From: Paul Jakma (paul at domain clubi.ie)
Date: Fri 21 May 1999 - 00:49:52 IST
On Thu, 20 May 1999, Kenn Humborg wrote:
The RAMDAC does _not_ contain the whole image.
i never said it did.
It still has to get the pixel data from the main RAM. If the
image data was stored in the RAMDAC, you'd have to upgrade the
RAMDAC to get higher resolution*colour_depth values.
the RAMDAC does not access frame buffer memory. It interfaces to the
graphics chipset which has to feed it data continously which it
converts to analogue continually. They're mostly off the shelf,
general purpose parts, (eg Sierra, IBM, Texas Inst.) they wouldn't
even be able to interface to framebuffer memory.
if the rendering part of the engine is bogged down, the RAMDAC still
needs the same constant stream of data. so RAM speed isn't an issue
to the ramdac. (except from the holistic 'balance' POV).
AFAIK, some cards use the 'spare' space in the video RAM for
acceleration purposes (and for textures and stuff on 3D cards).
Aye, I think XFree86 uses the extra RAM for XAA. Don't know about the
cards. But i think hardware would make use of on-chip buffers rather
than the framebuffer itself, doesn't make sense to use ram to draw to
the framebuffer in ram. the extra ram is more useful to the driver as
a cache than to the chipset imo.
2D acceleration is where instead of manually drawing multiple
boxes/triangles pixel by pixel into the framebuffer, you give the
card a command to 'draw boxes/triangles colour xyzw at x.y..' etc. So
i think there isn't much need for memory for the chipset to do that.
more useful for the driver to cache a complicated
triangle/box/polygon setup in the ram if the chipset supports it.
for 3d i imagine again it's more used as a texture/command cache by
the driver, than for the hardware acceleration. hardware acceleration
isn't really hardware acceleration if it's rendering in stages via
the onboard RAM. (might as well let your P-2 450 do it then).
(who's feeling even more argumentative than usual!).
PS: take everything above with a pinch of AFAIK :).
-- Paul Jakma paul at domain clubi.ie http://hibernia.clubi.ie PGP5 key: http://www.clubi.ie/jakma/publickey.txt ------------------------------------------- Fortune: In Blythe, California, a city ordinance declares that a person must own at least two cows before he can wear cowboy boots in public.
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