IT-101 Section 001 Lecture #13 Introduction to Information Technology

Computer Software  Overview

 Introduction Computer software consists of the instructions and application programs that permit computers to accomplish tasks It is called software because, unlike hardware that has fixed configurations, connections and operation, software is flexible and easily modified Software programs control computers at 4 different levels: Assembly code Operating system Programming language Application

Hierarchy of Software Operating System Assembly Code Programming Language (High Level Language) Application

Assembly Code Assembly Code, also called assembly language is the most basic language of the computer and consists of elementary instructions recognized by the CPU It is a a human-readable notation for the lowest level language-the machine language that a specific computer architecture uses. Machine language, a mere pattern of bits, are instructions directly recognized by the CPU Machine language is made readable (i.e. converted to assembly language) by replacing these pattern of bits with symbols called ‘mnemonics’ Unlike high-level languages, there is a 1-to-1 mapping from simple assembly to machine language, so that computers can translate in both directions without losing information. Assembly language is converted to machine language by an assembler

Examples of assembly code instructions (mnemonics) include: ADD and MOV Assembly code instructions perform the basic computer operations including moving data, changing contents of registers, adding the contents of two registers and jumping to different parts of a program For example, the computer performs the arithmetic operation: as follows: MOV 1, R0 (Move the number 1 into Register 0) MOV 2, R1(Move the number 2 into Register 1) ADD R0, R1 (Add R0 to R1 and put the result in Register 1) MOV 4, R0 (Move the number 4 into Register 0) ADD R0, R1 (Add R0 to R1 and put the result into Register 1) Assembly code is different for each type of computer and a code written for the Intel Pentium (IBM) microprocessor chip will not operate on the PowerPC (Macintosh) microprocessor

Operating System The operating system is a set of commands that performs common system tasks such as accepting data from the keyboard, displaying data on the monitor, sending data to the printer, examining the contents of a directory, etc. It is stored on the hard disk and is loaded to memory when the computer is switched on An operating system is conceptually broken into two components: A shell and a kernel. The shell is a command interpreter and as the name implies, is an outer wrapper to the kernel which in turn talks directly to the hardware Hardware Kernel Shell Applications In some operating systems the shell and the kernel are completely separate entities, allowing you to run varying combinations of shell and kernel (eg Unix), in others, the separation is only conceptual (eg Windows). Source: wikipedia.org

Examples of operating systems MS-DOS (Microsoft disk operating system) Introduced in 1981 Required basic familiarity with its operation and organization Intel computers such as commonly used MS-DOS Users were required to know basic abbreviations such as dir or del to perform tasks Uses a command line user interface-is non graphical MacOS Introduced in 1984 Operating system for apple Macintosh computers First commercially successful OS which used GUI (graphical user interface) Used icons to represent different data structures Microsoft Windows Dominant OS and is used in over 90% of PC’s Introduced in 1985 in response to MacOS

Generations of Windows: Windows 1.x, 2.x, 3.x Windows 95, 98 Windows NT Windows Me Windows 2000 Windows XP Windows Vista Unix and similar systems Unix MacOS X Linux Solaris operating environment Microsoft Xenix AIX by IBM

Linux Linux is causing a revolution in the world of computers Initially created as a hobby by a young student called Linus Torvalds at the University of Helsinki in Finland It may be used for a variety of purposes including networking and software development It is often considered to be an excellent, low-cost alternative to other more expensive operating systems The attractiveness of Linux is that its source code, which is a series of commands written in a programming language is freely available to anyone The source code can be modified to fit the individual needs of the user It is developed under the GNU General Public License, enabling the user to share and change free software in comparison to most other software whose source code is concealed, preventing users to share or modify it Source: linux.org

Operating Systems can be classified as: GUI: Graphical User Interface operating systems are operating systems that have the capability of using a mouse and are graphical Multi user: allows multiple users to utilize the computer and run programs at the same time Multi processing: allows multiple processors to be utilized Multi tasking: allows multiple software processes to be run at the same time Multi threading: allows different parts of a software program to run concurrently Operating system classification

Programming Language Higher level programming languages contain instructions that are more powerful, more general and better suited for programming a task than the basic assembly language instructions Each higher level language instruction might consist of hundreds of assembly language instructions Software programmers write the most efficient and fastest programs in assembly language However, it is tedious to write programs in assembly language and is easy to make errors To make program development easier, software programmers write in higher level computer programming languages such as FORTRAN, BASIC, PASCAL, etc

Programs written by programmers are translated into the machine language by a compiler Examples of programming languages: BASIC C C++ COBOL DBASE Java Pascal Perl Visual Basic XML

Application programs An application program is software written by a professional programmer for a specific operating system to perform a particular task such as word processing or graphic design Before ‘shrink-wrap’ software was available, computer users also needed to be computer programmers who knew the operating system and at least one higher level language Such knowledge is no longer necessary and software suitable for a particular application can be directly purchased Examples of application software: MS office Explorer Media player MSN messenger Adobe acrobat