1.2.1 - Systems 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 computer

Question 2

What do operating systems provide (management)?

Answer 2

Memory, resource, file, i/o, interrupt.

Question 3

What do operating systems provide (non-management)?

Answer 3

Utility software
Security
User interface

Question 4

What is paging?

Answer 4

Partitioning memory into fixed size sections.
Uses physical divisions.
Swapped in and out of virtual memory.

Question 5

What is segmentation?

Answer 5

Splitting memory into logical divisions which vary in size.

Question 6

What is virtual memory?

Answer 6

Virtual memory uses a section of the hard drive to act as RAM when the space in main
memory is insufficient to store programs being used. Sections of programs that are not
currently in use are temporarily moved into virtual memory through paging, freeing up
memory for other programs in RAM.

Question 7

What is the issue with memory management techniques?

Answer 7

Disk thrashing. This is when the
computer ‘freezes’ and occurs as a result of pages being swapped too frequently between
the hard disk and main memory.
As a result, more time is spent transferring these pages
between main memory and the hard disk then is spent actually running the program.
This
issue becomes progressively worse as virtual memory is filled up.

Question 8

What are interrupts?

Answer 8

Interrupts are signals generated by software or hardware to indicate to the processor that
a process needs attention

Question 9

How are interrupts stored?

Answer 9

Interrupts are stored in order of their priority within an abstract data
structure called a priority queue in a special register known as an interrupt register

Question 10

Give two examples of an interrupt.

Answer 10

A printer signalling

the completion of a print job or a peripheral signalling power failure.

Question 11

How is an ISR started?

Answer 11

End of FDE cycle, processor checks IR
If an interrupt exists that is of a higher priority to the
process being executed, the current contents of the
special purpose registers in the CPU are temporarily
transferred into a stack.
loads the appropriate interrupt service
routine (ISR) into RAM.
A flag is set to signal the ISR has begun.

Question 12

What happens once an interrupt has been serviced?

Answer 12

The flag is reset. The interrupt queue is checked
again for further interrupts of a higher priority to the process that was originally being
executed.

Question 13

What happens if there are more interrupts to be serviced?

Answer 13

The ISR process is repeated until all priority interrupts have
been serviced.
If there are no more interrupts or interrupts
are of a lower priority to the current process, the contents of
the stack are transferred back into the registers in memory.
The Fetch-Decode-Execute cycle resumes as before.

Question 14

What is the purpose of scheduling?

Answer 14

• Ensure all jobs receive fair processing time
• Process as many jobs in least possible time.
• Make efficient use of resources.

Question 15

What is the difference between pre and not pre-emptive scheduling?

Answer 15

Pre-emptive: Jobs are made to actively stop and start by the operating system.
Not pre-emptive: Once a job is started, it is left a lone til completed.

Question 16

How does the round robin algorithm work?

Answer 16

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, at which point
it is removed from the queue.

Question 17

Pros/cons of round robin

Answer 17

Although Round Robin ensures each job is
seen to, longer jobs will take a much longer time for completion due to their
execution being inefficiently split up into multiple cycles.
This algorithm also
does not take into account job priority.

Question 18

Describe the first come first served algorithm.

Answer 18

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.
Once a job starts it stops others being processed

Question 19

Describe multilevel feedback queues.

Answer 19

This makes use of multiple queues, each which is ordered based on a
different priority.
This can be difficult to implement due to deciding which job to prioritise
based on a combination of priorities.

Question 20

How does the shortest job first algorithm work (where is it most suitable)?

Answer 20

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.

Question 21

What is the danger with shortest job first?

Answer 21

It requires the processor to know or
calculate how long each job will take and this
is not always possible. There is also a risk of
processor starvation if short jobs continue
being added to the job queue.

Question 22

What is the shortest remaining time algorithm?

Answer 22

The queue storing jobs to be processed is
ordered according to the time left 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 23

What is a distributed operating system?

Answer 23

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 24

What is an embedded operating system?

Answer 24

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 25

What is a multi-tasking operating system?

Answer 25

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 26

What is a multi-user operating system?

Answer 26

Multiple users make use of one computer, typically a supercomputer, within a
multi-user system.
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 adequate processor time to execute and complete.

Question 27

What is a real-time operating system?

Answer 27

Commonly used in time-critical computer systems, a real time OS is
designed to perform a task within a guaranteed time frame.

Question 28

What is the BIOS?

Answer 28

It is the first program that runs when a computer system is
switched on, 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 29

What 3 checks does a BIOS do?

Answer 29

• POST (Power-on self test) which ensures that all
  hardware (keyboards, disk drives) are correctly
  connected and functional
• Checking the CPU clock, memory and processor is
  operational
• Testing for external memory devices connected to
  the computer

Question 30

What is the bootstrap?

Answer 30

The name given to the program loading the OS from the hard disk into main memory.

Question 31

What are device drivers?

Answer 31

Device drivers are computer programs which are provided by the operating system and
allow the operating system to interact with hardware.
They are specific to the computer’s architecture and operating system.

Question 32

What is a virtual machine?

Answer 32

This is a theoretical computer (software implementation of a computer system). It provides an environment with a translator for intermediate code to
run.

Question 33

What is intermediate code?

Answer 33

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 34

When are virtual machines commonly used?

Answer 34

To create a development environment for
programmers to test programs on different operating systems.
The advantage of this is
that it saves both the time and money of having to purchase multiple devices solely for
testing.
However, running intermediate code in a virtual machine can also be considerably
slower compared to running low-level code on the device it was designed for.

Question 35

How can virtual machines be used to protect from malware?

Answer 35

Malware will affect the virtual machine rather than the device being used.

Question 36

How can virtual machines be used when running incompatible software?

Answer 36

Programs specific to different operating systems or different versions of an
operating system can be run within a VM, saving time and money required
to purchase the hardware.
A common example is of games consoles being implemented on PCs via a
virtual machine.

Question 37

Why is memory management important?

Answer 37

It allocates memory and protects programs from each other.

It allows programs larger than main memory to run.