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