cache memory contains:
1. Copies of data from frequently used memory locations
2. Pre-fetched instructions
3. Data to be written to main memory

Question 20

How does increasing cache memory size affect processor performance?

Answer 20

1. Increasing cache size means more instructions and data can be stored in cache
2. So main memory needs to be accessed less frequently
3. Cache memory is faster to access than main memory
4. So throughput and response time is increased

Question 21

How does increasing the number of cores affect processor performance?

Answer 21

• Multiple tasks can be run at the same time
• A single task can be split over multiple processors
  
  • Many programs are now written to optimise the use of parallel processing
• This increases throughput
• However, increasing the number of cores greatly increasing complexity

Question 22

4 reasons secondary storage is needed

Answer 22

• Main memory is volatile, so secondary storage needed to store files that are needed multiple times
• Secondary storage can be used to store larger files, as it usually has a much higher capacity than main memory
• Secondary storage can be used for virtual memory
• Main memory is expensive

Question 23

What is the effect of increasing data bus width?

Answer 23

• more bits can be transferred between main memory and the processor in one operation
• so fewer read/write operations are needed
• so there are fewer delays when retrieving data

Question 24

What is the effect of increasing address bus width?

Answer 24

1. The processor can access a larger number of memory locations
2. so the CPU doesn’t have to access secondary storage as often
3. which reduces the reliance on virtual memory
4. Increased scalability and increased throughput

Question 25

What is the effect (and downside) of increasing word length?

Answer 25

• Longer word length means registers can store more bits, so the processor can process more bits in one go
• Also, complicated value splitting is required if the operand size is larger than the word length
• however if word length is increased, more memory is wasted when storing small values

Question 26

What is the effect (and drawbacks) of increasing clock speed?

Answer 26

• Higher clock speed means more instructions can be executed per unit time
  however
• All machine code instructions take a certain amount of time to execute anyway
• Heat dissipation becomes more of an issue as clock speed increases

Question 27

Give 4 advantages of Harvard architecture

Answer 27

• Instruction and data can be accessed simultaneously
• Instruction and data memory can have different word lengths
• Different technologies can be used to implement instruction and data memory
• For systems with a predetermined use, instruction memory can be implemented as ROM which protects the programs from hacking

Question 28

What three pieces of information are stored in a stack frame for a subroutine call

Answer 28

• Return address
• Parameters
• Local variables

Question 29

What is a parameter

Answer 29

A subroutine variable initialised with an input value (argument) when the subroutine is called.

It only while the subroutine is executing.

Question 30

What is a local variable?

Answer 30

A variable that is declared in a subroutine. It is only accessible within that subroutine.

It only while the subroutine is executing.

Question 31

What is a global variable?

Answer 31

A variable that is accessible by all subroutines (within a class)