1. Unit 1 - Computer Systems St Kentigern’s Academy
Computer Software
Unit 1 - Computer Systems
St Kentigern’s Academy

2. What I need to know…
Description of the function of a bootstrap loader;
Description and exemplification of the main functions of a single user operating system:
interpreting users commands,
file management,
memory management,
input/output management,
resource allocation,
Kernal/managing processes;
Definition of a utility program;
Description of utility programs (including virus checker, disk editor and defragmenter);
Description of the standard file formats for graphics files: jpeg, gif, TIFF;

3. What I need to know…
Description of a suitable selection of software to support typical tasks including production of a multimedia catalogue, setting up a LAN in a school, development of a school website;
Description and exemplification of software compatibility issues (including memory and storage requirements, and OS compatibility);
Classification of viruses by type of file infected:
file virus,
boot sector virus,
macro virus;

4. What I need to know… Description of the following virus code actions:
replication,
camouflage,
watching,
delivery;
Distinction between a virus, a worm and a trojan horse;
Description of anti-virus software detection techniques:
use of checksum,
searching for virus signature,
heuristic detection and
memory resident monitoring

5. Bootstrap Loader
A bootstrap loader is a small but important piece of software which is held in ROM.
Its function is to load the rest of the operating system from disk into main memory when the computer is switched on or restarted.

6. Single User Operating Systems
A single user operating system is only capable of supporting one user at a time. A network operating system is designed to support multi-access operation.
The main functions of a single user operating system are:
Interpreting users commands
File management
Memory management
Input/output management
Managing processes
Resource allocation

7. Command Language Interpreter
The CLI’s function involves taking instructions from the user, checking them, then passing them to the correct part of the OS to be carried out.
The CLI may be command driven, which means the user must know and be able to enter typed commands into it. Alternatively, the CLI may be menu-driven, which means that the user can choose commands from a list.
Most computers in current use have a menu-driven CLI which is part of the graphical user interface or GUI, e.g. Microsoft Windows

8. File Management
This is the part of the OS that is concerned with the efficient use of the computer’s backing storage devices and media.
The functions are:
Maintaining a directory (File Allocation Table) which keeps track of where files are stored on storage.
Providing a connection between the user’s logical view of the files and the actual physical location of the file.
Supporting the manipulation of the data in the file.
Requesting data transfer from the I/O devices.

9. Memory Management
This part of the OS organizes the storage of data in main memory. Its main functions are:
Allocates space in memory for programs and data.
To control where programs and data are placed in main memory
To keep track of the total amount of main memory available and which programs and data are currently loaded.
To ensure that any user programs do not interfere with the area of main memory which is used by the operating system.

10. I/O Management
This layer performs the actual transfer of data between peripherals and memory.
Its main functions are:
to communicate directly with the peripherals
to handle the transfer of data between the peripherals and the processor.
to control the timing to coordinate the flow of data between the CPU and external devices.
data buffering to regulate the speed between processor and MM.
To detect errors such as mechanical or electrical failures or transmission errors.

11. Kernal
The kernel is part of the OS responsible for managing processes and handling any interrupts.
Interrupts are signals used by peripherals to communicate with the processor.

12. Resource Allocation
Any process which is taking place in a computer system requires resources to be made available to it.
Resource Allocation is the way of managing which of these resources is available for use at any one time by a process.
Efficient resource allocation ensures that the processor is kept constantly busy, by maintaining a queue of processes that are always ready for the processor.
The scheduler makes the decisions as to which process from the queue is given the processor’s time.

13. Parallel Processing
A computer with a single processor can only run one process at any given time. But it may appear to run two or more.
What is actually happening is that the processor can divide its time between each process using a method called time slicing.
The processor does a bit of task 1, moves on to task 2 and does a bit of that and so on… Because a processor is very fast it appears to be doing many tasks at once.

14. Utility Programs
These are programs that are designed to carry out specific tasks related to the maintenance of a computer system.
Types of utilities are:
Virus Checker;
Disk editor;
Disk Defragmenter
Backup

15. Virus Checker
This utility checks your system for virus software using a range of techniques. It is an essential part of system maintenance and it should be run on a regular basis.
If your system is connected to a network it may be best to have it running in the background whenever you are online.
Virus checking software should be able to detect a virus infection and remove it from a computer system.
When copying a file from a disk or downloading a file from the Internet, the virus checking software should also check that the file does not contain a virus.
New viruses are constantly written, so most virus checking software companies allow you to download their updates.

