1.2.1 System Software pt 1

Question 1

Operating System

Answer 1

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

Question 2

Do you interact with the OS?

Answer 2

Only via applications

Question 3

OS Functions

Answer 3

User interface, memory management, interrupt management, processor scheduling

Question 4

What does a user interface do?

Answer 4

Hides the complexity of the hardware by providing an interface for the user to interact with

Question 5

4 types of user interface

Answer 5

Graphical User Interface
Command Line Interface
Menu-based Interface
Voice Interface

Question 6

Graphical User Interface (GUI)

Answer 6

A computer interacts with a series of graphics
e.g. iPhones
Convenient, easy to use, intuitive

Question 7

Command Line Interface

Answer 7

The user has a command line they can use
e.g. Command window in windows OS
Text-based, more power to use more commands, can use batch processing

Question 8

Menu-based Interface

Answer 8

The user can interact with a series of menus
e.g. ATM/PIN machine
Secure, limited functions, easy to use

Question 9

Voice Interface

Answer 9

You interact using your voice
e.g. Amazon Alexa/ google home
Can interact anywhere, easy, hands-free

Question 10

Paging

Answer 10

1. Memory is divided into fixed-size chunks (pages)
2. Each process is allocated x amount of pages - still 1 page if less than page size but may not be contiguous
3. A page table maps logical memory and physical memory location

Question 11

Contiguous

Answer 11

Where all the memory for a process is next to each other

Question 12

Paging advantages and disadvantages

Answer 12

+ Improves efficiency of space as data can be stored in all available space

• A whole page may not be full with a process
• Can increase reading time as data is separated

Question 13

Segmentation

Answer 13

Memory is divided into segments which can be of different lengths
All segments of the same process are contiguous
Could relate to part of a program

Question 14

Segmentation advantages and disadvantages

Answer 14

+ All memory for a process is together

• The remaining space may be smaller than the size of the segment required
• Swapping is more difficult

Question 15

Virtual Memory

Answer 15

An area of the hard disk drive is designated as virtual memory
Some current pages are stored there if RAM is full and are swapped back in when needed

Question 16

Swapping

Answer 16

Moving pages between RAM and Virtual Memory

Question 17

Disk Thrashing

Answer 17

Repeatedly swapping pages can slow down the computer as more time is spent swapping pages than running code

Question 18

Interrupt

Answer 18

• A signal to the CPU that something needs dealing with
• Can be sent by software, hardware or internal clock
• Stored in a priority queue

Question 19

Parity Error

Answer 19

An error caused by irregular changes in data - caused by electromagnetism

Question 20

Arithmetic Overflow

Answer 20

Where the answer to the calculation is bigger than the memory space available e.g. /0

Question 21

Buffer

Answer 21

A region of memory to temporarily store data

Question 22

Interrupt cycle

Answer 22

1. F/D/E
2. Are there any higher priority interrupts to be processed? No- F/D/E again
3. Push registers on memory stack
4. Load interrupt service routine into program counter and sets a flag to show the ISR has begun
5. F/D/E on interrupt
6. Are there any higher priority interrupts to be processed? No- pop register data off of the stack

Question 23

How do interrupts work?

Answer 23

They have different priorities and are processed in order of priority
Interrupts can be interrupted if a new interrupt has a higher priority

Question 24

What happens if an interrupt is interrupted?

Answer 24

The registers for the interrupt are also pushed onto the stack
They are retrieved on a last-in first-out basis

Question 25

Why is scheduling needed?

Answer 25

A single-core CPU can only process instructions for one application at a time so the OS must schedule when each process uses the CPU

Question 26

Pre-emptive

Answer 26

The job that is running can be interrupted

Question 27

What is scheduling used for?

Answer 27

a) Provide an acceptable response time to all users
b) Maximise the time the CPU is fully engaged
c) To ensure fairness on a multi-user system

Question 28

Round Robin (RR)

Answer 28

Each process is allocated a maximum amount of processor time, at which stage it is put back on the ready queue
Very inefficient for long jobs
Pre-emptive

Question 29

First-come First-served (FCFS)

Answer 29

The first job to enter the ready queue is the first to enter the running state
Not pre-emptive

Question 30

Shortest remaining time (SRT)

Answer 30

The ready queue is sorted based on how long each time each process has left, with the shortest running
The time for each process only has to be estimated when it enters the queue
Pre-emptive

Question 31

Shortest job first (SJF)

Answer 31

The user estimates how long each job will take and the shortest is run to completion
Suited to batch systems
Non pre-emptive

Question 32

Multilevel Feedback Queues (MLFQ)

Answer 32

• Multiple queues are created with different priority levels - jobs move between as their priority changes
• Jobs move down if they have uses up a lot of CPU time or up if they haven’t been recently used
• Pre-emptive

Question 33

Starvation

Answer 33

Where a job has to wait a long time before it gets running on the CPU

Question 34

Fragmentation

Answer 34

When memory or storage is used inefficiently, reducing capacity or performance

Question 35

Intermediate code

Answer 35

Code between machine code and object code, independent of the processor architecture. Used to create a development environment to test programs