Higher Computing Computer Systems

Computer Software

What is an operating system

The operating system is a program that controls the entire operation of the computer and devices which are attached to it

Most of the OS is permanently stored on hard disk and loaded in to RAM when the computer is on

The bootstrap loader

When the computer is switched on the initial part of the OS is loaded from chip (ROM)

The bootstrap loader’s function is to load the rest of the OS from disk

The single user operating system

A SUOS is only capable of being used by one person at a time

The main functions of a SUOS are:• Interpreting users commands (CLI)

• File management

• Memory management

• I/O management

• Managing processes

• Resource allocation

The Command Language Interpreter

The CLI is the layer with which the user interacts in order to give instructions to the computer – it interprets user commands

File management

The FMS is concerned with the efficient use of the computer’s backing storage devices and media

The FMS holds the details of where each file (or part of) is held on the disk

The FMS is also known as the Disk Operating System

Memory management

The MMS controls where programs and data are placed in main memory

The MMS keeps track of the total amount of main memory available and which programs and data are currently loaded

Input/Output management

The I/OMS communicates directly with the peripheral devices

The I/OMS handles the transfer of data between the peripherals and the processor

Process management – the kernel

The kernel is the part of the OS which is responsible for managing interrupts• e.g. from the printer

The kernel is responsible for managing processes• from the CLI e.g. copy. A process initiated by

the CLI may result in an interrupt

Resource allocation

RS is the way of managing which resource is available for use at any one time by a process

Efficient RS ensures that the processor is kept busy, by maintaining a queue of processes that are always ready for the processor

A “scheduler” prioritises the queue

The OS working - an example Loading the file “myfile” from disk

• Using the CLI you issue the command to open a folder• The kernel interrupts the current CLI process and passes

the request to the FMS• The FMS requests the I/O system to read the disk’s catalog

track and loads a list of file names• The CLI allows the user to choose the filename “myfile”

from the list• The FMS passes the physical location of the file to the I/O

system• The I/O system loads the file from disk and hands it over to

the MMS• The MMS places the file in the main memory and you again

have control of the computer

Utility programs

UP’s can be part of the OS or a separate program• Virus checker

• Disc editor

• Defragmenter

• Backup

• Emulators

• Converters, compressors and expanders

• Email and web filters

Virus checker

A virus checker is a utility program which is designed to check a computer system for viruses

A virus checker will then remove the virus

Virus checkers have to be kept up to date in order to be effective against new viruses

Disc editor

A disc editor allows the user to edit data directly on the surface of a disk

DE programs incorporate automatic disk repair and data recovery functions

Newer hard drives “smart drives” have their own software for which monitors the performance of the drive

Defragmenter

Files are often split and saved on different parts of a disk

This increases the time taken to access the file

A defragmenter gathers all the free space on the disk together and reunites all the fragmented files that can be accommodated

Before defragmentingAfter defragmenting

Usedblock

unused block

Backup

Backup software can help to automate the process of making regular backups

Backup software can allow you to synchronise files between two devices or media• When two copies of the same file exist on

different media the newer copy will replace the older version

Emulators

An emulator is a program that allows one type of computer to behave as if it was a completely different type

A computer running emulator software will run much slower than the computer it is emulating

Virtual PC allows apple computers to emulate Windows OS computers

Converters, compressors and expanders

Converters change the format of a data file from one application type to another

Compressors reduce the size of a data file

Expanders decompress a compressed data file

Printer drivers

A program that takes the codes in the document and translates them into the appropriate code for the printer in use

Email and web filters

An email filter is software that monitors emails which are sent on a network

The Regulation of Investigatory Powers Act (2000) gives employers the right to monitor employees email

A web filter monitors URLs and the content of web pages be accessed by the user

File formats

Standard file formats have been developed to ease the transfer of data between software developed by different manufacturers e.g. text files - RTF

Each type of application software has its own set of standard file formats e.g. JPEG used in photographic manipulation software

File formats - GIF

Graphics Interchange Format• For low resolution bit-map colour graphics

• 256 colours possible

• Compressed files

• Fast to load and transfer via internet

• Interlacing allowed

• Animations also possible

• GIF uses lossless compression

File formats – JPEG, MPEG

Joint Photographic Experts Group• Standard for still picture compression

• 16.7 million colours

• Uses lossy compression

• Typically achieves 10 to 1 compression with little perceivable loss in image quality

