1.2.1 Systems Software

Question 1

Operating system

Answer 1

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

Question 2

Functions of the OS

Answer 2

• 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)

Question 3

Utilities

Answer 3

• 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

Question 4

Memory management

Answer 4

• 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]

Question 5

Memory management techniques

Answer 5

• paging
• segmentation
• virtual memory

Question 6

Paging

Answer 6

• 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

Question 7

Paging benefits + drawbacks

Answer 7

• 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

Question 8

Segmentation

Answer 8

• 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

Question 9

Segmentation advantages + disadvantages

Answer 9

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

Question 10

PAGING VS SEGMENTATION similarities

Answer 10

• 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]

Question 11

PAGING VS SEGMENTATION differences

Answer 11

• 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

Question 12

Virtual memory

Answer 12

• 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]

Question 13

Virtual memory advantages

Answer 13

• allows more extensive programs to be run
• facilitates effective multitasking

Question 14

Virtual memory disadvantages

Answer 14

• 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]

Question 15

What is an interrupt?

Answer 15

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

Question 16

Interrupt service routine

Answer 16

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]

Question 17

Scheduling + why is it necessary ?

Answer 17

• 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

Question 18

Pre-emptive

Answer 18

• assigns the CPU to a process for a limited amount of time
• allows interruption of processes currently being handled
• it can result in low priority processes being neglected if high priority processes arrive frequently
• e.g round robin , STRF, multi level feedback queues

Question 19

Non pre-emptive

Answer 19

• 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

Question 20

Round robin

Answer 20

• 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]

Question 21

Round robin advantages

Answer 21

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

Question 22

Round robin disadvantages

Answer 22

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

Question 23

First come first served

Answer 23

• 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]

Question 24

First come first serve advantages+ disadvantages

Answer 24

• 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

Question 25

Multi level feedback queues

Answer 25

- 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

Question 26

MLFQ advantages

Answer 26

- shorter task are prioritised - helps avoid starvation by allowing processes to move between queues

Question 27

MLFQ disadvantages

Answer 27

difficult to implement/more complex than other algorithms

Question 28

Shortest job first

Answer 28

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

Question 29

SJF advantages

Answer 29

- minimise waiting time - efficient and fast for short processes

Question 30

SJF disadvantage

Answer 30

- requires knowing the burst of processes in advance - long processes can starve if short processes keep arriving

Question 31

Shortest time remaining First (strf)

Answer 31

- jobs with the shortest time remaining are processed first - If a shorter job arrives while a process is running, the current process will be preempted and requeued until it has the shortest remaining time.

Question 32

Strf Advantages

Answer 32

Ideal for jobs with short burst times

Question 33

Strf disadvantages

Answer 33

- can lead to processor starvation/ longer processes can starve if shorter processes keep arriving

Question 34

Types of OS

Answer 34

- distributed OS - Embedded OS - multi-tasking OS - multiuser OS - real-time OS

Question 35

Distributed OS

Answer 35

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

Question 36

Embedded OS

Answer 36

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

Question 37

Multitasking OS

Answer 37

runs multiple programs at the same time [1]

Question 38

Multi-user OS

Answer 38

- allows multiple users to access computer resources at the same time [1]

Question 39

Real time OS

Answer 39

- 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

Question 40

BIOS (basic input output system)

Answer 40

- 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]

Question 41

Device drivers

Answer 41

- 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

Question 42

Virtual machines

Answer 42

- 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

Question 43

Virtual machines uses

Answer 43

- enables multiple OS to be run on a single machine - can be used to create isolated test environments that leave the host operating system unaffected, debugging and development without affecting the main system [1]