What is system call? Explain the system call flow with the help of a block diagram.

A special signal or message generated and send by the user program or system itself to invoke a module of operating system so that it can work with it and access the control of the hardware. System call changes the mode of the application from user mode to kernel mode. Now when the mode is changed the application can complete its request to access the control of the hardware.

Let us consider a read system call. The system call constitutes of three parameters, file name, pointer to the buffer and number of bytes to read. The function call looks like,

count= read(fd, buffer, nbytes)

The count consist of total number of bytes to read and if some error occur it is set to -1 which is also indicated by a global variable errno. The program should check this error variable timely to check if any error has occurred. The 1^st and the 3^rd parameters are passed by value and the 2^nd parameter is passed by reference i.e. the buffer address is passed.

Now when the read procedure is called by the user program the control is transferred to the read procedure. At this stage the parameters are pushed onto the stack (step 1, 2, 3) and the control is now completely transferred to the read procedure. Then the system call number is put into the register(step 5) and a TRAP instruction is executed that switches from user mode to kernel mode(step 6). Now the system call number is examined and then the dispatcher dispatches to the correct system call handler via table of pointers to the system call handler(step 7). After that the system call handler works(step 8). After the system call handler completes its task the control may be returned to the user-space library procedure (step 9). Then the control is transferred to the user program from the read procedure (step 10).

Finally, SP is incremented to clean up the stack. In this way the job of read system call is completed.