Re: SVGA cards on PCI

["Thomas Schutt" <Thomas_Schutt@splashtech.com>]

Noam Halevy wrote:
> 
> Hello,
> 
> I am sorry if this is a very basic question for you video people ...
> 
> A video card which sits on the PCI has to refresh the screen at some rate.
> The screen data is (I guess) in it's intrnal vram.
> My question is:
> when the processor needs to change the data, how does it pass it to the
> video card memory ? Does it use the card's DMA (which would cause long READ
> bursts on the PCI which is bad for performance) or does it use it's own DMA
> for the job ?
> 
> In any case, I assume that the video data is never cached by the processor
> and that there is no copy of the video ram in the system memory.

I wouldn't say never. It is common in PowerPC machines like the Mac to make video 
buffers cachable in the eyes of the processor. This way the processor can use cache 
operations to move data to/from the video buffer in larger blocks. However the 
software deals with any cache coherancy so from the bridge point of view it is just 
prefetchable memory.
> 
> Thanks very much for any response.
> 
> Best regards,
> 
> Noam Halevy
> 
> email: noamh@msil.sps.mot.com
> Tel: 972-3-590534
> 
>                  MSIL
>                 ______
>                      / Motorola Semiconductor Israel.
>                     /________________________________
> 
- TJS
†ÜË