Software Interrupt

We can introduce a service call that lets one process cause a software interrupt in another:

Interrupt(process id, interrupt number)

and another that allows a process to associate a handler with an interrupt:

Handle(interrupt number, handler)

Software interruprts allow only one bit information to be communicate - that an event associated with the interrupt number has occurred. They are typically used by an operating system to inform a process about the following events:
The user typed the ``attention key''.
An alarm scheduled by the process has expired.
Some limit, such as file size or virtual time, has been exceeded.

It is important to distinguish among interrupts, traps, software interrupts, and exceptions. In all cases, an event is processed asynchronously by some handler procedure. Interrupt and trap numbers are defined by the hardware which is also responsible for calling the procedure in the kernel space. An interrupt handler is called in response to a signal from another device while a trap handler is called in response to an instruction executed within the cpu.

Software interrupt and exception handlers are called in user space. A software interrupt handler is called in response to the invocation of a system call. Software interrupt numbers are defined by the operating system. Exceptions are defined and processed by the programming language. An exception raised in some block, b, of some process p, can be caught by a handler in the same block, or a block/procedure (in p) along static/dynamic links from b, or by a process q that (directly or indirectly) forked p. The raiser of an exception does not identify which process should handle it, so exceptions are not IPC mechanisms.

The notion of software interrupts is somewhat confused in some environments such as the PC, where traps to kernel-provided I/O routines are called software interrupts. There is a special instruction on the PC called INT which is used to invoke these traps. For instance, the instruction

int 16H

executes the BIOS interrupt routine for processing the current character received from the keyboard. (It is executed by the interrupt handler of the Xinu kernel to ask the PC BIOS handler to fetch the character from the keyboard.) The term interrupt is used because these rountines are called usually by hardware interrupt routines. We are using the term software interrupts for what Unix calls signals, which are not to be confused with semaphores, though you invoke the signal operation on both!