1.2.1. Systems Software (Chp 3)

Question 1

Operating Systems

Answer 1

The terms refers to a collections of programs that work together to provide an interface between the user and computer.

• enables the user to communicate with the computer and perform certain low-level tasks involving the management of memory and resources.
• provides user interface between the user and hardware
• provides a platform in which applications can run.

Question 2

OS features

Answer 2

-memory management (paging, segmentation, VM)
-resource management(scheduling)
-File management
Input/output management (device drivers)
-Interrupt management
-Utility software (disk defragmenter, backup)
-security
-user interface

Question 3

Memory management

Answer 3

• computer memory must be shared fairly between multiple programs and applications being used. One of the roles of the OS is to determine and allocate the length of processor time each task receives through the use of paging, segmentation and VM

Question 4

Paging

Answer 4

when memory is split up into equal-sized sections known as pages, with programs being made up of a certain number of equally-sized pages. These can then be swapped between main memory and the HD as needed.
Pages are physical divisions, equal sizes, made to fit memory.

Question 5

Segmentation

Answer 5

splitting up the memory into logical sized divisions known as segments, which vary in size.
These are representative of the structure and logical flow of the programs.
Segments are logical divisions, different sizes, complete sections of programs

Question 6

Similarities between paging and segmentation

Answer 6

-both allow programs to run despite insufficient memory
pages and segments are stored on a disk
pages and segments are transferred into memory when needed.

Question 7

Virtual Memory

Answer 7

VM uses a section of the hard drive to act as RAM when the space in main memory is insufficient to store programs being used.
Sections of programs that aren’t currently in use are temporarily moves into virtual memory through paging freeing up memory for other programs.

Question 8

Key issue with VM, paging and segmentation

Answer 8

Disk thrashing - when the computer freezes and occurs as a result of pages being swapped too frequently between the HD and main memory. As a result more time is spent transferring the pages than is spent actually running the programs.
The issue becomes progressively worse as VM is filled up. If this continues programs will likely lock up and not run.
Improved by updating OS or getting more RAM

Question 9

What is an Interrupt?

Answer 9

Interrupts are signals sent through the control bus to indicate to the processor that a process needs attention.
Stored in order of their priority in a priority queue located in a special register called an interrupt register.

Question 10

Interrupt Service Routine (ISR)

Answer 10

1. The processor checks the contents of the interrupt register at the end of each FDE cycle.
2. If an interrupt exists of a higher priority than the process being executed, the current contents of the special purpose registers in the CPU are temporarily transferred into a stack.
3. A flag is sent to signal that the ISR has begun
4. Once interrupt serviced, flag is reset.
5. The Interrupt queue is checked again. If there are more interrupts of a higher priority, the ISR is repeated until all interrupts of higher priority have been serviced.
6. If there are no more, the contents of the stack are transferred back into the registers in memory.
7. The FDE cycle resumes as before.

Question 11

What is Round Robin (scheduling)

Answer 11

Each job is given an equal section of processor time - known as a time slice - within which it s allowed to execute. Once each job has used its first time slice, the OS again grants each job a time slice. This continues until a job has been completed at which point it is removed from the queue.

Question 12

Round Robin disadvantages

Answer 12

• longer jobs will take a much longer time for completion due to their execution being inefficiently split.
• This doesn’t take into account of priorities.

Question 13

What is First Come First Served

Answer 13

Jobs are processed in chronological order by which they entered the queue.

Question 14

Disadvantages of First Come First Served

Answer 14

Although this is straight forward to implement, it doesn’t take into account priority.

Question 15

Multi-level Feedback queues

Answer 15

Makes use of multiple queues which is ordered based on priority

Question 16

Multi-level Feedback queues disadvantages

Answer 16

This can be difficult to implement due to deciding which job to prioritise based on a combination of priorities
(Adv = services most interrupts first)

Question 17

Shortest job first

Answer 17

The queue storing jobs to be processed is ordered according to the time required for completion, with the longest jobs being serviced at the end.
This type is most suited to batch systems, where shorter jobs are given preference to minimise waiting time.

Question 18

Shortest job first disadvantages

Answer 18

• requires the processor to know or calculate how long each job will take and this is not always possible.
• also risk of processor starvation if shorter jobs continue being added to the job queue.

Question 19

Shortest time remaining

Answer 19

the queue storing jobs to be processed is ordered according to the time left for completion, with the jobs with the least time left for completion being serviced first.

Question 20

shortest time remaining disadvantages

Answer 20

• risk of processor starvation for longer jobs if short jobs are added to the job queue.
• requires the processor to know or calculate how long left each job has left for completion which is not always possible.

Question 21

Distributed OS

Answer 21

type of OS which run across multiple devices allowing the load to be spread across multiple computer processors when a task is run. e.g. internet

Question 22

Embedded OS

Answer 22

OS built to perform a small range of specific tasks, this OS is catered towards a specific device. They are limited in their functionality.
-hard to update although they consume significantly less power than other types of OS.

Question 23

Why is scheduling necessary / needed

Answer 23

• To give each task a sufficient amount of processor time/ memory in order to execute to completion
• reduces risk of processor starvation
• uses resources efficiently
• ensures that they are processed fairly
• so there is a quick response time