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Computer Science A Level OCR > 1.2.1 Systems Software > Flashcards

1.2.1 Systems Software Flashcards

1
Q

Operating system

A
  • a collection of programs that provide an interface between the user and computer
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2
Q

Functions of the OS

A
  • provides a user interface [1]
  • manages/controls hardware [1]
  • provides a platform for software to run [1]
  • handles interrupts [1]
  • provides security [1]
  • provides utilities for system maintenance [1]
  • input/output management (device drivers)
  • file management (moving editing deleting files and folders)
  • resource management (scheduling)
  • memory management (paging segmentation and virtual memory)
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3
Q

Utilities

A
  • software program with a specific function linked to the maintenance of the OS [1]
  • file encryption : encodes files before transfer allowing users to send files over networks securely
  • file compression : reduces file size to take up less storage space or for faster transfer over a network [1]
  • disk defragmentation : physically reorganises files on a hard disk so they can be found and accessed faster [1]
  • backup software : creates system snapshots and will restore them on demand /makes regular copies of files In case of loss [1]
  • disk clean up: scans the Hard disk for duplicate or corrupted files and removes them freeing up hard disk space
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4
Q

Memory management

A
  • is a fundamental role of the OS
  • deals with the allocation and deallocation of RAM to ensure RAM is used efficiently and not wasted [1]
  • benefit : enables multitasking allowing multiple programs to run at once [1]
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5
Q

Memory management techniques

A
  • paging
  • segmentation
  • virtual memory
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6
Q

Paging

A
  • main memory is split into fixed sizes sections called pages [1]
  • uses physical divisions
  • pages are swapped between main memory and hard disk as needed
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7
Q

Paging benefits + drawbacks

A
  • benefits : facilitates efficient memory management and enable use of virtual memory
  • programs/data can be held in non contiguous memory locations [1]
  • to avoid having to move content around to fit in new programs
  • drawbacks : can lead to internal fragmentation (pockets of wasted space within pages)
  • unused space in pages is wasteful as other unrelated data cannot be stored on this page
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8
Q

Segmentation

A
  • divides main memory into logical sized divisions called segments which can vary in size [1]
  • are complete sections of programs
  • segments represent the structure and logical flow of the program
  • eg. In a video editing application different segments may be created for video data, audio data, effects and UI elements
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9
Q

Segmentation advantages + disadvantages

A
  • benefits : is space efficient due to only allocating space depending on the amount an application needs
  • drawbacks : can result in external fragmentation [1]
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10
Q

PAGING VS SEGMENTATION similarities

A
  • both allow programs to run despite insuffienct memory [1]
  • both are ways of splitting up memory [1]
  • both allow programs to be stored non continuously in memory [1]
  • both pages and segments use virtual memory (are stored on a disk) [1]
  • both pages and segments are transferred into memory when needed [1]
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11
Q

PAGING VS SEGMENTATION differences

A
  • paging uses fixed sized memory blocks , segmentation uses variable length memory block [1]
  • paging uses physical divisions, segmentation use logical divisions [1]
  • pages are made to fit sections of memory , segments are complete sections of programs
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12
Q

Virtual memory

A
  • virtual memory is a section of hardrive/secondary storage device [1]
  • when memory intensive applications exceed available RAM
  • the OS moves less frequently accessed pages to virtual memory [1]
  • this frees up space for other programs in RAM [1]
  • pages are transferred back into RAM from virtual memory as needed [1]
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13
Q

Virtual memory advantages

A
  • allows more extensive programs to be run
  • facilitates effective multitasking
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14
Q

Virtual memory disadvantages

A
  • slower to access than physical memory which degrades performance if overused
  • can lead to disk thrashing, when the computer freezes due to pages being swapped too frequently between the hard disk and main memory [1]
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15
Q

What is an interrupt?

A

a signal to the processor indicating that a device/process needs attention [1]

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16
Q

Interrupt service routine

A
  1. The processor checks the interrupt register at the end of each FDE cycle [1]
  2. If an interrupt exists with a higher priority to the current process :
    - the current contents of the registers in the CPU are transferred into a stack [1]
    - the relevant ISR is loaded into RAM by loading the relevant value into the PC [1]
    - when the ISR is complete and if there are no interrupts with a higher priority to the current process [1]
    - the contents of the stack are popped/loaded back into the register [1]
    - the FDE cycle resumes [1]
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17
Q

Scheduling + why is it necessary ?

