UNIT 2: Systems Software

Question 1

What is an OS?

Answer 1

-boot loader in ROM loads the OS into RAM when computer is switched on
-OS manages the hardware and provides an interface and the application software

Question 2

What functions do the OS provide?

Answer 2

-functions it provides- GUI, MEMORY MANAGEMENT, INTERRUPTS and PROCESSOR SCHEDULING

Question 3

OS- User Interface

Answer 3

what the user interacts with
hides the complexity of the hardware from the user by providing a UI

Question 4

OS- Memory Management

(Paging, segmentation,VM,)

Answer 4

programs and their data needs to be loaded into RAM
OS must manage the allocation of RAM to the different programs
may not be sufficient RM to load all the program at once

Question 5

MM- Paging

Answer 5

Available memory is divided into fixed sized chunks called pages
each page has an address
process loads into RAM is allocated sufficient pages- they may not be contiguous in physical terms

Question 6

MM- Segmentation

Answer 6

Memory can also be divided into segments which can be different lengths
they can relate to parts of a program- for example functions of subroutine may occupy a segment.

Question 7

MM- Virtual memory

Answer 7

computer has fixed amount of RAM- demand often exceeds this amount
an area of the hard disk can be designated as VM
some pages are stored in VM until they are needed- then swapped into RAM
if many are running- lots of time is spent swapping as there is insufficient RAM

Question 8

VM - DISK THRASHING

Answer 8

Repeatedly swapping can noticeably slow down the computer- DISK THRASHING

Question 9

OS- Interrupts

Answer 9

Vital CPU can be interrupted when necessary
They can be sent to the CPU by software, hardware devices or CPU internal clock

Question 10

When does the CPU check for interrupts?

Answer 10

CPU checks at the end of each clock cycle whether they are any interrupts to be processed

Question 11

Interrupts EXAMPLES

Answer 11

EXAMPLE- printer runs out of paper, error, scheduled interrupt or power failure

Question 12

Interrupts - using a stack

Answer 12

when an interrupt is detected- the processor stops fetching and pushes the current content of its registers into a stack
uses an interrupt service routine

Question 13

Interrupts - Priority

Answer 13

they have priotires and will be processed in order
interrupts can themselves be interrupted if the new interrupts is of higher priority

Question 14

OS - Processor scheduling

Answer 14

single CPU can only process instructions for one application at a time
OS must schedule when each app can use the CPU
gives the illusion of multitasking - appear to be running simultaneously

Question 15

Processor scheduling AIMS

Answer 15

provide acceptable response times to all users
maximise the time the CPU is usefully engaged
ensure fairness on a multiuser system

Question 16

PS Round Robin

Answer 16

Each job is allocated(BY FIFO) a time slice during which it can use the CPU
if job ain’t complete by the end of time slice- next job is allocated a time slice

Question 17

What is FIFO? (Round Robin)

Answer 17

First in, first out

Question 18

PS First come first served

Answer 18

First job that arrives is executed until its complete

Question 19

PS Shortest remaining time

Answer 19

time to completion is estimated as each new job arrives
job with SRT is executed, a new shorter job can take over from current process

Question 20

PS Shortest job first

Answer 20

also called - shortest process next
total execution time is estimated by the user
waiting job w the smallest total execution time is executed when the current job completes

Question 21

PS Multifeedback queues

Answer 21

created w different priority levels
if uses too much CPU time its moved to lower priority
processor can move if waited a long time