Transcript Course Notes

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
10.3.2
What does an Operating System do?
1.
2.
3.
4.
5.
6.
7.
It checks all input devices: keyboard for key presses
and mouse for movement and button presses.
It manages the sending of data to output devices like
the screen and the printer
It controls where programs and data are placed in the
main memory.
It manages the filing system
In some network systems it controls the security of the
system.
Provides a human computer interface (HCI) to the user
Lets the user know if any mistakes have occurred.
Summary of OS Functions

Memory Management


File Management


Organises data that is sent to backing storage and
controls the loading of files into memory.
Input/Output


Controls where data and programs are placed in
memory.
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 Credit
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.
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
Credit
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.




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.
Credit
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.
Example of Hierarchical
Filing System
Diary
Moon
Root
WordProcessing
Soccer
Folder
Spreadsheet
Folder
Folder
Database
Folder
Soccer
Tables
Tables 2
Soccer
File
Example of Hierarchical Filing System
Root
Folder
Sub
Folder
Ranger Server
G Atherton
K Winters
Computing
St Grd
WP
ex1
ex2
SS
ex3
ex1
WP
Maths
S1
circles1
SS
DB
ex2
File
circles2
Computing
Area
WP
Diary
Moon
SS
Soccer
Soccer
Tables
DB
Tables2
Soccer
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)
Credit
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.
John Walsh Questions

Answer the Foundation, General and
Credit questions on pages 276 and 277.