DOS Internal - Move file pointer

        Entry:
          AX = 1228h
          BP = 4200h, 4201h, 4202h (see INT 21/AH=42h)
          BX = file handle
          CX:DX = offset in bytes
        Return:
          ???

        Notes:  can be called only from within DOS switches critical error
                stack, moves BP to AX, and calls INT 21/AH=42h