Operating Systems

Question 1

What is an operating system?

Answer 1

• Software on comp that manages its day to day use

- Deals with security, provides a GUI, manages the hard and the software of the system

Question 2

What is a multi-tasking OS?

Answer 2

• Allows multiple programs running simultaneously
• CPU’s can deal with more programs than they have cores
• CPU is much faster than peripherals so it creates illusion that multiple programs are running at once when in fact one instruction is executed at a time

Question 3

What is a multi user OS?

Answer 3

• More than one user can access computers resources at the same time
• Common on mainframe comps where multiple users will need to use it at same time

Question 4

What is a distributed OS?

Answer 4

• Controls multiple comps, presenting them to user as 1 system
• Used to tackle difficult tasks were more power is needed

Question 5

What is an Embedded OS?

Answer 5

• Only has one job and won’t need multitasking
• Designed for a specific device and will run efficiently on low CPU’s with little RAM
• eg: ATM or TV

Question 6

What is a real time OS?

Answer 6

• Used to carry out actions in real time
• eg: Hospital equipment such as heart rate monitors will alert the staff as soon as sudden changes occur
• Auto pilot, traffic lights

Question 7

What is a special feature of these OS’s

Answer 7

OS’s can have more than one of these properties. Most real time OS’s are also embedded

Question 8

What is the Kernel and what is its function?

Answer 8

• Core of an OS
• Manages the system, memory management and scheduling
• Allows apps to send and receive data from devices
• Lies below UI

Question 9

What does the memory manager allow?

Answer 9

• Programs to run simultaneously, programs to share memory to run at the same time
• Programs to share data if they have a valid reason

Question 10

What does the memory manager disallow?

Answer 10

• Programs to collide and overwrite one another
• Restricts Programs from accessing and amending there own area of data
• Malicious software from attacking the data of another program

Question 11

What two processes allow programs to split up across the memory if there is not room for the whole program?

Answer 11

Paging = Division of data into blocks of the same physical size. Each block is a page. OS uses page table to keep track of Where the pages are stored meaning all the pages of a process don't have to be stored together. More precise but not as efficient, used for smaller files and smaller sections of memory. Data would be good for paging. Page files usually 4kb
Segmentation = Logical division of data into blocks containing routines or modules. Programs would be good for segmentation as they need to be logically divided into segments

Question 12

What is virtual memory and why is it used?

Answer 12

• System takes area of hard drive and treats it as RAM
• OS identifies a page(s) in the RAM that are unlikely to be needed in the future and moves them to virtual memory. When needed they are moved back to physical memory
• Used when the RAM is low, it free ups space by moving pages or segments that are not likely to be used in the near future into the virtual memory
• RAM is expensive so instead of buying more this method is used
• Slower than actual RAM

Question 13

What is disc thrashing

Answer 13

• If RAM is full the OS can move pages back and fourth between RAM and VM often.
• Slows down system

Question 14

What is a schedular?

Answer 14

• Program that managers the time different processes have in the CPU
• They use algorithms to decide this

Question 15

What is the advantage of scheduling?

Answer 15

• Allows multiple process to be ran at the same time

- Algorithms ensure that all processes get seen

Question 16

What type of OS’s uses schedulers

Answer 16

Multi-user and multi-tasking

Question 17

What is the Round robin algorithm?

Answer 17

• Each process given set amount of time

- If process not completed it goes to the back of the que and waits for its turn again

Question 18

What is the First come first served algorithm?

Answer 18

• 1st process is dealt with start to finish

- other processes que up and wait their turn

Question 19

What is the Shortest job first Algorithm?

Answer 19

• Picks shortest job to deal with first and deals with it start to finish
• Needs to know time before hand

Question 20

What is the shortest remaining time algorithm?

Answer 20

• Estimates how long each will take then picks shortest one

- If new job is added with shortest time it then chooses that one

Question 21

What is polling? -?

Answer 21

• CPU checks each peripheral to see if it needs attention
• Waste of time
• like a teacher coming round classroom asking each student if they need help

Question 22

What is an interrupt?

Answer 22

• Device sends a signal to the processor to get its attention
• Similar to when a kid puts a hand up in class to get attention
• Has a priority in dictating how urgently it requires attention
• When an interrupt is raised it runs the correct ISR

Question 23

What is an Interrupt service routine (ISR) and what are the steps involved?

Answer 23

Determines what happens when a particular interrupt is carried out

1. At end of each fetch-decode-execute cycle the CPU checks to see if there are any interrupts. If they are and they are of a higher priority of the current task;
2. The contents of the program counter and the other registers are copied at the stack
3. The relevant ISR is loaded by changing to program counter to the value where the ISR starts in the memory
4. When complete the previous values of the programs counter and the other registers can be restored from memory to CPU

Question 24

Problems with ISR’s?

Answer 24

• It is possible that the interrupt could itself be interrupted by a higher priority interrupt.
• When this happens the current interrupt is copied to the stack and the new interrupt is carried out
• When finished (assuming it hasn’t been interrupted again) the original interrupt is loaded from the stack and carries on

Question 25

What is a device driver?

Answer 25

Piece of software, usually supplied with device, that tells the OS how to communicate with a device.

Question 26

What are multi-level feedback queues?

Answer 26

- Uses a number of queues each with a different priority level - The algorithm can move jobs between these queues depending on the jobs' behaviour

Question 27

What is a virtual machine? +/-

Answer 27

A program that has the same functionality as a physical machine + Can be backed up and duplicated + more than one can be ran at one time on a physical machine.... +...meaning that you only need on physical machine - Not as good as the real thing -Not as efficient

Question 28

What are common uses of Virtual machines?

Answer 28

- Run OS's within OS's = Parallels for MAC emulates Windows OS on MAC. Useful when testing a program being developed on multiple platforms without buying the platform - Virtualising Networks = Many organisations make their servers a group of virtual machines running from a cluster of physical machines - Interpreting intermediate code - Test new software/configurations - Backup OS'

Question 29

What is the BIOS?

Answer 29

- Basic Input Output System - What allows the computer to start up - Checks comp is functional, memory is installed and accessible and that the processor is working - It then uses Boot loader program to load OS's Kernel into memory - Stored in RAM

Question 30

Why does increasing memory increase performance?

Answer 30

-Have to rely less on slow VM meaning your system will be faster

Question 31

What is a Java VM?

Answer 31

- Usually you have to tailor code to a specific platform (Windows, Linux etc) but the JVM allows you to run the Java code on any device that can run the JVM - Offers platform independence. (The code is independent but the JVM itself is device specific) - .java contains classes, methods etc - JVM interrupts the byte code and executes the program - You write the code, compile it into byte code (intermediate code) and then anything with the correct JVM will be able to run the code

Question 32

What is open source software? +/-?

Answer 32

-Source code made public ally available -User can modify it to suit there needs -Anyone can have part in dev of software -Eg: Linux +Cheaper (usually free) than closed source +mass support +Regular updates -Lacks polish of closed source -Less secure

Question 33

What is Closed source software? +/-?

Answer 33

-Sold commercially with source code restricted from access -Compiled to machine code so user can run it without having to translate it +Security increased (copyright and malicious) +Simpler +Dedicated support and professional finish -non-customisable -Expensive