Port 3C0  -  Address Register

        The attribute controller registers do not have an input bit
        to control selection of the address and data registers. An
        internal address flip-flop controls this selection. Reading
        Input Status Register 1 (port 3DA) clears the flip-flop and
        selects the Address register.

        After the Address register has been loaded with the index, the
        next Write operation to 3C0 loads the data register. The
        flip-flop toggles for each Write operation to address 3C0. It
        does not toggle for Read operations to 3C1.

                11xx xxxx  Reserved
                xx1x xxxx  Internal Palette Address Source
                             This bit is set to 0 to load color values
                             to the registers the internal palette. For
                             normal operations of the attribute controller,
                             this bit is set to 1, which allows the video
                             data to use the internal palette for output.
                xxx1 1111  Register Index
                             These bits contain the index to the data
                             registers in the attribute controller.