On Wed, 3 May 2000, Ian C.Sison wrote:
..
>
> > That said, the 2D is decent. However a friend with an i740 (and take note,
> > the i740 is a BETTER video chipset) said that XFree86 is slow on the i740,
> > Accelerated/X is faster. But the 2D performance isn't staggering. It'd
> > suggest to stick with 16 bit color depth to help the speed.
>
> I seem to recall you once give a long post explaining that the # of
> colors does not affect display speed updates. Is this still so?
Oh, they do. The # of colors affects the memory bandwidth. For example, if
you have 16bpp vs 32bpp, you need twice the memory bandwidth and RAMDAC
speed to get identical refresh rate with 32bpp than with 16bpp at a given
resolution.
For MOST video cards this is a nonissue because they have 128- or 256-bit
memory paths and 350MHz RAMDACS.
But the Intel i752 SHARES THE SYSTEM SDRAM (takes part of it as video
memory). So you are competing with the CPU for RAM access. If you have a
high-res, high-bit depth, high-refresh display, you are stealing memory
read cycles from the CPU.
For example, if you have 1024x768x32bpp at 85Hz, that figures out to
(1024x768 x 4bytes x 85) = 267Mbytes/second. What's the data rate of PC100
SDRAM? 400Mbytes/second? (32-bit wide DIMM x 100MHz system bus). Your
video card is taking an unholy chunk of that memory bandwidth.
The "hub architecture" in the i810 does NOT help. It's similar to a
crossbar switch such as in the SGI Octane and VisWS and the Sun UPA, but
it doesn't help you because your memory bandwidth is restricted (Octane
for example has a 1.6Gbyte/sec memory bandwidth using standard PC100 RAM..
makes you wonder what's the fuss with RAMBUS).
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