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Computer Science Revision > Systems Software > Flashcards

Systems Software Flashcards

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1
Q

What is an Operating System (OS)?

A

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

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2
Q

What are the jobs of the Operating System?

A

Memory management
Resource management ie. scheduling
File management
Input/Output management
Interrupt management
Utility software
Security
User Interface

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3
Q

What is Memory management?

A

Making use of paging, segmentation and virtual memory to ensure memory is shared effectively by programs.

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4
Q

What is Paging

A

Memory is split up into equal-sized sections known as “pages”.
Pages are swapped between main memory and the hard disk as needed

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5
Q

What is Segmentation

A

Memory is split up into logically-sized divisions called segments.

These will vary in size equivalent to the number of lines of code.
For example, a while loop may have a larger segmentation than a variable assignment

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6
Q

What is Virtual Memory? (not virtual storage)

A

A section of the hard drive that acts as RAM when there isn’t enough space in main memory.

Sections of programs not being used are temporarily moved into VM through paging.

Frees up memory in RAM for other programs.

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7
Q

What is the issue with using Virtual Memory?

A

Disk Thrashing

  • when a computer “freezes” due to pages being swapped too frequently between hard disk and main memory.
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8
Q

What are Interrupts in memory management?

A

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

The process is then stored within a priority queue in an Interrupt register.

CPU checks the interrupt register at the end of each Fetch-Execute cycle.

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9
Q

What are the steps of the Interrupt Service Routine if an interrupt exists within a higher priority than the current process?

A
  1. Contents of the register transferred into the stack
  2. Relevant interrupt service routine (ISR) is loaded into RAM
  3. Flag is set to signal the ISR has begun.
  4. Flag is reset once the ISR is over
  5. Process is repeated
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10
Q

What are the steps of the Interrupt Service Routine if an interrupt DOES NOT exist within a higher priority than the current process?

A
  1. The contents of the stack are popped back into the registers
  2. Fetch-Execute cycle resumes
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11
Q

What is Scheduling?

A

Operating System ensure all sections of program being run receive a fair amount of processing time.

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12
Q

What are the two Scheduling algorithms?

A

Pre-emptive
Non pre-emptive

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13
Q

What happens in the Pre-emptive scheduling algorithm?

A

Jobs are actively made to start and stop by the operating system.

e.g.
Multilevel Feedback Queues, Shortest Remaining Time, Round Robin

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14
Q

What happens in the Non pre-emptive scheduling algorthm?

A

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

E.g.
First Come First Served, Shortest Job First

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15
Q

What happens in Round Robbin?

A

Each job is given a section of processor time.

Once each job in the queue has used its first time slice, they are given another slice until a job has been completed.

Completed jobs are removed from the queue.

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16
Q

What is an advantage of Round Robin

A

All jobs will eventually be attended to.

17
Q

What are the disadvantages of Round Robin

A

Longer jobs will take a much longer time for completion
Round Robin does not take job priority or urgency into account.

18
Q

What happens in First Come, First Served?

A

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

19
Q

What is the advantage of First Come, First Served?

A

Straightforward to implement

20
Q

What is the disadvantage of First Come, First Served?

A

Does not take job priority or urgency into account

21
Q

What happens in multilevel feedback queues?

A

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

22
Q

What is the advantage of Multilevel Feedback Queues?

A

Takes job priorities into consideration

23
Q

What is the disadvantage of Multilevel Feedback Queues?

A

Difficult to implement

24
Q

What happens in Shortest Job First

A

The queue storing jobs to be processed is ordered according to the time require for completion, with the longest jobs completed last.

25
Q

What is the advantage of Shortest Job First?

A

Suited to batch systems, as waiting time is reduced

26
Q

What are the disadvantages of Shortest Job First?

A

Requires processor to calculate how long each job will take

Processor starvation if short jobs are continuously added to the queue

Does not take job priority or urgency into account

27
Q

What happens in Shortest Remaining Time

A

The queue storing jobs to be processed is ordered according to the time left for them to be completed. Jobs with the least remaining time are serviced first.

28
Q

What is the advantage of Shortest Job First?

A

Throughput increases as shorter processes can be quickly completed

29
Q

What are the disadvantages of Shortest Job First?

A

Does not take job priority or urgency into account

Processor starvation if short jobs are continuously added to the queue.

30
Q

What are the different types of Operating System?

A

Distributed
Embedded
Multi-tasking
Multi-user
Real Time

31
Q

What is a Distributed OS?

A

Runs across multiple devices

Means the load of a task is spread across multiple processors

32
Q

What is a Embedded OS?

A

Built to perform a small range of specific tasks

Consumes less power than other types of OS, but is harder to update

33
Q

What is a Multi-tasking OS?

A

Enables user carry out tasks simultaneously

Time slicing is using to switch quickly between programs and applications in memory.

34
Q

What is a Multi-user OS?

A

Multiple users make use of one computer

A scheduling algorithm is used to allocate processor time fairly between jobs and prevent processor starvation.

35
Q

What is a Real Time OS?

A

Used in time-critical computer systems and designed to perform a task within a guaranteed time frame

Computer Science Revision (4 decks)

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