26Aug What Is INTERRUPT????

Although I/O devices may be treated like memory locations, there is one big difference between the two; I/O devices generally have the capability to interrupt the microprocessor while it is executing a program. The I/O device does this by issuing an interrupt request input signal to the microprocessor. Each device in a PC-compatible system that is capable of interrupting the microprocessor must be assigned it’s own unique IRQ number. The system uses this number to identify which device is in need of service.

If the microprocessor is responding to INT signals and a peripheral device issues an interrupt request on an IRQ line, the microprocessor will finish executing it’s current instruction and issue an Interrupt Acknowledge(INTA) signal on the control bus. The microprocessor suspends its normal operation and stores the contents of its internal registers in a special storage area reffered to as the stack.

The interrupting device responds by sending the starting address of a special program called the interrupt service routine to the microprocessor. The microprocessor uses the interrupt service routine to service the interrupting device. After it finishes servicing the interrupting device the contents of the stack are restored to their original locations, and the microprocessor returns to the original program at the point where interrupt occured. If two interrupt signals occur at the same instant, the interrupt that has the highest priority will be serviced first.

Two varities of interrupts are used in microcomputers:

Maskable Interrupts (MI): Which the computer can ignore under certain conditions.

Non maskable interrupts (NMI): Which it must always respond to.