Semantics of System Call Execution is described as follows
The following series of events takes place when a process invokes the system call:
1. The user procedure makes a call to the library function
2. The library routine puts the appropriate parameters at a well-known place, like a register or may be on the stack. These parameters comprises of arguments for the system return, address call, and call number. Three basic methods are used to pass parameters between the running program and the operating system is stated below.
- Pass parameters in the registers.
- Save the parameters in a table in the main memory and the address of table is passed as a parameter in the register.
- Push (store) parameters onto the stack by the program, and pop off the stack by the operating system.
3. A trap instruction is executed to change the mode from the user to kernel and give control to an operating system.
4. The operating system determines which system call is to be carried out by examining one of the parameters passed to it by the library routine.
5. The kernel uses call number to index the kernel table or the dispatch table which contains pointers to service routines for all the system calls.
6. The service routine is executed and control given back to the user program by means of return from trap instruction; the instruction also changes mode from system to the user.
7. The library function executes the instruction following the trap; interprets the return values from the kernel and returns back to the user process.
Figure gives a pictorial view of the above written steps.
Figure Pictorial view of the steps needed for execution of a system call
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