IRQ2 - EGA VERTICAL RETRACE

        Notes:  On the Tandy 1000, this interrupt is used by the hard disk
        the TOPS and PCnet adapters use this interrupt request line by
        default DOS 3.2 revectors IRQ2 to a stack-switching routine on ATs
        and above, the physical data line for IRQ2 is labeled IRQ9 and
        connects to the slave 8259.  The BIOS redirects the interrupt for
        IRQ9 back here.