1.2.1 - Systems Software

Question 1

What does ‘operating system’ mean?

Answer 1

The term operating system refers to a collection of programs that work together to provide an interface between the user and the computer.

Question 2

Name four tasks that the operating system does.

Answer 2

Memory management (paging, segmentation, virtual memory);
Resource management (scheduling);
File management;
Input / output management;
Interrupt management;
Utility software;
Security;
User interface.

Question 3

Name the two types of memory management.

Answer 3

Paging and segmentation.

Question 4

Describe the purpose of memory management.

Answer 4

Computer memory must be shared fairly between multiple programs and applications to ensure the computer doesn’t crash and each application runs properly.

Question 5

Describe how paging works.

Answer 5

Paging is when memory is split up into equal size sections called pages.

Paging uses physical divisions of memory.

Question 6

Describe how segmentation works.

Answer 6

Segmentation is the process of splitting up memory into segments of varying size.

Segmentation uses logical memory divisions.

Question 7

What is meant by ‘virtual memory’?

Answer 7

Virtual memory uses a section of the hard drive to act as RAM when the RAM is insufficient.

Question 8

What is the main issue with virtual memory?

Answer 8

Virtual memory can cause disk thrashing to occur. This is where the computer freezes as a result of pages being swapped too frequently between the hard disk and main memory.

Question 9

What is an interrupt?

Answer 9

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

Question 10

How are interrupts stored?

Answer 10

Interrupts are stored in order of their priority.

Question 11

Whose job is it to ensure interrupts are serviced fairly by the processor?

Answer 11

The operating system through interrupt service routines.

Question 12

How does an interrupt service routine work?

Answer 12

• The processor checks the contents of the interrupt register at the end of each fetch-decode-execute cycle.
• If an interrupt exists that is a higher priority to the process being executed, the current contents of the registers in the CPU are transferred onto the stack.
• The processor then loads the correct interrupt service routine into RAM.

Question 13

What happens when an ISR has finished?

Answer 13

The contents of the registers are taken from the stack and reloaded into the registers in the CPU. The processor then carries on as normal.

Question 14

What is scheduling?

Answer 14

Scheduling ensures that all sections of the programs being run receive a fair amount of processing time.

Question 15

Name the five scheduling algorithms.

Answer 15

Round robin, first come first served, shortest job first, shortest remaining time and multilevel feedback queues.

Question 16

Describe how the round robin scheduling algorithm works.

Answer 16

Each job is given a section of processor time called a time slice. If the job has not been completed when the time slice has ended, the next job is given a time slice.

Question 17

Describe how the first come first served scheduling algorithm works.

Answer 17

Jobs are processed in chronological order by which they entered the queue.

Question 18

Describe how the shortest job first scheduling algorithm works.

Answer 18

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.

Question 19

Describe how the shortest remaining time scheduling algorithm works.

Answer 19

The queue storing jobs to be processed is ordered according to the time left for completion, with the jobs with the lowest remaining time being serviced first.

Question 20

Describe how the multilevel feedback queue scheduling algorithm works.

Answer 20

This makes use of multiple queues which store different priority jobs. The higher priority, the faster the job is completed.

Question 21

What is a disadvantage of using the round-robin scheduling algorithm?

Answer 21

Longer jobs have a much longer time to completion due to the execution being split up into multiple cycles.

Question 22

What is a disadvantage of using the first come first served scheduling algorithm?

Answer 22

There can be lots of small jobs waiting behind a larger job.

It doesn’t take into account the priority of the job.

Question 23

What is a disadvantage of using the multilevel feedback queue scheduling algorithm?

Answer 23

It is difficult to implement as it has to decide which job to prioritise.

Question 24

What is a disadvantage of the shortest job first algorithm?

Answer 24

It requires the processor to estimate how long each job will take.

There is a risk of processor starvation if short jobs keep on being added to the queue.

Question 25

What is a disadvantage of the shortest remaining time algorithm?

Answer 25

There is a risk of processor starvation if short jobs are added to the queue.

Question 26

Name the five different types of operating system.

Answer 26

Distributed, embedded, multi-tasking, multi-user, real time.

Question 27

Describe how a distributed operating system works.

Answer 27

A distributed operating system runs across multiple devices which allows the load to be spread across multiple computer components when a task is run.

Question 28

Describe the features of an embedded operating system.

Answer 28

An embedded operating system is built to support a small range of tasks and catered towards a single device.

Question 29

Describe how a multi-tasking operating system works.

Answer 29

Multi-tasking operating systems allow the user to carry out tasks seemingly simultaneously. This is done by switching quickly between programs and applications in memory.

Question 30

Describe how a multi-user operating system works.

Answer 30

Multiple users make use of a single computer (usually a supercomputer) from different terminals.

Question 31

Describe how a real time operating system works.

Answer 31

A real time operating system is designed to perform a task within a guaranteed time frame.

Question 32

Name two features of an embedded operating system.

Answer 32

They are limited in functionality

Hard to update

No memory management

Consume less power that a traditional operating system

Question 33

Name an important feature of a multi-user operating system.

Answer 33

A scheduling algorithm must be used to ensure processor time is shared fairly between jobs. This is to reduce the risk of processor starvation.

Question 34

What does the BIOS stand for?

Answer 34

Basic Input Output System.

Question 35

What is the first program run when a computer is loaded?

Answer 35

The BIOS.

Question 36

Name a test that is run as part of the BIOS.

Answer 36

Power On Self Test (POST)

Checking the CPU clock, memory and processor to ensure that it is operational.

Testing for external memory devices.

Question 37

What happens during the power on self test (POST)?

Answer 37

Ensures that all hardware are correctly connected and functional.

Question 38

What is a device driver?

Answer 38

A computer program that is provided by the operating system and allows the operating system to interact with hardware.

Question 39

What are device drivers specific to?

Answer 39

The computer’s architecture and the operating system.

Question 40

What is a virtual machine?

Answer 40

A theoretical computer that is a software implementation of a computer system. It also provides an environment with a translator for intermediate code to run.

Question 41

What is intermediate code?

Answer 41

Code that is halfway between machine code and source code is called intermediate code.

Question 42

What is intermediate code independent of? What advantage does this give?

Answer 42

The computer architecture. This means it can be used across different machines and operating systems.

Question 43

Why are virtual machines commonly used?

Answer 43

They create a development environment for programmers to test programs on operating systems.

Question 44

What are the advantages of using virtual machines?

Answer 44

It saves time and money for developers as they do not have to purchase multiple devices solely for testing purpose.

Question 45

What is a disadvantage of using a virtual machine?

Answer 45

Intermediate code can be considerably slower than using low level code on the device it was designed for.

Question 46

Name another use of a virtual machine.

Answer 46

Protection from malware.

Running incompatible software.