Interrupts are an important capability of modern computers. They allow the processing of several independent tasks by the CPU. On large computers they allow multiuser and time sharing activities. On microprocessors they allow the running of a main program while periodically taking data or sending data to a slow device like a printer. Also computer start up, the keyboard, and TOD functions make use of the interrupt function.

In the discussion which follows, we will first trace the steps taken by the CPU when it receives an Interrupt Request (INTR) from other parts of the computer and then look into the ways we can cause interrupts to be generated and serviced.

Interrupts and the CPU

The interrupt sequence is similar to a jump to a subroutine except that it occurs when signalled by a wire leading to the CPU (INTR). When an interrupt signal is present on the INTR wire and the interrupt enable bit (IF) of the Flag register (see Figure 8.1) is 1, the CPU begins processing the interrupt. The interrupt enable bit is used to prevent the CPU from beginning to process the same interrupt again before it has completed the first one. Without it the computer would go into a continuous regression. The IF bit is set equal to 0 during an interrupt sequence and further interrupts are ignored until this bit is returned to 1. This can be done with the SEI instruction but is done automatically at the return from the interrupt service routine.