DOS idle (INT 28h - internal)

        This interrupt is called from inside the "get input from
        keyboard" routine in DOS, if and only if it is safe to use
        INT 21 to access the disk at that time. It is used primarily
        by the PRINT.COM routines and TSR programs, but any number
        of other routines could be chained to it by saving the original
        vector, and calling it with a FAR call (or just JMPing to it)
        at the end of the new routine.

        The INT 28h handler may invoke any INT 21h function except
        functions 00h through 0Ch.

        Until some program installs its own routine, this interrupt
        vector simply points to an IRET opcode.