Programming Embedded Systems, 2nd Edition

Interrupt Service Routine

Let’s take a closer look at the ISR. The ISR is the function called when a particular interrupt occurs. Its central purpose is to process the interrupt and then return control to the main program. Typically, ISR functions have no arguments passed into them; they can never return a value.

In order to keep the impact of interrupts on the execution of the main program to a minimum, it is important to keep interrupt routines short. If additional processing is necessary for a particular interrupt, it is better to do this outside of the ISR. Keeping ISRs short also aids in ISR debug, which can be difficult. When it is done by a specific function, completion of the interrupt handling outside the ISR is called a deferred service routine (DSR).

Regardless of the specific processing required by the ISR, the ISR is responsible for doing the following things:

Saving the processor context

Because the ISR and main program use the same processor registers, it is the responsibility of the ISR to save the processor’s registers before beginning any processing of the interrupt. The processor context consists of the instruction pointer, registers, and any flags. Some processors perform this step automatically.

Acknowledging the interrupt

The ISR must clear the existing interrupt, which is done either in the peripheral that generated the interrupt, in the interrupt controller, or both.

Restoring the processor context

After interrupt processing, in order to resume the main program, ...