16. Disk Editor
A disk editor allows the user to edit data directly on the surface of a disk, by-passing the normal loading and saving features of the operating system.
This is a very powerful feature, but must be used with caution, since you run the risk of completely destroying all your data if you do not know what you are doing.
However, most disc editor programs incorporate automatic disk repair functions, which will repair a damage disk and recover any data.

17. Disk Defragmenter
When the OS saves a file to disk it uses the first available empty sectors it finds. This means that the disk will eventually have parts of files scattered across the disk surface which can decrease system performance. 1 2 4 7 5 6 3
Disk before defragmentation

18. Disk Defragmenter
To rectify this you can use a defragmenter.
This utility rearranges the contents of the hard disk so that the data blocks that make up a file are contiguous.
Defragmenting a disk frees up space and speeds up access time.
This process may result in an improvement in the performance of the disk, especially if it is nearly full. 1 2 3 4 5 6 7
Disk after defragmentation

19. Backup
If you back your files up regularly, then you shouldn’t need to worry too much about disk recovery programs.
Backup software can help to automate the process of making regular backups.
Backup software allows you to select the type of backup media, e.g. removable disk or tape.
Other choices include the frequency of backup, for instance daily or weekly, and the scheduled time, such as overnight.

20. Standard File Formats
Standard formats allow files to be compatible. (This is Int 2). This means that they can be used with a larger number of packages.
A word processed document saved on a PC in MS Word as an rtf file can be opened and understood by Appleworks on a Mac.
Some standard file formats are:
Txt - text file
Rtf - text file
Mp3 - music file
Wav - music file

21. Tag(ged) Image File Format (TIFF)
A file format for bitmaps is TIFF. This can represent every type of graphic from monochrome or greyscale graphics up to 24-bit colour. TIFF is used mainly in DTP.
TIFF files are very large and are not used on a web pages, unlike JPEG or GIF, because of the time taken to download them.
TIFF is an uncompressed file format.
TIFF is not a truly universal standard format and there are limits to the number of packages that can open these formats.

22. Bit-mapped (BMP)
This is the standard bitmap graphics format used by the windows OS.
The file contains data on the pattern of display pixels that need to be illuminated to recreate the image in its original size.

23. Graphics Interchange Format (GIF)
This format:
Is an 8-bit colour code giving a maximum of 256 colours;
Is used on the internet because the files compress well using a lossless technique;
Uses compression to save on storage and transfer time.
Is often found in web pages to represent charts cartoons etc.
Is not suitable for photos because of the 8-bit format.

24. Joint Photographic Experts Group (JPEG)
This is the standard format for data compressed, digitised still images.
It uses lossy compression techniques, cutting out data which the human eye won’t miss such as shades of background colour;
It works well on photos but not so well on lettering or simple cartoons.

25. Joint Photographic Experts Group (JPEG)
JPEG has a range of possible compression rates.
The more compressed a file is, the poorer the quality of the image.
Normally JPEG’s are compressed 10:1 or 20:1 where the loss of data is hardly noticeable.
Graphics saved in jpeg are much smaller than the equivalent saved as a gif.
They make less demands on storage and are faster to transmit.

26. Software Compatibility Issues
Software that runs on a particular system is said to be compatible with that system. There are a number of issues that affect software compatibility.
Software compatibility issues include:
Memory requirements
Storage requirements
OS compatibility

27. Memory Requirements
RAM is used to hold the programs and data in a computer system while they are being processed.
As OS and programs become more complex, the demands for more RAM increase.
If there is insufficient free RAM ten the program cannot be loaded and therefore cannot run.

28. Memory Requirements
If insufficient RAM is available to hold a complete program, then part of the program may be stored on disk and swapped in and out of RAM when required.
This is called virtual memory
Using virtual memory slows down processing because it is around a thousand times slower to access data from disk than from RAM.
The best method of solving this problem is to add more RAM.

29. Storage Requirements
The storage requirement refers to the amount of backing storage space which is required to hold software after it has been installed.
Some application programs require hundreds of megabytes of backing storage.
Such programs usually offer a range of installation options.
Full or complete installation, customised or a minimal installation.