Motion Pictures Expert Group• Standard for moving pictures

File formats - TIFF

Tag(ged) Image File Format• For high resolution bit-map images

• Large files unsuitable for WWW

• Uncompressed

• Used by DTP, book and magazine printing

Matching software to tasks (1)

Production of a multimedia catalogue• Multimedia authoring package e.g. Macromedia Director

Additional software for editing individual elements• Video

• Audio

• Graphic

• Text

And• CD or DVD writing software

Matching software to tasks (2)

Setting up a LAN in a school• Modern OS capable of peer to peer

networking

• Client server needs network OS software

• Database servers need specific software e.g. Microsoft SQL

• Database application such as Filemaker Pro are available in single or network versions

Matching software to tasks (3) Development of a school website

• Web page creation software with WYSIWYG page editing

Additional software for editing individual elements• Video• Audio• Graphic• Text

And• Testing using a range of browser software

Software compatibility issues

Software that works on a computer system is said to be compatible

Issues that can affect the compatibility of the software are:• Memory requirements

• Storage requirements

• OS compatibility

Memory requirements (RAM)

If a system cannot meet the minimum RAM requirements to run the software the software will not run

If there is insufficient RAM to hold the complete program then part of it will be stored on disk, virtual memory

Running programs using VM will slow the program

Storage requirements

Full installation - The amount of backing storage needed to hold the complete program after installation (and decompression)

Minimal installation – The least amount of storage requirement needed to run the program, normally the installation CD is still needed to run the program

OS compatibility

Three main OS used on desktops and laptops• Windows

• Unix

• Apple OS Users need the correct version of

software for the different OS Windows OS not always backward

compatible

Viruses

A virus is a program which can destroy or cause damage to data stored on a computer system

A virus infects host files and is distributed along with them

Worm

A worm is a virus that can make copies of itself and spread between computers without having to be attached to a file

Trojan horses

A Trojan horse is disguised as another type of file

What viruses infect

A virus may infect different parts of the computer system• File viruses

• Boot sector viruses

• Macro viruses

File viruses

File viruses attach themselves to files They add malicious code into the files Typical files that are infected are

Windows OS files with .COM and .EXE extensions

Boot sector viruses

All magnetic disks contain a boot sector The boot code can become infected with

a virus and is loaded when the machine is rebooted

Less common now that floppy discs are no longer in popular use

Macro viruses

A macro virus infects documents e.g. spreadsheets or word processing documents

A macro virus causes a malicious series of events once the document is opened

The macro virus will also infect any new documents created by the same application

Virus code actions

Virus act differently according to how they are programmed

The action the virus may carry out when run include:• Replication

• Camouflage

• Watching

• Delivery

Replication

A worm can make copies of itself this is known as replication

Camouflage

Mixed in with good code Malicious or false code is changed every

time it runs to avoid detection Disguise the code so that it looks

harmless All these techniques are used to avoid

detection by anti-virus programs

Watching

Sleeper virus lays in wait for a particular sign which will activate it

A date or time or action Time bomb (triggered by a date) Logic bomb (triggered by an action)

Delivery

The method used by the virus to enter the system

Shared backing storage devices Connected to the internet Web pages Downloads Emails Email attachments

Anti-virus detection

Anti-virus software detection techniques include:• Use of checksum

• Searching for virus signature

• Heuristic detection

• Memory resident monitoring

Use of checksum (1)

A checksum is a value calculated for a given set of data

The answer to the calculation is the checksum

It is used to check that data has been transmitted and received correctly

Use of checksum (2)

The checksum is transmitted along with the data

The receiving computer performs the same calculation and checks the sum

If they are the same its ok If the don’t match the anti-virus software

will issue a virus warning

Searching for virus signature

Anti-virus programs can find viruses inside programs by scanning them for virus signatures

A virus signature is a characteristic pattern in a file

These files can then be disinfected Polymorphic viruses modify their code to

avoid virus signature detection

Heuristic detection

Detect viruses a heuristic or rule of thumb method

Imprecise method although useful If it looks like a virus and behaves like a

virus then its probably a virus Used to detect new viruses

Memory resident monitoring

Anti-virus software that loads in to the computers memory on start up

Monitors all programs and data loaded in to the computers memory

Can slow down the processing speed of a computer as everything is checked for viruses

The End