SYSTEM CALL.............

System calls:

                                  System calls provide the interface between a process and the operating system. These calls are generally available as assembly-language instructions, and they are usually listed in the various manuals used by the assembly-language programmers. Certain system allow system calls to be made directly from a higher level language program, in which case the calls normally resemble predefined function or subroutine calls. They may generate a call to a special run-time routine that makes the system call, or the system call may be generated directly in line.

             How system calls are used, consider writing a simple program to read data from one file (input file) and to copy them to another file (output file). In an interactive system, this approach requires a sequence of system calls. First system call is to write a prompting message on the screen, then to read from the keyboard the characters that define the two files. Once the two file names are obtained, the program must open input file and create output file. Each of these operations require another system call. 

Various type of system calls are following:

• Process control
• File management
• Device management
• Information maintenance
• Communications

Function of process control:

• Process control performs abort or end of program.
• It executes and loads the program.
• The process should be created and terminated by the process control.
• Process control system calls set and get process attributes.
• Process control perform wait and signal event function.
• Process control perform wait for time function and allocate the memory to program.

Function performs by file management:

• File management performs read, write and repositioning of the program.
• The creation and deletion of file function should be performed by the file management.
• Set and get process attributes function also performed by the file management.
• Open and close of the program/file function occurred by file management.

Function performs by device management:

• Device logically attachment and detachment function performed under device management.
• The request and release of device function performs by the device management.
• Device management performs the set and get device attributes function.
• Read, write and reposition of the devices should be performs by device management.

Function of communication:

• Communication system calls perform creation and deletion communication connection.
• Send and receive massages function performed by the communication system calls.
• Transfer status information and attach or detach remote devices function performed by the communication system calls.

Function of information maintenance:

• Information maintenance system calls perform set or get time or date.
• System date should be set or get by the information maintenance system calls.
• The attributes of the process, file and device are set and get by the information maintenances.