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OCR A2 Computing F453 > Functions of Operating Systems > Flashcards

Functions of Operating Systems Flashcards

1
Q

What is an operating system?

A

A piece of software that provides a platform on which the applications software can run. It controls the hardware and provides communication with the outside world.

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2
Q

What is an interrupts?

A

Messages sent to the processor by some external entity asking them to stop what they are doing and do another job.

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3
Q

What is a scheduler?

A

A program, or set of instructions, used by the OS to decide where each of the jobs which are in its control should be and in what order to manipulate.

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4
Q

What is a High-Level Scheduler(HLS)?

A

Scheduler that places the jobs in the ready queue, and ensures that the system never gets overloaded.

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5
Q

What is the definition for a Medium-Level Scheduler(MLS)?

A

Scheduler that swaps jobs between main memory and backing store.

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6
Q

What is a Low-Level Scheduler(LLS)?

A

Moves jobs in and out of the ready state.

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7
Q

What is a Pre-Emptive Scheduler?

A

Scheduler that is allowed to move jobs out of the ready state, and decides which order to place them in the running state.

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8
Q

What is a Non-Pre-Emptive Scheduler?

A

Scheduler that does not have the power to move jobs until they are finished.

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9
Q

What is First Come First Serve (FCFS)?

A

A priority method where the first job to enter the running queue is the first job to enter the running state. It favours long jobs.

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10
Q

What is Shortest Job First (SJF)?

A

A priority method where jobs are sorted in the ready queue in ascending order of time needed.`

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11
Q

What is Round Robin (RR)?

A

A priority method which gives each job a maximum length of processor time, it will then go to the back of the queue again.

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12
Q

What is Shortest Remaining Time (SRT)?

A

A priority method were the ready queue is sorted on the amount of expecting time still to do.

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13
Q

What is a Multi-Level feedback queue?

A

A priority method which involves several queues of different priorities with jobs migrating

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14
Q

What is the purpose of an operating system?

A
  • Provide and manage hardware resources
  • Provide an interface between the user and the machine
  • Provide an interface between application software and the machine
  • Provide security for data on the system
  • Provide utility software to allow maintenance to be done on the system
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15
Q

What are 4 types of interrupts?

A
  • I/O
  • Timer
  • Hardware error
  • Program error
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16
Q

What is the purpose of scheduling?

A
  • to ensure all jobs are processed/changing priorities where necessary
  • process as many jobs as possible in the least possible time
  • maximise number of interactive users receiving fast response times
  • utililise resources/processor time efficiently
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17
Q

What is a linker?

A

A utility program included in the OS which links parts of the program together through the memory

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18
Q

What is a loader?

A

A small utility program that loads jobs into memory and adjusts the addresses.

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19
Q

What are pages?

A

Equal sizes sections of memory.

20
Q

What are segments?

A
  • A way of partitioning memory
  • Segments are not fixed size
  • Segments are logical divisions…
  • …which hold complete sections of programs.
21
Q

What is disk threshing?

A

Where code contains many jump instructions so the processor spends most of its time switching between virtual memory and main memory. It involves the disk continually searching for pages.

22
Q

What is virtual memory?

A
  • use of backing store…
  • …as additional memory
  • uses paging / swapping pages (between memory & backing store)
  • holds part of the program not currently in use
  • allows large programs to run (when memory size is insufficient)
23
Q

What is spooling?

A

Where an I/O device is slower than the processor so the job is moved into another storage location while being used. This is kept track of using a spool queue which references the jobs. It also allows for priorities.

24
Q

What does FAT stand for?

A

File Allocation Table

25
Q

What is the File Allocation Table?

A

A table that uses a linker to point to the clusters on the disk that contain files.

  • A map of where files are stored…
  • …in backing store/hard disk
26
Q

How is a File Allocation Table set up and used?

A
  • The disk must first be formatted, done by dividing the disk into sectors and into concentric circles called tracks.
  • To find a file, the OS looks for the file name, and gets the corresponding cluster number, which can be used to find the file.
  • When a file is deleted, the clusters that were used to find the file can be set to zero.
27
Q

What makes up a cluster?

A

Two or more sectors on a single track make up a cluster.

28
Q

What is a POST routine?

A

Power on self-test.

  • resides in the permanent memory
  • clears the registers
  • loads the address of the instruction into the program register
  • sends signals to the necessary hardware to check it’s all there and working properly
29
Q

What is the BIOS?

A

Basic Input Output System.

- instructs the computer on how to perform a number of basic functions such as booting and keyboard control

30
Q

What is CMOS RAM?

A

Part of RAM that stores the BIOS.

31
Q

What does the boot program do?

A
  • Gets the system ready to accept an operating system.

- Provides personal settings

32
Q

Where is the boot program stored?

A

ROM

33
Q

What is the boot file?

A

A file that contains basic parameters to which the system will operate. It can be altered.

34
Q

What are the benefits of spooling?

A
  • Keeps output of different queues separate
  • Saves the user having to wait for the processor
  • Lets the processor get on with other jobs while others are queued
35
Q

What is the purpose of interrupts?

A
  • To obtain processor time for a higher priority task
  • To avoid delays
  • To avoid loss of data
  • As an indicator to the processor that a device needs to be serviced
36
Q

Why is memory management necessary?

A
  • To allocate memory to allow separate processes to run at the same time to deal with allocation when paging/segmentation
  • To reallocate memory when necessary
  • To protect processes/data from each other
  • To protect the operating system/provide security to enable memory to be shared
37
Q

What problem may occur from paging and segmentation?

A

Disk Threshing

38
Q

Why may a jobs priority be changed by the operating system?

A
  • to allow it to be processed

- otherwise if higher priority jobs keep occurring it would have to wait indefinitely

39
Q

How is an interrupt detected during the fetch-execute cycle?

A
  • Interrupt register is checked…
  • when each cycle completed
  • by comparing priority of the current task with interrupt
    register
40
Q

State the steps that need to be taken before resuming processing after an interrupt.

A
  • reset flag(s) to inactive state
  • check for further interrupts…
  • … & service them if necessary
  • restore contents of registers (from stack)
41
Q

How are paging and segmentation similar?

A
  • ways of partitioning memory
  • allow programs to run despite insufficient memory/used for virtual memory
  • segments and pages are stored on backing store
  • segments and pages are assigned to memory when needed
42
Q

How are paging and segmentation different?

A
  • segments are different sizes but pages are fixed size
  • segments are complete sections of programs, but pages are made to fit sections of memory
  • segments are logical divisions, pages are physical divisions
43
Q

What problems can occur when using paging and segmentation?

A
  • disk threshing
  • more time spent swapping pages than processing
  • computer may ‘hang’
44
Q

When is the boot file used?

A
  • while the operating system is loading
  • when the computer is switched on
  • after POST
45
Q

Explain the purpose and use of FAT.

A
  • a map of where files are stored…
  • …in backing store/hard disk
  • provides addresses/pointers to (start of) files
  • stores file names
  • stores file sizes
  • stores access rights
  • identifies free space
  • is updated by the operating system when files are saved/deleted
  • Is used by the operating system when files are accessed
46
Q

Describe how a data structure is used during the servicing of an interrupt.

A
  • Stack
  • To store the contents of the registers
  • To return values to registers
  • so that processing can resume after the interrupt has been serviced
47
Q

What is the purpose of virtual memory?

A

To allow programs to run when there is insufficient

memory available.

OCR A2 Computing F453 (9 decks)

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