The 8259A Programmable Interrupt controller was introduced by Intel in 1972. It accepts
interrupts from up to eight
different devices. If any one of the devices requests service, the 8259 will
toggle an interrupt output line (connected to the CPU) and pass a
programmable interrupt
vector to the CPU. You can
cascade the device to support up to 64 devices by connecting nine 8259s
together: eight of the devices with eight inputs each whose outputs become the eight inputs of the ninth device. A
typical PC uses two of these devices to provide 15 interrupt inputs (seven on the
master PIC with the eight input coming from the slave PIC to process its eight inputs}.
The 8259 PIC is a very
complex chip to program. Fortunately, all of the
hard stuff has already been done for you by the BIOS when the
system boots.
Most computers these days have more complicated PIC's than the 8259, but they are all
compatible with the 8259