Operating Systems |
The operating system is a very special piece of software. All the other applications software such as databases and spreadsheets communicate with the computer hardware through the operating system. The operating system controls the overall operation of the computer. It is the most important part of the systems software.
An operating system (or OS) is a computer program which controls the entire operation of the computer. Computer software applications communicate with the computer hardware via the operating system.
Example Operating Systems
When you buy a computer it is usually supplied with an operating system already installed. You will not be able to use a computer without an operating system. Different operating systems are available for different types of computer and different purposes. Some popular operating systems are :
| Operating System | Description |
| MS-DOS® | A simple, command-driven operating system used on personal computers. |
| Windows 2000® | A WIMP based operating system used on most PC computers. |
| UNIX | A multi-user, multi-computer operating system which is used on large mainframe computers and networks. |
The choice of operating system will affect how a computer works. It will probably determine the type of user interface (command line, menu driven or graphical) that the computer uses and the types of processing (batch, transaction or real-time) that the computer can carry out.
Because an application such as a spreadsheet works by communicating with the operating system the application will probably only work with one particular operating system. If you buy Microsoft® Works® for Windows® 2000 it will only operate on computers with the Windows 2000 operating system. If you want to use Microsoft Works with the MS-DOS® operating system you will have to buy a different version of the application.
MS-DOS®, Windows® 2000 and Microsoft® Works® are registered trademarks of Microsoft Corporation in the United States and other countries.
There are several tasks which are performed by practically all operating systems, regardless of the complexity of the computer the operating system is being used on. These tasks include :
The range of tasks that an operating system performs increases if the computer using the operating system is on a network or if the operating system is a multi-tasking or multi-user operating system.
Managing Communications Between Computer and Connected Peripherals
Many peripherals can be connected to a computer. It is the job of the operating system to manage the communication between the application programs that are running on the computer and the peripheral devices. For example the operating system will :
There has to be some control over these activities to make sure that problems do not occur. Consider what might happen if a computer program was trying to load a file from a floppy disk and, at the same time, another program was trying to save data. Therefore the operating system manages all of these tasks. If, for example, a program wants to save or print a file it must send a request to the operating system asking it to do so. The operating system will then carry out the task.
Provision of a User Interface
The operating system will provide a user interface that the user uses to interact with the computer. Modern operating systems usually provide a graphical user interface. Alternatives to this are command line and menu driven interfaces.
Allocation of Computer Memory
Computer memory must be allocated to (divided between) the different tasks that the computer is performing. Even if your computer appears to be doing only one task such as word processing there will be several different items to which memory must be allocated. For example memory would have to be allocated to storing the word processor program, storing your word processed document and storing a copy of the image that is being displayed on the monitor. And of course the operating system itself will also use up some memory.
Organising Data on Backing Storage Devices
Work and programs must be saved on a backing storage device so that they can be kept when a computer is turned off. The operating system is responsible for organising data on backing storage devices. Work and programs are saved as files which must be organised so that they can be found and loaded when required. Each file is identified by a filename.
Most operating systems divide a backing storage device up into directories (also known as folders). Files can be stored within each directory. The diagram below shows how data has been organised on a particular hard disk drive.
The user has chosen the names of the directories and files but the operating system has decided where and how to store the files on the disk. The operating system has ensured that no two files are stored in the same place on the disk and that rules such as having a maximum length for filenames have been enforced.
MS-DOS®, Windows® 2000 and Microsoft® Works® are registered trademarks of Microsoft Corporation in the United States and other countries.
The most basic type of operating system can only carry out one task at a time and can only be used by one person at a time. Sometimes a more sophisticated operating system is required. The three most common advanced operating systems are :
| Operating System | Use | |
| Network OS | Network operating systems are used on computers which are connected to a network to manage the security of the network. | 
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| Multi-Tasking OS | A multi-tasking operating system can appear to carry out more than one task at the same time. |
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| Multi-User OS | A multi-user operating system is used on a mainframe or minicomputer which may be used by more than one person at a time. |
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These operating systems are more complicated and may require faster hardware and more memory to work than more basic operating systems.
