IV.

Program Function

Modern computers usually store programs on some form of magnetic storage media that can be accessed randomly by the computer, such as the hard drive disk permanently located in the computer, or a portable floppy disk. Additional information on such disks, called directories, indicate the names of the various programs on the disk, when they were written to the disk, and where the program begins on the disk media. When a user directs the computer to execute a particular application program, the operating system looks through these directories, locates the program, and reads a copy into RAM. The operating system then directs the CPU (central processing unit) to start executing the instructions at the beginning of the program. Instructions at the beginning of the program prepare the computer to process information by locating free memory locations in RAM to hold working data, retrieving copies of the standard options and defaults the user has indicated from a disk, and drawing initial displays on the monitor.

The application program requests a copy of any information the user enters by making a call to a system routine. The operating system converts any data so entered into a standard internal form. The application then uses this information to decide what to do next—for example, perform some desired processing function such as reformatting a page of text, or obtain some additional information from another file on a disk. In either case, calls to other system routines are used to actually carry out the display of the results or the accessing of the file from the disk.

When the application reaches completion or is prompted to quit, it makes further system calls to make sure that all data that needs to be saved has been written back to disk. It then makes a final system call to the operating system indicating that it is finished. The operating system then frees up the RAM and any devices that the application was using and awaits a command from the user to start another program.