X11 expertise on ancient HW sought... (4-plane visual (overlay) via X-server on MS-WIndows)

Mouse mouse at Rodents-Montreal.ORG
Sun Jun 7 08:27:04 CDT 2015

> Mouse, thanks very much for taking the time to comment.

You're welcome!  I've received so much help over the years from the
net, paying it back in some small part is the least I can do.  Even in
a case like this where I can't really help except in generalities.

>> [...] comparatively small code changes [...]
> [...] does not have the source.

Oh.  Yeah, that runs the cost of that option up.  Substantially. :-(

>> You probably do not need a GPU.  The era when 4-bit visuals were
>> common was full of dumb memory-mapped framebuffers; modern CPUs are
>> fast enough that they can probably fake up a 4-bit overlay visual
>> and still run at least as fast as the hardware your client software
>> was designed to run against.

>> I'm not sure how hard it would be to do.  [...]

> Depending on the cost, hacking the X-Server might be an option.  I
> have presented MS-Windows as a given here, but in fact I could also
> use Linux.  Do any possibilities involving Linux occur to you?

Nothing specific, because I don't use Linux - and, given what I've seen
in my brushes with Linux-land, I am inclined to doubt that anyone has
built out-of-the-box support for such a thing.

But I would hope Linux is capable of presenting a dumb 24bpp
memory-mapped framebuffer to userland, in which case an X server could
be built which presents whatever it wants to clients and then composits
it all into the framebuffer, with the main CPU if necessary.  (This
would make colourmap changes comparatively expensive if the underlying
hardware is TrueColor instead of DirectColor, but only _comparatively_
expensive - I'd guess such a change could probably be done within a
single vertical retrace.)

However, as I said, this is not something I've ever personally tried,
so I have only wild guesses at how easy it would be.  At least with
Linux on the (X) server host, I become much more confident it's just
("just", hah!) a SMOP.

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