Operating Systems

Operating System  A program running in a computer from the moment it is switched on.  Part of the Systems Software of the computer.  It is needed to control how the computer works and to control any devices attached to the computer.  The CPU works much faster than its peripheral devices. The operating system co-ordinates all of the activities of a computer system so that the CPU is used efficiently.  For example Windows XP and MacIntosh OS

What does an Operating System do? 1.It checks all input devices: keyboard for key presses and mouse for movement and button presses. 2.It manages the sending of data to output devices like the screen and the printer 3.It controls where programs and data are placed in the main memory. 4.It manages the filing system 5.In some network systems it controls the security of the system. 6.Provides a human computer interface (HCI) to the user 7.Lets the user know if any mistakes have occurred.

Summary of OS Functions Memory Management Controls where data and programs are placed in memory. File Management Organises data that is sent to backing storage and controls the loading of files into memory. Input/Output Controls input and output devices. Job Scheduler Organises the way in which different tasks are carried out.

Error Reporting Informs the user if any problems occur. Human Computer Interface Most computer systems use a graphical user interface (GUI). One type of graphical user interface is a WIMP system. The Operating System controls the way the WIMP system works. The user may choose to tailor or customise the WIMP system to suit their own preference.

Types of Operating Systems Real Time Systems Interactive Systems

Real Time Systems Immediate response. Provides accurate and up to date information at any one time although many alterations are made continuously. Examples:  Booking an aeroplane seat. If the information is not up to date then more than one person could book the same seat.  Guidance systems for rockets and space crafts.  Monitoring equipment for nuclear or chemical plant.

Interactive Systems An interactive system, used by most computers, responds immediately to user commands. Example:  Bar code read at the checkout till gives the price and name of item.

Interactive Systems with Background Job Capability Interactive operating systems can spend a large amount of time waiting for something to happen. Other tasks can be carried out in this idle time. These tasks are called background jobs/tasks and are suspended as soon as the main task requires the processor. Credit

Example of Background Job Most time in a word processor is spent waiting for characters to be typed in at a keyboard. The OS can use these gaps to share out the processor between word processing and background printing. The sending of data to the printer is carried out by the CPU in-between each key press. This means that you don’t have to wait for the printer to finish before you can continue using the computer. See Walsh page 273 for more info example

Device Driver A program which is an add-on to the Operating System to allow the computer to operate a particular device which is attached to it. Eg) if you buy a new printer, scanner or digital camera for your computer system then you will need to load a new device driver so that the device will operate correctly. This is usually supplied on a CD-ROM and must be loaded when the new device is installed for the first time. Credit

Printer Driver A printer driver takes the codes used in the document and translates them into the appropriate code for the type of printer in use. Credit

Printers can produce many different printing styles such as underline, bold, italic, and different type faces such as Times New Roman, Courier, Comic Sans, Arial, Harrington, Chiller, Gigi Before a printer can produce these styles, it must have the correct code sent to it from the computer. The code which is required for any given style is different depending on the style of printer that is in use. Eg) one code may produce bold print on a Brother laser printer and italic print on an Epson inkjet printer. A program driver program is used to overcome this problem.

Filing Systems The OS is responsible for the information stored on backing storage. It has to maintain information on:  How many files there are  Where the files are located on the disc  How large the files are Each time a file is modified, added or deleted the catalogue is updated.

Directories and Catalogues A catalogue is a list of all the files or programs stored in a directory. A directory (folder) is an area on backing storage where files are stored.

Types of Files 2 main types of files :  Data File  Program File

Data File Contains the data to be operated on by programs or applications. Example: Text produced by a word processing program.

Program File These contain the instructions telling the computer what to do make a particular program or application work. Example : Word Processing Programs like Microsoft Word, Claris Works etc.

Hierarchical Filing System This filing system allows different levels to be set up and so files can be grouped together as the user wishes. It is organised into sub-directories and folders. Credit

Example of Hierarchical Filing System Root WordProcessing Folder Database Folder Spreadsheet Folder Diary Moon Soccer Tables Tables 2 File Folder

Example of Hierarchical Filing System Folder Root Sub Folder File

Advantages – Hierarchical Filing Systems Files can be organised better into folders and sub-folders. Files can have the same names if they are in different folders. Each folder will have fewer files in it, so its easier to view the complete catalogue on screen at one time.

Disadvantages – Hierarchical Filing Systems Easy to over-organise your work and have too many levels of folders, making it hard to find things. More memory used.

Sequential + Direct (random) access devices Tape = Sequential access as the data can only be read back in sequence. Disks=Direct access because the the read/write head on the disc can go straight to the track where the data is stored, without having to read all the data in between. Credit

John Walsh Questions Answer the Foundation, General and Credit questions on pages 276 and 277.