1.2.1 Systems Software

Question 1

What is the OS?

Answer 1

• A piece of software held in permanent storage that controls the software and hardware of the computer
• Acts as a bridge between the user and computer’s hardware as the user cannot communicate with the hardware directly

Question 2

Why is the OS needed?

Answer 2

• Provides the interface between the user and the hardware
• Provides a platform in which users can run programs

Question 3

What are utility programs?

Answer 3

• Some are built into the OS that help maintain the computer
• Encryption, Compression and Defragmentation software

Question 4

Features of the OS: Multitasking

Answer 4

• When you more than one program opened and running at the same time
• Processor allocates a small amount of time to each process and cycles between them

Question 5

Features of the OS: File management

Answer 5

• Data is stored in files and the extension of the filename tells the OS which application to load the file into
• Allows user to rename, delete, copy and move files
• Client-server network may impose a fixed/roaming profile and manage login requests to the network

Question 6

Features of the OS: User management

Answer 6

• Allows users to log onto the same computer
• OS retains settings for each user as each user may have different access rights to files and programs

Question 7

Features of the OS: User interface

Answer 7

• WIMP
• Visual
• Intuitive
• Optimised for touch and mouse gesture input

Question 8

What is paging?

Answer 8

• Where memory is split into equal sized sections (pages)
• Physical divisions in which programs are split up to fit in a given number of pages

Question 9

What is an advantage of paging?

Answer 9

• Allows memory to be allocated in a non-contiguous manner - pages of the same process don’t need to be stored together but can be allocated whenever there’s free space in memory

Question 10

What is a disadvantage of paging?

Answer 10

• Inefficient: could separate instructions inside a looping condition and are in different pages

Question 11

What is segmentation?

Answer 11

• Memory is spit into logical sized divisions known as segments, which are complete sections of programs and vary in size

Question 12

What are the advantages of segmentation?

Answer 12

• Segments do not need to be stored continuously across a fixed address space
• Can be moved in and out of memory as required

Question 13

What are the similarities between paging and segmentation?

Answer 13

• Both allow programs to run despite insufficient memory
• Stored on disk
• Both pages and segments are transferred into RAM when needed

Question 14

What are the differences between paging and segmentation?

Answer 14

• Pages are fixed size, Segments vary in size
• Pages are made to fit sections of memory, segments are complete sections of programs
• Pages are physical divisions, segments are logical divisions

Question 15

What is virtual memory?

Answer 15

• An allocated area of a hardware/secondary storage device which is used when there’s not enough physical RAM to store open programs

Question 16

What is an advantage of virtual memory?

Answer 16

• Extends memory available: swapping instructions and data out of RAM into secondary storage means a vast quantity of memory is potentially available

Question 17

What are the disadvantages of virtual memory?

Answer 17

• Accessing a secondary storage medium is slower than accessing RAM, may slow down performance
• Disk thrashing: When the computer ‘freezes’ due to pages/segments being swapped out too frequently between the hard disk and RAM

Question 18

What is an interrupt?

Answer 18

• A way of signaling the processor when other devices and applications require attention

Question 19

What is the Interrupt Service Routine (ISR)?

Answer 19

Program with a set of instructions that need to be fetched, decoded and executed in order to carry out the operations of the interrupt

Question 20

Where can ISRs be found?

Answer 20

Built into the OS or provided via device drivers

Question 21

Why may the processor be interrupted?

Answer 21

• Hardware device has signaled it has data to be processed
• Hardware failure has occurred and needs to be addressed
• Software process needs a service to be provided/ OS function needs to be performed

Question 22

Are ISR priorities important?

Answer 22

Yes, interrupts always have a higher priority than normal programs

Question 23

Examples of interrupts for hardware

Answer 23

• Power supply failure
• Power/reset button pressed

Question 24

Examples of interrupts for the user

Answer 24

• Keyboard presses
• Tapping an icon to load a new program

Question 25

Examples of interrupts for software

Answer 25

- Arithmetic overflow - New log on request

Question 26

Examples of interrupts for I/O devices

Answer 26

- Printer ink supply notification - Buffer nearly empty

Question 27

What is scheduling?

