INT 2F U - DOS 3.2+ - SET DISK INTERRUPT HANDLER
        AH = 13h
        DS:DX -> interrupt handler disk driver calls on read/write
        ES:BX = address to restore INT 13 to on system halt (exit from root
                 shell) or warm boot (INT 19)
Return: DS:DX from previous invocation of this function
        ES:BX from previous invocation of this function
Notes:  IO.SYS hooks INT 13 and inserts one or more filters ahead of the
          original INT 13 handler.  The first is for disk change detection
          on floppy drives, the second is for tracking formatting calls and
          correcting DMA boundary errors, the third is for working around
          problems in a particular version of IBM's ROM BIOS
        before the first call, ES:BX points at the original BIOS INT 13; DS:DX
          also points there unless IO.SYS has installed a special filter for
          hard disk reads (on systems with model byte FCh and BIOS date
          "01/10/84" only), in which case it points at the special filter
        most DOS 3.3+ disk access is via the vector in DS:DX, although a few
          functions are still invoked via an INT 13 instruction
        this is a dangerous security loophole for any virus-monitoring software
          which does not trap this call (many Bulgarian viruses are known to
          use it to get the original ROM entry point)
SeeAlso: INT 19