shared memory

[Lennart Sorensen]

On Fri, Jan 28, 2005 at 12:36:09PM -0500, John McGregor wrote:
> There are basicly two types of motherboards -- integrated and 
> non-integrated. In the integrated boards, video, sound, nics, modems are 
> all included on the board itself rather than being separate add in pci 
> cards et al.The chipsets for the motherboards can be made by various 
> maunfacturers with the more common ones being Intel, SIS, Nvidia and 
> ATI. The advantage to integrated boards is that they generally cost less 
> as a total solution than a box that uses a non-integrated board.. The 
> disadvantage is the memory that is shared with the video card. If the 
> video card is slated to use <= 32MB, then the performance hit that you 
> can expect in a 512 MB system will be negligible. If on the other hand 
> the card is configured to use 128MB, the performance hit can be 
> substantial, especially in graphics intensive applications.What may be 
> confusing you is that a standard add in video card is supplied with its 
> own memory chips and has no need to access system memory.

Loosing memory is a minor problem.  Loosing bandwidth to access your
memory is a big problem.  The video card will be constantly reading the
framebuffer 60 to 75 times per second to draw the display, all of which
cuts into the amount of data the cpu can read and write from memory.

ie: 1024*768*32bpp = 3MB/fps * 75Hz refresh = 225MB/s to keep the
display running.

Lennart Sorensen
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