30. Storage Requirements
A program called a wizard is often used to guide the user through the process of installing software.
The wizard normally contains code which checks whether or not there is sufficient backing storage space before it will permit the installation to proceed.

31. OS compatibility
The three main types of operating system used on desktop and laptop computers are, Windows, Unix and MacOS.
Software which is OS compatible is suitable for one type of operating system only, and will not run on any other.
To combat this problem many software companies supply more than one version of their products, sometimes on the same installation CD-ROM or DVD-ROM.
Like any other software operating systems are constantly under development and new version appear regularly.
Some programs are only compatible with a particular version of the OS.

32. Viruses, Worms and Trojans
What is a Virus?
A virus program must be run in order toi nfect a computer system.
Viruses can attach themselves to other programs in order to ensure that this happens.
Common Symptoms:
Reboot
Display unwanted messages
Loss of data

33. Viruses, Worms and Trojans
Viruses can spread through file downloads or infected storage media such as floppies.

34. Viruses, Worms and Trojans
What is a Worm?
A worm can spread itself to other computers without needing to be transferred as part of a host program.
Worms generally come through , but computers can also become infected by a Trojan Horse containing a worm.
Worms are often used to activate a “Denial of service” attack.
This means they can flood a network with useless traffic which overwhelms a network’s processing capability and halts communications.
(Die Hard 4.0)

35. Viruses, Worms and Trojans
What is a Trojan Horse?
A Trojan Horse is used to refer to a program that appears to be safe, but hidden inside is usually something harmful, like a worm or a virus.
A backdoor Trojan may be placed on your computer by a virus. Once in place the person who sent the Trojan can take control of your computer.
One of the most common uses of a backdoor Trojan is to make your computer into a source of spam messages!

36. What is a trojan horse?
This is software which appears to be doing one thing but actually secretly does another job.
A classic trojan horse activity is to pretend to be a network login screen so it can steal an id and password which it either s to a hacker or stores in a file that the hacker can get access to.
Others pass themselves off as graphics files or adverts.

37. Types of Virus File Virus Boot Sector Virus
This type of virus attaches itself to an application program such as a game or .exe file. When you run the program the virus instructions are also carried out.
Boot Sector Virus
Every disk drive, both hard and floppy, contains a boot sector, whether or not the disk is bootable. The boot sector contains information about the format of the disk and a small program – the boot code.
The boot code can become infected by a virus.

38. Types of Virus Program Virus Macro Virus
A macro virus is a computer virus that infects a document and causes a malicious sequence of actions to be performed when the document is opened.
Program Virus
Virus attaches to the file and can carry out a number of actions.

39. Virus Techniques Replication Camouflage
The virus inserts copies of itself into other program files. Each time the infected program is run by the computer it reproduces itself, copying itself into another program, often into that part of the code containing information about the program running: the program header.
Camouflage
A virus can disguise itself to avoid detection by anti-virus software by adding fake instructions to its code so that the anti-virus software is unable to spot the pattern of instructions which identify it.

40. Virus Techniques Watching Delivery
Some viruses copy themselves to memory and wait there checking for a condition before carrying out their destructive action. Meantime it replicates.
Delivery
This is the method used by the virus in order to enter the computer system and cause the infection.
Sharing an infected floppy disk among computers

41. Virus Detection Techniques
Searching for virus signatures - the anti-virus uses a table, which has to be regularly updated, containing virus signatures. It scans and tries to find a match for these bits of code (signatures) in its table.
Use of checksum - this technique scans an uninfected program file and calculates a checksum using the binary values of the data in the file. It then scans the file whenever the program is run and repeats the calculation. If the answers are different there could be a virus.

42. Virus Detection Techniques
Memory Resident Monitoring - this is when the software resides in RAM which monitors all a computer’s actions for suspicious activity, e.g. copying files. If it finds anything suspicious it throws up an error.
Heuristic detectors - this is software that looks for code that is triggered by time or date events, for code that searches for .com or .exe files, and for instructions that try to write to disk without going through the normal OS procedures.

43. We have finished all notes for Computer Systems Your Systems Exam will be on Tuesday 30th of September, Your NAB will be on Monday 29th of September! Remember you have to pass to stay in HIGHER! Happy Studying!