Operating systems that are used on computers connected to networks usually have to carry out some additional tasks. Probably the most important of these is managing system security. Important aspects of systems security that an operating system might manage are :
Making a user log on with a user name and password before a computer can be used.
Allow a user on one computer to access files stored on another computer (e.g. a server).
Controlling who has access to which files.
Keeping a log of all of the files that a user changes.
Automatically backing up important files.
A multi-tasking operating system is capable of appearing to carry out more than one task at a time.
Windows® 2000 and UNIX®; are examples of multi-tasking operating systems. If you have a computer with a multi-tasking OS you could be using a spreadsheet and a word processor at the same time. You could also have the computer testing every five minutes to see if you had any new email and displaying a television programme on screen. Most multi-tasking operating systems have a WIMP user interface with each task being displayed in its own window.
In reality because the computer only has one microprocessor it can only be doing one thing at once. The computer fools you into thinking it is doing more than one task at once by dividing the processor time up between each of the tasks. Each task is given a small amount of processor time in turn. The amount of time allocated to a task is known as a time slice. These time slices are very short, usually less that 1/1000 of a second. Because the slices are so small, the user is fooled into thinking that the computer is doing many things at once.
The time can be allocated to the different tasks on an equal basis (each tasks gets the same amount of time) or on a priority basis with more important or more urgent tasks getting most time. Commonly the task that the user is currently working on (such as entering data into a spreadsheet) has the highest priority. It is allowed exclusive use of the processor whenever it wants it and is known as the foreground job. Other less urgent tasks such as sending data to a printer are only permitted to use the processor when the foreground job does not need it. These tasks are known as background jobs.
If an operating system is carrying out more than one task then the job of allocating memory also becomes harder because the computer’s memory must be allocated between the tasks.
Multi-tasking operating systems are also sometimes known as multi-program operating systems.
Windows® 2000 is a registered trademark of Microsoft Corporation in the United States and other countries.
Multi-user operating systems are used on large mainframe computers. A mainframe computer system has one very powerful processing unit which may contain many microprocessors. Many users will all share this processing unit. They use terminals (a keyboard and a monitor) to access the mainframe computer.
A multi-user operating system lets more that one user work on a mainframe computer at the same time.
A multi-user operating system allocates the CPU time of the mainframe computer between the different users. Each user in turn is given a small time slice of the CPU time. Because these time slices are very small the users believe that they are all using the mainframe at the same time.
This diagram shows how a mainframe computer with four terminals connected to it divides its processing time up.
Time can be shared equally between each user or can be allocated to users using a priority system so that some users get more time than others. Multi-user operating systems rely on having very powerful mainframe computers to process data.
Sometimes operating systems are also classified according to the types of processing that can be carried out by software packages running on them. As well as the categories
of network, multi-tasking and multi-user a particular operating system could fall into these four categories :
| OS Type | Description |
| Batch | A batch operating system can perform batch processing of data. Data to be processed is grouped together into batches. No data is processed until a complete batch has been collected, then all of the data in a batch is processed in one go. There may be a long time delay between data being input into the system and the data being processed. Each task that is waiting to be carried out is known as a job. |
| Interactive | An interactive operating system carries out a dialogue with the user. Once the user has entered a command or selected an option the operating system will process the user's input and react with some output before the user can input any more data. Interactive operating systems can carry out transaction processing. |
| On-Line | An on-line operating system allows the user of a computer to interact directly with another central computer. On-line operating systems are commonly used by transaction processing systems that require many users at different computers to interact with a central computer. An on-line operating system could be used in a chain of travel agents where computers located at many branches make bookings on one central computer. |
| Real Time | A real time operating system can perform real time processing of data. This means it can process data extremely quickly to meet a real world deadline. Real time operating systems are usually used in control systems and embedded systems. |
Note that a particular operating system could fall into more than one category. e.g. it could be network, multi-tasking, interactive and on-line.
GCSE ICT Companion 04 - (C) P Meakin 2004