1 Software

2 What is software ► Software is the term that we use for all the programs and data on a computer system. ► Two types of software ► Program - the instructions that the computer follows (e.g. a word processor or a game). ► Data - what the programs process (e.g. a word processing document or a saved game)

3 More on software ► Systems Software - programs used to organise the computer, handle its devices, manage memory etc i.e. The Operating System ► Applications software programs designed to carry out a manual task (Word Processors, Databases, Graphics etc.)

4 The Operating System ► The operating system (O/S) is the most important program used by the computer.  Usually the O/S is stored on hard disc and is loaded into RAM when the computer starts up.  Some computers store their O/S in ROM so that it cannot be erased by accident. ROM operating systems load very quickly.

5 The Operating System  The operating system controls and monitors the operation of the computer system. Examples of operating systems which are: ► Windows XP and Vista for the PC ► OS X for Apple ► Unix for large fileservers and networks.

6 Standard Functions of the OS. ► User interface  The means of communicating with the computer system (Icons are provided by the operating system). ► Manages memory  The way that memory is allocated to programs is controlled by the OS. ► Controls input/output  The access to all devices attached to the computer is controlled by the OS.

7 More OS - Standard Functions ► The filing system manages backing storage  The filing system will prevent two programs trying to access the same files at the same time.

8 Resource allocation ► Various techniques are used to prevent two programs demanding the same resource at the same time (e.g. two programs can't both send files to the printer at the same time). The resource allocation system also ensures that all programs running on the system receive as much processing time as possible.

9 OS - Special Functions ► Multi-programming  Operating Systems allow more than one program to run on the system at the same time.  Multi-access Some operating systems allow many users to use the same computer system at the same time. Any computer system used in this way will have a multi-access operating system.

10 OS - Different modes of processing ► Interactive Interactive computer systems are programs that allow users to enter data or commands. Most popular programs, such as word processors and spreadsheet applications, are interactive. In interactive processing, the application responds to commands as quickly as it can once they have been entered.

11 OS - Different modes of processing ► Real Time Real-time operating systems are systems that respond to input immediately. Automatic teller machines for banks are an example of real time processing.

12 OS - Different modes of processing ► Background job capability ► Some operating systems allow a background process to occur at the same time as a foreground interactive process. ► The foreground process is the one that accepts input from the keyboard, mouse, or other input device. Background processes cannot accept interactive input from a user, but they can access data stored on a disk.

13 The Filing System Catalogue/Directory ► The contents of a hard discs are held in the directory (also called catalogue). The filing system reads this information so that it can find any file on the disc straight away. ► Software can be stored on backing storage as files. There are two types of files.  Program files (such as the operating system, word processors etc).  Data files (such as graphics images, documents etc).

14 Types of filing system ► A hierarchical filing system is one that uses directories to organise files into a tree structure. C:/D:/ Computer Drives Folders Files ► Hierarchical filing system ► A filing system in which directories have files and subdirectories beneath them. A Hierarchical Filing Structure

15 Types of access ► Sequential Access ► To go from file A to file Z in a sequential-access system, you must pass through all intervening files. ► Sequential access is sometimes called serial access. ► Random Access ► Refers to the ability to access data at random. In a random-access system, you can jump directly to file Z. Disks are random access media, whereas tapes are sequential access media.

16 Systems Software ► The operating system is just one example of systems software. ► Other examples of systems software are:  Anti-virus tools  Disk utilities  Programming tools  Printer manager utilities ► All these are examples of programs. Programs can be written in a number of different ways...

17 Low Level Languages ► Computers only understand binary! ► All programs have to end up in binary language in order work. ► The binary language that the computer understands is called MACHINE CODE ► Machine code looks like this… , , ► NOT very easy to understand. That’s why High Level Languages have been produced...

18 High Level Languages ► We use High Level Languages (HLLs) because programming in low level languages like machine code is VERY difficult. ► High level languages make it easier for us to write programs because we can use commands that mean something (PRINT, END, IF, THEN). ► High level language programs still have to be converted into machine code to work. This is done by a translator.

19 Common features of High Level Languages ► English like words and phrases as instructions eg. PRINT, IF, THEN, END, STOP, ELSE, OPEN etc. ► Arithmetic operations such as multiply, divide, add subtract etc.

20 Common features of High Level Languages ► Must be translated into machine code. A special program (a type of systems software) called a translator does this….

21 Types of translator ► 1. Compiler ► A program that translates source code into machine code. ► Compilers create an executable program. Programs produced by compilers run much faster than the same programs executed by an interpreter.

22 Types of translator ► Interpreter ► An interpreter translates line by line. Each HLL statement is converted, in turn, into machine code and then executed. ► Interpreters are sometimes used during the development of a program, when a programmer wants to add small sections at a time and in education because they allow students to program interactively.