1.2.1 system software

Question 1

what is an operating system

Answer 1

A collection of programs that work together to provide an interface between the user and a computer.

Operating systems enable the user to communicate with the computer and perform certain low-level tasks involving the
management of computer memory and resources

Question 2

describe how virtual memory is used when there is not enough RAM

Answer 2

-A section of the hard drive is used to act as RAM
-sections of programs that are not currently in use are temporarily moved to virtual memory through paging.
-this frees up memory for other programs in RAM

Question 2

similarities and differences between paging and segmentation

Answer 2

BOTH are used when RAMs space is inefficient, can cause disk thrashing

paging uses equal sized sections called pages

segmentation uses variable sized, logical sections called segments

Question 2

give 3 functions of an operating system

Answer 2

-memory management
-resource management
-i/o management
-security
-user interface
-utility software

Question 2

what is an interrupt

Answer 2

a signal generated by software or hardware to indicate to the processer that a process needs attention.

Question 3

Explain the stages of the interrupt service routine

Answer 3

-interrupt register is checked at the end of every FDE cycle
-if there is an interrupt that is of higher priority compared to the current process the current contents of the registers in the CPU are transferred into a stack
- the relevent interrupt service routine (ISR) is loaded into RAM
-A flag is set to signal the ISR has begun
-the flag is reset once the ISR has finished
-interrupt register checked again. If interrupts are of a higher priority the process is repeated.
-if there are no interrupts with a higher priority, the contents of the stack are popped and placed back into the special memory registers.
-the fetch decode execute cycle renews as before

Question 4

are the scheduling algorithms pre-emptive or non pre-emptive

Answer 4

pre-emptive- RR, SRT, MLFQ
non pre-emptive-FCFS, SJF

Question 5

Give one advantage and one disadvantage of using multilevel feedback queues for scheduling

Answer 5

Advantages- services most urgent interrupts
Disadvantages-hard to implement

Question 6

define a real time operating system and when it may be used

Answer 6

real time operating systems perform a task within a guaranteed time frame. They are used in systems were a response within a given time frame is critical e.g life support systems, self driving cars, power systems

Question 7

Describe three functions of the BIOS

Answer 7

-POST(power on self test) which ensures that all hardware is correctly connected and functionality.
-checking the CPU clock, memory and processor
-testing for external memory devices

Question 8

what is the role of a device driver

Answer 8

enables the operating system to interact with hardware devices

Question 9

describe an instance in which a virtual machine may be used

Answer 9

-testing programs
-protecting from malware
-running software compatible with different versions and types of operating systems

Question 10

Give one advantage and one disadvantage of intermediate code

Answer 10

Advantage - platform independent, portable
Disadvantage-slower execution

Question 11

Pre-emptive

Answer 11

Jobs are actively made to start and stop by the
operating system.

Question 12

Non pre-emptive

Answer 12

Once a job is started, it is left alone until it is
completed.

Question 13

Round robin

Answer 13

Each job is given a section of processor time - known as a time slice - within
which it is allowed to execute. Once each job in the queue has used its first
time slice, the operating system again grants each job an equal slice of
processor time. This continues until a job has been completed

Question 14

First come first served

Answer 14

Jobs are processed in chronological order by which they entered the queue.
Although this is straightforward to implement, FCFS again does not allocate
processor time based on priority.

Question 15

Shortest job first

Answer 15

The queue storing jobs to be processed is ordered according to the time
required for completion, with the longest jobs being serviced at the end.
This type of scheduling is most suited to batch systems, where shorter jobs
are given preference to minimise waiting time.
However it requires the processor to know or
calculate how long each job will take and this
is not always possible

Question 16

Shortest remaining time

Answer 16

The queue storing jobs to be processed is
ordered according to the time eft for
completion, with the jobs with the least
time to completion being serviced first.
Again, there is a risk of processor starvation for longer jobs if short jobs are
added to the job queue.

Question 17

Distributed type of os

Answer 17

This is a type of operating system which is run across multiple devices,
allowing the load to be spread across multiple computer processors when a
task is run

Question 18

Embedded type of os

Answer 18

Built to perform a small range of specific tasks,
this operating system is catered towards a
specific device. They are limited in their
functionality and hard to update although they
consume significantly less power than other
types of OS.

Question 19

Multi-tasking type of os

Answer 19

Multi-tasking operating systems enable the user to carry out tasks seemingly
simultaneously. This is done by using time slicing to switch quickly between
programs and applications in memory.

Question 20

Multi-user type of os

Answer 20

Multiple users make use of one computer. Therefore a scheduling algorithm must be used to ensure processor time is shared fairly between jobs. Without a suitable scheduling algorithm, there is a risk of processor starvation, which is when a process is not given enough processor time to execute and complete.

Question 21

why is the BIOS critical

Answer 21

The BIOS is critical to the computer system as it is only
after these checks are completed that the operating system
can be loaded into RAM from the hard disk

Question 22

Intermediate Code

Answer 22

Code that is halfway between machine code and object code is called intermediate code.
This is independent of the processor architecture so can be used across different
machines and operating systems.

Question 23

Paging

Answer 23

Paging is when memory is split up into equal-sized sections known as pages, with programs being made up of a certain number of equally-sized pages. These can then be swapped between main memory and the hard disk as needed.

Question 24

Segmentation

Answer 24

Segmentation is the splitting up of memory into logical sized divisions, known as segments, which vary in size. These are representative of the structure and logical flow of the program, with segments being allocated to blocks of code such as conditional statements or loops

Question 25

Emmulator

Answer 25

They trick a program into thinking its running on its native hardware when in fact it is running on an entirely different machine

The programs themselves are not aware they are being emulated

Question 26

Bios

Answer 26

Responsible for loading the operating system when the computer first turns on

Question 27

Multi level feedback queues

Answer 27

Several ready quesues are used, each with a different scheduling algorithm. Jobs can move between queues as their priorities change