Answer 27

- Part of the OS that is responsible for managing the order of processes and how they will be managed to achieve multitasking

Question 28

What does a scheduler manage?

Answer 28

- Which process to execute next - The length of time the next process can execute for

Question 29

How does the First Come, First Served scheduling algorithm work?

Answer 29

- Like a supermarket queue / first job is the first to enter the running state - Processes are executed in the order they arrive - If a process takes a long time, the others behind have to wait - Not pre-emptive

Question 30

How does the Shortest Job First scheduling algorithm work?

Answer 30

- Picks the process that takes the shortest amount of time and runs them until they finish - Scheduler needs to know how long each process will take - Not pre-emptive

Question 31

How does the Round Robin scheduling algorithm work?

Answer 31

- Each process is allocated a fixed amount of time (quantum) - If the process is not complete by the end of its time slice, it returns to the back of the ready queue - Pre-emptive

Question 32

How does the Shortest Remaining Time scheduling algorithm work?

Answer 32

- Ready queue is sorted by the estimated time to complete the process - Processes that arrive with a shorter time to complete the current running process are moved to the running state - Pre-emptive

Question 33

How does the Multi-Level Feedback Queues scheduling algorithm work?

Answer 33

- Several ready queues are used, each with a different scheduling algorithm - Jobs are able to move between queues as their priorities change - Pre-emptive

Question 34

What are the different types of operating systems?

Answer 34

- Distributed OS - Embedded OS - Multitasking OS - Multiuser OS - Real Time OS

Question 35

What is the purpose of the distributed OS?

Answer 35

- You can combine the processing power of multiple computers across a network for a single task - In distributed computing, the OS controls and co-ordinates the computers, presenting them to the user as if they were a single system

Question 36

What is the purpose of the embedded OS?

Answer 36

- Specialised OS with limited resources and functionality, built in to control a single machine - Tend to run on dedicated hardware so they run with maximum efficiency, using low-powered processors

Question 37

What is the purpose of the multitasking OS?

Answer 37

- Each active program is scheduled to receive a tiny time slice in quick rotation, giving the impression they are operating at the same time

Question 38

What is the purpose of the multiuser OS?

Answer 38

- Provides the facilities for multiple users to access the same system - Controls the consumption of resources so users can access the system at the same time, without affecting other users

Question 39

What is the purpose of the Real Time OS?

Answer 39

- Processes are guaranteed to executed during a known time frame - Processes in real time OS rarely run at capacity

Question 40

What is the BIOS (Basic Input Output System?

Answer 40

- Responsible for loading the OS when the computer first turns on - Checks all hardware it needs is connected and working using a Power On Self Test

Question 41

What is the purpose of the boot loader program (bootstrap)?

Answer 41

- Used to load the OS kernel into memory

Question 42

What would happen without the BIOS?

Answer 42

- Nothing could be loaded as RAM doesn't contain any instructions that could start the boot program

Question 43

Where are the BIOS settings stored?

Answer 43

- In flash memory so they can be changed and are retained when power is turned off

Question 44

What is a device driver?

Answer 44

- Software that tells the OS how to communicate with a device - Translates the OS instructions into a series of instructions that a specific piece of hardware will understand

Question 45

What is a virtual machine?

Answer 45

- Program that has the same functionality as a physical computer

Question 46

What is an emulator?

Answer 46

A hardware or software that enables one computer system (host) to behave like another computer system (guest)

Question 47

What is intermediate code?

Answer 47

Code partly translated between high level and machine language produced by a compiler

Question 48

State some uses of virtual machines

Answer 48

- Creating a test system - Running software compatible with different versions and types of OS - Running multiple servers

Question 49

How is intermediate code executed?

Answer 49

- At run time, the og program is translated to IC which is passed to the VM - Using the VM as an intermediary between the system and source code allows the same code to be used on any program that can run the VM

Question 50

What is the purpose of the real time OS?

Answer 50

- An OS where data is processed as it comes, with responses generated with a guaranteed timeframe - Used in safety critical environments, e.g. autopilot, hospitals