Chapter 7 - Functions of an Operating System (1.2)

Question 1

What is an operating system?

Answer 1

program/set of programs that manages the operations of the computer for user

Question 2

Where is the operating system held?

Answer 2

permanent storage

Question 3

What are some functions of an operating system? (6)

Answer 3

1. provides a user interface
2. memory management
3. interrupt service routines
4. processor scheduling
5. backing store management
6. management of all input and output

Question 4

What is paging?

Answer 4

When memory is split up into equal sized sections known as pages, can be swapped between main memory and the hard disk as needed

Question 5

What is segmentation?

Answer 5

Splitting up memory in segments which can vary in size, represents the structure and logical flow of the program

Question 6

What is virtual memory?

Answer 6

Uses a section of the hard drive in RAM when there’s no more space in main memory to run apps, moves rarely used programs here through paging which frees up memory for other programs in RAM

Question 7

What’s the issue with paging, segmentation and virtual memory?

Answer 7

Disk thrashing (when pages are swapped too frequently between the hard disk and main memory, causing the computer to slow down)

Question 8

Describe the process of interrupts.

Answer 8

At the end of the fetch decode execute cycle (during the execute phase), the processor checks for any interrupts in the interrupt register. if there are any, if the priority of the interrupt is higher than the priority of the current instruction being executed, the current instruction’s contents are pushed onto a stack and the interrupt is executed. If the interrupt is of same or lower priority, its contents are pushed onto a stack and is fetched once the current instruction’s contents are executed. If an interrupt of higher priority is in the interrupt register when the first interrupt has been executed, the first interrupt’s contents are pushed onto a stack and the second interrupt is executed. Stacks work in a FIFO (first in first out) order, so the first interrupt’s contents would be put back into registers before the original instruction.

Question 9

What is a scheduler?

Answer 9

The operating system module responsible for making sure that processor time is used as efficiently as possible

Question 10

Scheduling algorithms can either be..? (2)

Answer 10

Pre-emptive (jobs are actively made to start and stop by the operating system)
Non pre-emptive (once a job is started, it’s left alone until completion)

Question 11

Describe the round robin scheduling algorithm.

Answer 11

Each job is given​ a time slice​ within which it is allowed to execute. If the process hasn’t completed before before the time slice ends or a higher priority interrupt occurs, the OS gives the CPU the next process. Once each job in the queue has used its first time slice, the OS again grants each job a time slice, until a job has been completed, where it’s then removed from the queue.

Question 12

What is an advantage of the round robin scheduling algorithm?

Answer 12

Ensures every job is executed.

Question 13

What are 2 disadvantages of the round robin scheduling algorithm?

Answer 13

1. Longer jobs will take way more time to complete due to it being inefficiently split into multiple cycles
2. Doesn’t take into account job priority

Question 14

Describe the first come first served scheduling algorithm.

Answer 14

Jobs are processed in the order in which they arrive.

Question 15

What is a disadvantage of the first come first served scheduling algorithm?

Answer 15

Doesn’t take into account job priority.

Question 16

Describe the shortest remaining time scheduling algorithm.

Answer 16

The queue storing jobs to be processed is ordered according to the time left for completion, jobs with the least time to completion are executed first. If a job with a shorter remaining time joins the queue, the current job being executed is pushed back into the queue.

Question 17

What is a disadvantage of the shortest remaining time scheduling algorithm?

Answer 17

Risk of processor starvation if shorter jobs keep on getting added, leaving the longer jobs with no execution time

Question 18

Describe the shortest job first scheduling algorithm.

Answer 18

Process with the estimated shortest running time is run next until execution

Question 19

What system is the shortest job first algorithm suited for?

Answer 19

Batch systems.

Question 20

What are some disadvantages of the shortest job first algorithm? (2)

Answer 20

1. Processor won’t always be able to estimate how long each job will take
2. Risk of processor starvation for longer jobs if shorter jobs keep getting added

Question 21

Describe the multi-level feedback queues scheduling algorithm.

Answer 21

Uses multiple queues, with jobs moving between queues depending on how much processor time they use

Question 22

What is an advantage of multi-level feedback queues?

Answer 22

Maximises processor use and prevents bottlenecks from occurring

Question 23

What is a disadvantage of multi-level feedback queues?

Answer 23

Can be difficult to implement due to deciding which job to prioritise

Question 24

What is backing store management?

Answer 24

Transferring files and applications from backing storage (e.g. hard drive) into memory

Question 25

What is peripheral management?

Answer 25

The operating system communicating with peripherals (e.g. printers or keyboards)