A
  • the method used by the OS to ensure all jobs get sufficient/fair processor time ensuring efficient use of resources /processor time [1]
  • as many jobs as possible are processed in the least possible it time [1]
  • scheduling algorithms can be pre- emptive or non-pre-emptive
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18
Q

Pre-emptive

A
19
Q

Non pre-emptive

A
  • Allocates the CPU for unlimited time slots
  • so once the CPU is given to a process that process keeps using it until it’s completed
  • a process can’t be interrupted unless it completes or it’s burst time is reached
20
Q

Round robin

A
  • is pre-emptive
  • each process is given a fixed/equal amount of time [1]
  • if the process hasn’t been completed by the end of its time quantum, it returns to the back of the ready queue [1]
21
Q

Round robin advantages

A
  • all processes/jobs will eventually be attended to
  • all process get an equal/fair share of the CPU
22
Q

Round robin disadvantages

A
  • choosing the right time quantum can be difficult
  • doesn’t take into account job priority/urgency
23
Q

First come first served

A
  • is non-pre-emptive
  • jobs are processed on a FIFO basis [1]
  • whichever process is received first , is executed first
  • each job is equal priority [1]
  • e.g for printer queue : all documents have equal priority [1]
  • whichever document is received first is printed first [1]
24
Q

First come first serve advantages+ disadvantages

A
  • benefits: is easy to implement
  • drawbacks: does not take into account job priority/urgency
  • short process can starve if a long process arrives before a short process
25
Q

Multi level feedback queues

A
  • is pre emptive
  • uses queues with different priorities [1]
  • jobs can be moved between queues [1] e.g if they exceed the time quantum
  • shorter and more critical task are processed first
26
Q

MLFQ advantages

A
  • shorter task are prioritised
  • helps avoid starvation by allowing processes to move between queues
27
Q

MLFQ disadvantages

A

difficult to implement/more complex than other algorithms

28
Q

Shortest job first

A
  • non- pre-emptive
  • process which has shortest time remaining is completed first [1]
29
Q

SJF advantages

A
  • minimise waiting time
  • efficient and fast for short processes
30
Q

SJF disadvantage

A
  • requires knowing the burst of processes in advance
  • long processes can starve if short processes keep arriving
31
Q

Shortest time remaining First (strf)

A
  • jobs with the shortest time remaining are processed first
  • if a job/task is not completed by the end of its time slice it will be requeued for further processing
32
Q

Strf Advantages

A

Ideal for jobs with short burst times

33
Q

Strf disadvantages

A
  • can lead to processor starvation/ longer processes can starve if shorter processes keep arriving
34
Q

Types of OS

A
  • distributed OS
  • Embedded OS
  • multi-tasking OS
  • multiuser OS
  • real-time OS
35
Q

Distributed OS

A

Allows multiple computers to work together on a single task (sharing the load) [1]

36
Q

Embedded OS

A

has a dedicated function and is read only/cannot be changed + stored in the ROM [1]

37
Q

Multitasking OS

A

runs multiple programs at the same time [1]

38
Q

Multi-user OS

A
  • allows multiple users to access computer resources at the same time [1]
39
Q

Real time OS

A
  • designed for immediate data processing and can ensure tasks are processed in specific/guaranteed time frames
  • commonly used in time critical computer systems eg. Automotive - self driving cars where low latency is critical to safety
40
Q

BIOS (basic input output system)

A
  • basic input output system is the first program that runs when a computer is switched on
  • is stored in ROM [1]
  • it performs a power on self test (POST) that ensures all the hardware components and working properly [1]
  • is used to call the bootstrap which boots up the operating system [1]
41
Q

Device drivers

A
  • A piece of software that enables communication between an OS and (external) hardware [1]
  • eg printers, graphics cards, network cards
  • are specific to the computers architecture
  • are specific to the OS
42
Q

Virtual machines

A
  • a theoretical computer and a software implementation of a computer system [1]
  • VMs are entitled OSs running inside another OS
  • running a VM helps access software that is only designed to run on specific OSs
43
Q

Virtual machines uses

A

Computer Science A Level OCR (16 decks)

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