Int 2F Fn 13  U - DOS 3.2+ - Set Disk Interrupt Handler                    [D]

   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 set by previous invocation of this function
   ES:BX set by 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 3.2+ 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 ("INT13", "Nomenklatura", and many
     Bulgarian viruses are known to use it to get the original ROM entry
     point)

See Also: INT 13/AH=01h,INT 19,INT 9D"VIRUS"