Other uses of interrupts

An OS providesa variety of services to aplication programs. To facilitate the implementation of these services, most processors have several different software interrupt instructions, each with its own interrupt vector. They can be used to call different parts of the OS, depending on the service being requested. Alternatively, a processor may have only one software instruction with an interrupt, and also with an immediate operand that can be used to specify the desired service. In a computer that has both a supervisor and a user mode, the processor uses or switches its operation to supervisor mode at the time it accepts an interrupt request. It does so by setting a bit in the processor status register after saving the old contents of that register on that stack. Thus when an application program calls the OS by a software interrupt instruction, the processor automatically switches to supervisor mode giving the os complete access to the computer’s resources.