NAME
        WriteExpansionByte - write a byte nybble by nybble.

   SYNOPSIS
        WriteExpansionByte( board, offset, byte )
                            A0     D0      D1

   FUNCTION
        (Not typically called by user code)

        WriteExpansionByte writes a byte to a new-style expansion
        board.  These boards have their writeable data organized
        as a series of nybbles in memory.  This routine writes
        two nybbles in a very careful manner to work with all
        types of new expansion boards.

        To make certain types of board less expensive, an expansion
        board's write registers may be organized as either a
        byte-wide or nybble-wide register.  If it is nybble-wide
        then it must latch the less significant nybble until the
        more significant nybble is written.  This allows the
        following algorithm to work with either type of board:

                write the low order nybble to bits D15-D12 of
                        byte (offset*4)+2

                write the entire byte to bits D15-D8 of
                        byte (offset*4)

        The offset is a byte offset into a expansionrom structure.
        The actual memory address read will be four times larger.
        The macros EROFFSET and ECOFFSET are provided to help get
        these offsets from C.

   INPUTS
        board - a pointer to the base of a new style expansion board.
        offset - a logical offset from the configdev base
        byte - the byte of data to be written to the expansion board.

   EXAMPLES
        WriteExpansionByte( cd->BoardAddr, ECOFFSET( ec_Shutup ),  0 );
        WriteExpansionByte( cd->BoardAddr, ECOFFSET( ec_Interrupt ), 1 );

   SEE ALSO
        readexpansionbyte(), readexpansionrom()