Vladimir Ivanov <vla...@xxxyahooxxx.com
>> The Apple IIgs Hardware Reference gives a small amount of information less
>> than a page and half including the usual table for figuring the color
> The Hardware Reference claims 4-bit sliding window.
Hmm, my observations suggest that it uses 5 or 6 bits. Also, the color of
certain pixels appears to depend on whether _future_ bits are set. I would
expect a "normal" sliding window to consider only the past bits.
1000 0000 gives a single blue pixel followed by black pixels
1000 1000 gives five blue pixels followed by black pixels
1000 0100 gives five blue pixels, followed by one green pixel, followed by black pixels
In the last two cases, adding a 1 to the second block of four causes the space
in between to be filled. I would expect to see black in between the blue and
green in the third case, and four blue pixels in the first case.
I've written the bits as they appear at the output of the shift register. The
fact that the actual bytes in video memory may be different doesn't seem to
The hardware does appear to use some sort of sliding window as the same
behavior occurs if the patterns are shifted over 1, 2, or 3 bits -- just the
actual colors are different.