The Interrupt Handler

Although the CPU normally progresses sequentially through memory, carrying out the instructions and working on the data it finds, there are some situations in which this is not necessarily a good thing.

For example, if the CPU had to continually keep checking if a key has been pressed on the keyboard or a floppy disk has been inserted in its drive, it could waste a lot of its time which could otherwise be used executing the program in memory.

A special hardware mechanism, called Interrupts, is provided whereby certain time-critical devices can interrupt the CPU's normal program sequence and divert the CPU's program counter to whichever device caused the interruption.

Only 16 interrupts are provided. The first 8 of those are reserved for system functions and the remaining 8 are provided for the use of external hardware such as the keyboard, modems, the mouse, soundcards etc.