NAME
        WritePotgo -- write to the hardware potgo register.

   SYNOPSIS
        WritePotgo(word, mask)
                   D0    D1

        void WritePotgo( uword, uword );

   FUNCTION
        The WritePotgo routine sets and clears bits in the hardware
        potgo register.  Only those bits specified by the mask are
        affected -- it is improper to set bits in the mask that you
        have not successfully allocated.  The bits in the high byte
        are saved to be maintained when other users write to the
        potgo register.  The START bit is not saved, it is written
        only explicitly as the result of a call to this routine with
        the START bit set: other users will not restart it.

   INPUTS
        word - the data to write to the hardware potgo register and
            save for further use, except the START bit, which is
            not saved.
        mask - those bits in word that are to be written.  Other
            bits may have been provided by previous calls to
            this routine, and default to zero.