Unit 2: Systems Software

Question 1

What is meant by the term ‘operating system’?

Answer 1

The term ‘operating system’ refers to a collection of programs that provide an interface between the user and computer. Operating systems enable the user to communicate with the computer and perform tasks involving the management of computer memory and resources.

Question 2

Name 5 features provided by Operating systems.

Answer 2

• File management
• Utility software
• Security
• User interface

Question 3

What is Paging?

Answer 3

Paging is when memory is split up into equal-sized sections known as pages. These can then be swapped between main memory and the hard disk as needed.

Question 4

What is Segmentation?

Answer 4

Segmentation is the splitting up of memory into logical sized divisions, known as segments, which vary in size. These represent the structure and logical flow of the program.

Question 5

What is Virtual Memory and when is it used?

Answer 5

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 not currently being used are temporarily moved into virtual memory through paging, freeing up memory for other programs in RAM.

Question 6

What is disk thrashing and when does it occur?

Answer 6

Disk thrashing is when the computer ‘freezes’ due to pages being swapped too frequently between the hard disk and main memory. This issue becomes progressively worse as virtual memory is filled up.

Question 7

What are Interrupts?

Answer 7

Interrupts are signals generated by software or hardware to indicate to the processor that
a process needs attention. Interrupts have different priorities and their urgency is taken into account by the OS when allocating processor time. They are stored in a priority queue in the interrupt register.

Question 8

Explain how an Interrupt Service Routine is used.

Answer 8

The processor checks the interrupt register at the end of each Fetch-Decode-Execute cycle. 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. The processor then uses an interrupt service routine to process the interrupt. The interrupt queue is then checked for further interrupts of a higher priority to the process that was originally being executed. When all priority interrupts have been serviced, the Fetch-Decode-Execute cycle resumes as before.

Question 9

What is Scheduling and why is it used by the OS?

Answer 9

The operating system ensures all sections of programs being run (known as ‘jobs’) receive a fair amount of processing time. This is done through various scheduling algorithms.

Question 10

What is starvation (Scheduling)?

Answer 10

Starvation is when a process cannot complete its execution because it is constantly denied processor time.

Question 11

What are the two types of scheduling algorithms?

Answer 11

• Pre-emptive: Jobs are actively made to start and stop by the
  operating system (e.g Shortest Remaining Time, Round Robin)
• Non pre-emptive: Once a job is started, it is left alone until it is completed. (e.g First Come First Served, Shortest Job First)

Question 12

Describe the ‘Round robin’ scheduling algorithm.

Answer 12

Each job is given a section of a time slice in which it is allowed to execute. Once each job in the queue has used its first time slice, this process is repeated until the job has been completed, at which point it is removed from the queue. This means that all jobs will eventually be attended to, however longer jobs will take a much longer time for completion as execution is inefficiently split up into multiple cycles; starvation may occur

Question 13

Describe the ‘First come first served scheduling’ algorithm.

Answer 13

Jobs are processed in chronological order by which they entered the queue. It is straightforward to implement

Question 14

Describe the ‘multi-level feedback queue’ algorithm.

Answer 14

Processes are chosen from different queues based on the priority of each queue and how much processing time each process has already had. If a process uses too much CPU time, it is moved to a lower priority queue and if a process has been idle for a long time, it is moved to a higher priority queue. This helps avoid starvation, however it is difficult to implement.

Question 15

Describe the ‘Shortest job first’ algorithm.

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 is suited to batch systems as waiting time is reduced, however it can result in processor starvation and delays; the processor needs to calculate how long each job will take.

Question 16

Describe the ‘Shortest remaining time’ algorithm.

Answer 16

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. This means throughput is increased as shorter processes can be quickly completed, however as this does not take into account job priority or urgency, starvation may occur if short jobs are continuously added to the queue.

Question 17

What are the 5 types of operating systems?

Answer 17

• Distributed
• Embedded
• Multi-tasking
• Multi-user
• Real Time

Question 18

Describe the function and use of a Distributed OS.

Answer 18

Run across multiple devices, allowing the load to be spread across multiple computer processors when a task is run.

Question 19

Describe the function and use of an Embedded OS.

Answer 19

Built to perform a small range of specific tasks and catered towards a specific device such as a household appliance. They are limited in their functionality and hard to update although they consume significantly less
power than other types of OS.

Question 20

Describe the function and use of a Multi-tasking OS.

Answer 20

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 21

Describe the function and use of a Multi-user OS.

Answer 21

Multiple users make use of one computer. A scheduling algorithm is used to ensure processor time is shared fairly between jobs and prevent processor starvation.

Question 22

Describe the function and use of a Real Time OS.

Answer 22

Commonly used in time-critical computer systems, a real time OS is designed to perform a task within a guaranteed time frame. Examples of use include the management of control rods at a nuclear power station or within self-driving cars: any situation where a response within a certain time period is crucial to safety.

Question 23

What is the BIOS and what is it responsible for?

Answer 23

The Basic Input Output System is the first program that runs when a computer system is
switched on. The BIOS is responsible for running various key tests before the operating
system is loaded into memory, such as:
- POST (Power-on self test) which ensures that all hardware is correctly connected and functional
- Checking the CPU clock, memory and processor is operational
- Testing for external memory devices connected to the computer

After these checks are completed, the operating system can be loaded into RAM from the
hard disk by the bootstrap/ bootloader.

Question 24

What are Device Drivers?

