INT 21 - DOS 1+ - SET INTERRUPT VECTOR
        AH = 25h
        AL = interrupt number
        DS:DX -> new interrupt handler
Notes:  this function is preferred over direct modification of the interrupt
          vector table
        some DOS extenders place an API on this function, as it is not
          directly meaningful in protected mode
        under DR-DOS 5.0+, this function does not use any of the DOS-internal
          stacks and may thus be called at any time
        Novell NetWare (except the new DOS Requester) monitors the offset of
          any INT 24 set, and if equal to the value at startup, substitutes
          its own handler to allow handling of network errors; this introduces
          the potential bug that any program whose INT 24 handler offset
          happens to be the same as COMMAND.COM's will not have its INT 24
          handler installed
SeeAlso: AX=2501h,AH=35h

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