A Level Computing for AQA Teacher’s Resource CD-ROM 42 CHAPTER: Operating systems Nick Sims Basic functions Classifying operating systems

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray What does the operating system (OS) do? The operating system is a collection of software that acts as an interface between the hardware, the software and the user. It hides the complexities of the computer from the user, creating a virtual machine. The OS starts running at the point when you switch on and is always operating, even if your machine is apparently idle. Common operating systems include: –Windows – Microsoft –Linux – Linus Torvalds –Unix – Ken Thompson at Bell Labs –MacOS – Apple Modern operating systems are very large programs, because they carry out many tasks.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Common OS tasks Controls the start-up of the computer Recognises that an input device (e.g. the mouse) is being used Loads and unloads programs from memory as needed Allocates memory to programs Attempts to cope with errors (e.g. hardware failure) Manages all the users on a network Controls print queues Closes down the computer properly

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Resource management The OS controls all of the resources on the computer, for example: the amount of processor time that will be spent on certain tasks the amount of memory allocated to programs that are currently running access to secondary storage devices, such as hard disks access to peripherals, such as printers.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Multitasking This is the ability for one user to carry out several tasks at the same time. It usually involves having several programs open at the same time, although the user may only be able to use one at a time. A typical multitasking activity could be downloading from the Internet while printing out a document and using an application all at the same time.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray File management The OS also controls the location of all of the files stored on a computer. It uses directories (folders) to store files in and the files are identified by the name given them by the user and a suffix that indicates the file type. For example, you might set up a folder called coursework and then store coursework.doc inside it. The user sees this as folders and is able to create, edit and delete folders into which the work is stored and rename files. The OS stores folders as directories using a pathname. The pathnames are stored on the drive that contains them (e.g. C drive is the hard disk or D drive is the CD disk).

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Pathnames To take the coursework example, you might set up two folders under MyDocuments – one for coursework and one for theory. You might them create a Word document that you save into the coursework folder called coursework.doc. The pathname for this file would be: C:\MyDocuments\coursework\coursework.doc C is the drive (hard disk). \ is used to separate the parts so there is a folder called MyDocuments and within it another folder called coursework. Coursework.doc is the file itself.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Classifying operating systems There are four main types of operating system: Batch (Off-line) Interactive (On-line) Real-time Network.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Batch (off-line) A number of instructions or tasks are passed to the OS. The OS then carries out the tasks without any further need for user intervention. For example, an electricity company prints thousands of bills. The instructions for printing them can be sent to the OS and the process of handling this request may take place over the next few hours or overnight. Where the OS is given several jobs, it will decide which jobs to do when. The main characteristic of a batch OS is that it is used for repetitive processes where an instant response is not needed.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Interactive (on-line) The user interacts with the OS. This means that the OS responds directly to the instructions it is receiving. This is the most common OS and is used on PCs at home and within organisations. For example: you select Print and the computer sends your document to the printer you select Shut Down and the computer shuts itself down. Effectively, the user is having a dialogue with the computer.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Real-time The OS responds instantly to instructions that it receives. A feature of real-time systems is that the input may be from sensors rather than directly from a user (e.g. a central heating system). Real-time systems can be further classified as: –critical: where any delay in response could cause damage or death (e.g. missile control, ABS brakes) –non-critical: where a delay in response would not be critical (e.g. computer controlled greenhouse, central heating system) Some real-time systems do require human intervention (e.g. on- line airline bookings need to respond instantly so that the same booking cannot be made twice from two different computers).

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Network Networks have multiple workstations and users so the OS needs to cope with this complexity. For example: –logging users on and off –allocating resources to users –controlling access right to data –handling data being transmitted between users –controlling shared peripherals such as printer.

Operating System © 2005 Bob Reeves, Dave Fogg/Hodder Murray Multi-user This term is normally associated with network operating systems which allow more than one user access to the computer at the same time. As more users use the system, the central computer (i.e. the server) will handle the requests more slowly. A supermarket EPOS system is multi-user as all the tills are connected to a server. At the same time, the stock control department could be analysing the data – this is known as multi-access.