1. Functions of an Operating System

Question 1

Define the term operating system

Answer 1

A collection of programs that provide an interface between the user and the computer

Question 2

Name the 8 features that an operating system provides

Answer 2

1. Memory management
2. Resource management
3. File management
4. I/O management
5. Interrupt management
6. Utility software
7. Security
8. User interface

Question 3

Name the 3 techniques used by the OS to ensure memory is shared effectively by programs

Answer 3

1. Paging
2. Segmentation
3. Virtual memory

Question 4

Describe how paging works

Answer 4

1. Memory split up into equal-sized sections (pages)

2. Pages swapped between main memory and hard disk as needed

Question 5

Describe how segmentation works

Answer 5

1. Memory split up into logical sized divisions (segments)
2. Segments vary in size
3. Segments represent structure and logical flow of program

Question 6

Describe how virtual memory works

Answer 6

1. A section of hard drive acts as RAM when space in main memory is insufficient
2. Sections of programs not currently being used are temporarily moved to virtual memory through paging
3. This frees up memory for other programs in RAM

Question 7

Define the term disk thrashing

Answer 7

A disadvantage of virtual memory when the computer freezes due to pages being swapped too frequently between hard disk and main memory

Question 8

Define the term interrupt

Answer 8

Signals generated by software or hardware to indicate to processor that a process needs attention

Question 9

What is considered when allocating processor time?

Answer 9

The interrupt’s priority

Question 10

Where are interrupts stored?

Answer 10

Within an abstract data structure called a priority queue in an interrupt register

Question 11

What is an ISR (Interrupt Service Routine)?

Answer 11

A software process invoked by an interrupt request

Question 12

Describe what happens when an interrupt exists with a higher priority to the current process

Answer 12

1. Current contents of registers in CPU are transferred into stack
2. Relevant ISR is loaded into RAM
3. Flag is set to signal ISR has begun
4. Flag is reset once ISR has finished
5. Process is repeated

Question 13

Describe what happens when there are no interrupts with a higher priority to the current process

Answer 13

1. Contents of stack are put back into registers

2. FDE cycle resumes

Question 14

What is scheduling?

Answer 14

How the OS ensures all sections of programs being run (jobs) receive a fair amount of processing time

Question 15

What is a preemptive scheduling algorithm?

Answer 15

When jobs are actively made to start and stop by the OS

Question 16

What is a non-preemptive scheduling algorithm?

Answer 16

Once a job is started, it is left alone until it is completed

Question 17

Name the 3 examples of preemptive scheduling algorithms

Answer 17

1. Multilevel Feedback Queues
2. Shortest Remaining Time
3. Round Robin

Question 18

Name the 2 examples of non-preemptive scheduling algorithms

Answer 18

1. First Come First Served

2. Shortest Job First

Question 19

Describe how round robin works

Answer 19

1. Each job is given a section of processor time (time slice) within which it runs
2. Once each job in queue has used its first time slice, they are given another slice of processor time until a job is completed
3. Completed jobs are removed from queue

Question 20

What is the advantage of round robin?

Answer 20

All jobs will eventually be attended to

Question 21

What are the 2 disadvantages of round robin?

Answer 21

1. Longer jobs take much longer time for completion

2. Does not account for job priority or urgency

Question 22

Describe how first come first served works

Answer 22

Jobs processed in chronological order by which they entered queue

Question 23

What is the advantage of first come first served?

Answer 23

Straightforward to implement

Question 24

What is the disadvantage of first come first served?

Answer 24

Does not account for job priority or urgency

Question 25

Describe how multilevel feedback queues work

Answer 25

Makes use of multiple queues, each ordered based on different priority

Question 26

What is the advantage of multilevel feedback queues?

Answer 26

Takes into consideration different job priorities

Question 27

What is the disadvantage of multilevel feedback queues?

Answer 27

Difficult to implement

Question 28

Describe how shortest job first works

Answer 28

The queue storing jobs to be processed is ordered according to time required for completion so longest jobs are serviced at the end

Question 29

What is the advantage of shortest job first?

Answer 29

Suited to batch systems since waiting time is reduced

Question 30

What are the disadvantages of shortest job first?

Answer 30

1. Requires processor to calculate how long each job will take
2. Processor starvation if short jobs are continuously added to queue
3. Does not account for job priority or urgency

Question 31

Define the term processor starvation

Answer 31

When a particular process does not receive enough processor time to execute and be completed

Question 32

Describe how shortest remaining time works

Answer 32

Queue storing jobs to be processed us ordered according to time left for completion so jobs with least time left are serviced first

Question 33

What is the advantage of shortest remaining time?

Answer 33

Throughput is increased as shorter processes can be quickly completed

Question 34

What are the disadvantages of shortest remaining time?

Answer 34

1. Processor starvation if short jobs are continuously added to queue
2. Does not account for job priority or urgency