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INT 2F - Multiplex - DOS 3.3+ - 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)
Return: DS:DX from previous invocation of this function
        ES:BX from previous invocation of this function
Notes:  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 (at least two viruses are known to use
          it to get the original ROM entry point)