What Is a System Call?

A currently running process has to make what's called a system call. That basicly just means asking the operating system to do something on its behalf. A section of code stored in memory will receive the call that's just been made by the process and read the values that have been placed on those registers. The trap handler starts executing its code, looking at the registers where the message has been left by the process. And on the basis of tever data is stored there, it executes the correct instructions,. Once that instruction has been completed, the processo is returned to use a mode and is e control is handed back to whatever process the operating system hands the control back to.