[argyllcms] Re: Help: We don't have access to the RAMDAC

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 21 Jan 2007 12:08:01 +1100

Stefan Döhla wrote:
My old Toshiba Portege notebook was in former times running with a
64 entry RAMDAC. The range was from 0x0000 to 0xffff - and it worked!

The code will work as long as the per component frame buffer depth
matches the number of entries in the RAMDAC, so if the frame buffer
is 6 bits (total 18 for RGB), then 64 entries will be fine.

The problem is in dealing with a limited depths screen, and having
any knowledge of how the frame buffer is wired to the RAMDAC.
If you've got 6 bits and 256 entries, how is it arranged ? Which
entries get used, and which don't ?

This was with XFree86 some time ago until I updated to X.org (some current
version - not exactly sure which one) - and there setting RAMDAC
values only works with a 24 bit depth visual.

I suspect some of the driver writers are a bit slack on how they
handle the RAMDAC access, because they don't see it as important.
The lack of individual access to CRTC RAMDAC's on the nVidia and
ATI multi-screen drivers is another example of this. The new version
of RandR promises to fix this, although access to the DDC channel
for each display is still an issue (the X server reads the EDID, but
doesn't then facilitate further communications.)

Graeme Gill.

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