Answer 24

Device drivers are computer programs which are provided by the operating system and allow the operating system to interact with hardware. When a piece of hardware is used, the device driver communicates this request to the operating system which can then produce the relevant output.

Question 25

What is a Virtual Machine?

Answer 25

A virtual machine is a theoretical computer in that it is a software implementation of a computer system. It provides an environment with a translator for intermediate code (code that is halfway between machine code and object code) to run; however, running intermediate code in a virtual machine can be considerably slower.

Question 26

What is the purpose of Applications software?

Answer 26

Designed to be used by the end-user to perform one specific task. Application software
requires systems software in order to run.
Examples: desktop publishing, word processing, spreadsheets, web browsers.

Question 27

What is the purpose of Systems software ?

Answer 27

Low-level software responsible for managing the computer resources and maintaining
consistently high performance.
Examples: library programs, utility programs, operating system, device drivers.

Question 28

Name and describe 4 examples of Utility software.

Answer 28

• Compression: Used to decompress and compress files.
• Disk defragmentation: As the hard disk becomes full, read/write times slow down. This is because files can no longer be stored contiguously. The disk defragmenter utility rearranges the contents of the hard drive so they can be accessed faster.
• Antivirus: Detects potential threats to the computer, alerts the user and removes these threats.
• Backup: Routinely creates copies of files specified by the user; in an unfortunate event files can be recovered.

Question 29

What is meant by ‘Open Source’?

Answer 29

Can be used by anyone without a license and is distributed with the source code.

Question 30

What is meant by ‘Closed Source’?

Answer 30

Requires the user to hold an appropriate license to use it. Users cannot access the source code as the company owns the copyright license.

Question 31

What are 3 advantages of an Open Source program?

Answer 31

• Can be modified and improved by anyone
• Technical support from online community
• Can be modified and sold on

Question 32

What are 3 advantages of a Closed Source program?

Answer 32

• Thorough, regular and well-tested updates
• Company owning software provides expert support and user manuals
• High levels of security as developed professionally.

Question 33

What are 2 disadvantages of an Open Source program?

Answer 33

• Support available online may be insufficient or incorrect. No user manuals.
• Lower security as may not be developed in a controlled environment

Question 34

What are 2 disadvantages of an Closed Source program?

Answer 34

• License restricts how many people can use the software at once
• Users cannot modify and improve software themselves

Question 35

What is a translator and what are the three types?

Answer 35

A translator is a program that converts high-level source code into low-level object code,
which is then ready to be executed by a computer.

The three types are:
- Compiler
- Interpreter
- Assembler

Question 36

Describe the 3 characteristics of a Compiler.

Answer 36

• Translates high-level code into machine code all at once.
• The initial compilation process is longer than using an interpreter or an assembler.
• Compiled code is specific to a particular processor type and operating system but can be run without a translator present.

Question 37

Describe the 4 characteristics of an Interpreter.

Answer 37

• Translate and execute code line-by-line. They stop and produce an error if a line contains an error.
• Initially appear faster than compilers, but are slower than running compiled code as code must be interpreted each time it is executed. Code also requires the correct interpreter in order to run on different devices.
• Interpreters are useful for testing code, as time is not wasted compiling code with errors.
  -Code is also platform-independent, making interpreted code more portable.

Question 38

Describe the main function of an Assembler.

Answer 38

Assemblers translate assembly code into machine code. Each line of assembly code is equivalent to almost one
line of machine code.

Question 39

Name the four Stages of compilation high level code is subjected to (before it is fully compiled).

Answer 39

• Lexical Analysis
• Syntax Analysis
• Code Generation
• Optimisation

Question 40

In compilation, describe the 3 main processes of Lexical Analysis.

Answer 40

• Whitespace and comments are removed from the code.
• Keywords and names of variables and constants are replaced with tokens.
• Information about tokens is stored in a symbol table.

Question 41

In compilation, describe the 3 main processes of Syntax Analysis.

Answer 41

• Tokens are analysed against the rules of the programming language; tokens that break the rules are flagged up as syntax errors.
• An abstract syntax tree is produced, which is a representation of the source code in the form of a tree.
• Semantic analysis is carried out where logic mistakes within the program are detected.

Question 42

In compilation, describe the main process of Code Generation.

Answer 42

The abstract syntax tree is used to produce machine code.

Question 43

In compilation, describe the 3 main processes of Optimisation.

Answer 43

• Searches through the code for areas it could be made more efficient to reduce execution time but is a very time-consuming part of the compilation.
• Redundant parts of code are removed. Repeated sections of code are replaced with more efficient code.
• Excessive optimisation may alter the way in which the program behaves.

Question 44

What is a Linker?

Answer 44

A piece of software responsible for linking external modules and libraries included within the code.

Question 45

Name and describe the two types of Linkers.

Answer 45

Static Linker: Modules and libraries are added directly into the main file. This increases the size of the file.

Dynamic Linker: Addresses of modules and libraries are included in the file. File size does not change and external module/library updates automatically feed through to the main file.

Question 46

What are Loaders?

Answer 46

Programs provided by the operating system. When a file with dynamically linked addresses is executed, the loader retrieves the library or module from the specified memory location.

Question 47

Describe how Libraries can be used in programming.

Answer 47

Pre-compiled programs which can be incorporated within other programs. They are ready-to-use and error free, so save time developing and testing modules. Libraries can be reused across multiple programs and save programmers from having to ‘reinvent the wheel’ and instead make use of others’ expertise.