C Program Phases Writing And Executing A Program

System Software
The system software controls the computer. It communicates with computer’s hardware (key board, mouse, modem, sound card etc) and controls different aspects of operations. Sub categories of system software are:
  • Operating system
  • Device drivers
  • Utilities
Operating system

An operating system (sometimes abbreviated as "OS") is the program that manages all the other programs in a computer. It is a integrated collection of routines that service the sequencing and processing of programs by a computer. Note: An operating system may provide many services, such as resource allocation, scheduling, input/output control, and data management.

Device drivers

The device driver software is used to communicate between the devices and the computer. We have monitor, keyboard and mouse attached to almost all PC’s; if we look at the properties of these devices we will see that the operating system has installed special software to control these devices. This piece of software is called device driver software. When we attach a new device with the computer, we need software to communicate with this device. These kinds of software are known as device drivers e.g. CD Rom driver, Sound Card driver and Modem driver. Normally manufacturer of the device provide the device driver software with the device. For scanners to work properly with the computers we install the device driver of the scanner. Nowadays if you have seen a scanner, it comes with TWAIN Drivers. TWAIN stands for Technology Without An Interesting Name.

Utility Software
Utility software is a program that performs a very specific task, usually related to managing system resources. You would have noticed a utility of Disk Compression. Whenever you write a file and save it to the disk, Compression Utility compresses the file (reduce the file size) and write it to the disk and when you request this file from the disk, the compression utility uncompressed the file and shows its contents. Similarly there is another utility, Disk Defragmentation which is used to defragment the disk. The data is stored on the disks in chunks, so if we are using several files and are making changes to these files then the different portions of file are saved on different locations on the disk. These chunks are linked and the operating system knows how to read the contents of file from the disk combining all the chunks.
Similarly when we delete a file then the place where that file was stored on the disk is emptied and is available now to store other files. As the time goes on, we have a lot of empty and used pieces on the disk. In such situation we say that the disk is fragmented now. If we remove this fragmentation the chunks of data on the disk will be stored close to each other and thus reading of data will be faster. For the purpose of removing fragmentation on the disk the Defragmentation utility is used. The compilers and interpreters also belong to the System Software category.

Application software

A program or group of programs designed for end users. For example a program for Accounting, Payroll, Inventory Control System, and guided system for planes. GPS(global positioning system) is being used in vehicles which through satellite determines the position of vehicle.

History of C language
The C language was developed in late 60’s and early 70’s, in Bell Laboratories. In those days BCPL and B languages were developed there. The BCPL language was developed in 1967 by Martin Richards as a language for writing operating systems software and compilers. In 1970 Ken Thompson used B language to create early versions of the UNIX operating system at Bell Laboratories. Thus both the languages were being used to develop various system software even compilers. Both BCPL and B were ‘type less’ languages, every data item occupied one ‘word’ in memory and the burden of treating a data item as a whole number or real number, for example was the responsibility of the programmer.
Dennis Ritchie developed a general purpose language, called C language, by using different features of BCPL and B languages. C uses many important concepts of BCPL and B while adding data typing and other features. In the start C became widely known as the development language of the UNIX operating system, and the UNIX operating system was written by using this C language. The C language is so powerful that the compiler of C and other various operating systems are written in C. C language has almost unlimited powers to do with computers. You can program to rn on or off any device of computer. You can do a lot to hard disk and other peripherals. It is very easy to write a program in C that stops the running of computer.
The C language and UNIX operating system widely spread in educational and research institutions. There was C and UNIX everywhere. Due to the wide spread of C, different researchers started to add their features in the language. And thus different variations in C came into existence. Many universities developed their own C by adding different features to the C language developed by Ritchie. These variations led to the need of a standard version of C. In 1983 a technical committee was created under the American National Standards Committee on Computer and Information Processing to provide an unambiguous and machine-independent definition of the language. In 1989 the standard was approved. ANSI cooperated with international Standard Organization (ISO) to standardize C worldwide.

Different types of Tools
As programmer we need different tools to develop a program. These tools are needed for the life cycle of programs.


Editors
First of all we need a tool for writing the code of a program. For this purpose we used Editors in which we write our code We can use word processor too for this, but word processors have many other features like bold the text, italic, coloring the text etc, so when we save a file written in a word processor, lot of other information including the text is saved on the disk. For programming purposes we don’t need these things we only need simple text. Text editors are such editors which save only the text which we type. So for programming we will be using a text editor.

Compiler and Interpreter
As we write the code in English and we know that computers can understand only 0s nd 1s. So we need a translator which translates the code of our program into machine language. There are two kinds of translators which are known as Interpreter and Compilers. These translators translate our program which is written in C-Language into Machine language. Interpreters translates the program line by line meaning it reads one line of program and translates it, then it reads second line, translate it and so on. The benefit of it is that we get the errors as we go along and it is very easy to correct the errors. The drawback of the interpreter is that the program executes slowly as the interpreter translates the program line by line. Another drawback is that as interpreters are reading the program line by line so they cannot get the overall picture of the program hence cannot optimize the program making it efficient.
Compilers also translate the English like language (Code written in C) into a language (Machine language) which computers can understand. The Compiler read the whole program and translates it into machine language completely. The difference between interpreter and compiler is that compiler will stop translating if it finds an error and there will be no executable code generated whereas Interpreter will execute all the lines before error and will stop at the line which contains the error. So Compiler needs syntactically correct program to produce an executable code. We will be using compiler in our course.

Debugger
Another important tool is Debugger. Every programmer should be familiar with it. Debugger is used to debug the program i.e. to correct the logical errors. Using debugger we can control our program while it is running. We can stop the execution of our program at some point and can check the values in different variables, can change these values etc. In this way we can trace the logical errors in our program and can see whether our program is producing the correct results. This tool is very powerful, so it is complex too.

Linker

Most of the time our program is using different routines and functions that are located in different files, hence it needs the executable code of those routines/functions. Linker is a tool which performs this job, it checks our program and includes all those routines or functions which we are using in our program to make a standalone executable code and this process is called Linking.

Loader
After a executable program is linked and saved on the disk and it is ready for execution. We need another process which loads the program into memory and then instruct the processor to start the execution of the program from the first instruction (the starting point of every C program is from the main function). This processor is known as loader. Linker and loaders are the part of development environment. These are part of system software.

The following figure represents a graphical explanation of all the steps involved in writing and executing a program.
 
 
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