8259A Programmable Interrupt Controller

        The 8259A prioritizes up to eigth interrupts and presents
        them to the 8088 in their priority sequence. Pending lower
        priority interrupts are held until they may be processed.
        INT 08-0F are associated with the eight 8259 interrupts
        (IRQ 0-7). IRQ 0 is highest priority, while IRQ 7 is lowest.
        Interrupt types may be individually enabled or disabled by
        setting 8259 mask bits. The associated interrupt routine is
        responsible for notifying the 8259 when other interrupts may
        be processed by sending 20h to port 20h. On the IBM PC AT,
        two 8259A has been cascade cobbled together, therefore the
        AT has 16 IRQs.

             IRQ      Used by
             ------------------------------------------
              0       Timer interrupt
              1       Keyboard interrupt
              2       EGA vertical retrace
              3       Serial port 2 (com2:)
              4       Serial port 1 (com1:)
              5       Fixed disk (PC/AT: LPT2)
              6       Diskette interrupt
              7       PPI interrupt (lpt1:, lpt2:)
              8       Real-time clock (AT/XT286/PS)
              9       LAN adapter 1 (AT,XT286/PS, rerouted to
                      INT 0Ah by BIOS)
              A       Reserved (AT/XT286/PS)
              B       Reserved (AT/XT286/PS)
              C       PS50+ mouse interrupt (AT/XT286/PS)
              D       80287 error (AT/XT286/PS, rerouted
                      to INT 2 by BIOS)
              E       Fixed disk (AT/XT286/PS)
              F       Reserved (AT/XT286/PS)

        See Intel Microprocessor and Peripheral Handbook, for
        specifications.