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.
0 Comments