What is meant by a system call?



A system call is an interrupt to the operating system to perform some activity. On receiving a system call CPU suspend the current activity and branch to an interrupt service routine and executes it. It results in performing the task desired by the caller. On receiving a system call, the program status word which contains the state of CPU is stored in stack along with the return address.

System calls are traps to operating system. They are handled by the operating system. Normal function call does not save program status work in stack. Only the return address is stored in stack. Normal function calls are not managed by operating system. In normal function call, program control is transferred to a subroutine and after the execution of subroutine, program control returns to the main program. System calls provide an interface between the process and operating system calls allows user level processes to request some services from operating system which process itself is not allowed to do. When a system call is made, operating system enter into Kernel mode where it has access to privileged instructions and can perform the desired service on behalf of user level process.   

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