Re: 16bpp graphics modes on old iMac, what is the MSB function?

From: Tom Stiller (tomstiller_at_comcast.net)
Date: 06/01/05 

Date: Tue, 31 May 2005 23:15:18 -0400

In article <1117591516.935208.152430@g43g2000cwa.googlegroups.com>,
 larwe@larwe.com wrote:

> Hi Tom,
>
> Tom Stiller wrote:
>
> > > Is there some special function assigned to the MSB of video memory in a
> > > Mac running at "thousands" of colors, specifically a 16bpp mode on the
> > > kernel framebuffer device?
> >
> > IIRC, the 2^15 bit was an alpha plane overlay bit. I think there's
> > something wrong with your mapping. It makes no sense to have more bits
> > representing blue than representing red.
>
> I did post a reply saying I agree it makes no sense, but I'd verified
> it experimentally. Well, that turns out to be a lie. In fact the mode
> that the normal console runs in is utterly beyond weird. The color
> components are in basically the right positions, but it's as if the DAC
> is still in a palettized mode.
>
> If I execute X & sleep 10 ; ./myapp then the graphics appear totally
> normal.
>
> It's really, really bizarre. Any thoughts?

It's been too long since I worked at that level of graphic hardware and
my brain's fuzzy. 

-- 
Tom Stiller
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