On Thu, 10 May 2007, Tom Evans wrote:

On Thu, 2007-05-10 at 08:49 -0700, Garrett Cooper wrote:
A good rule of thumb: Don't buy a video card with more RAM than 1/8 to
1/4 of the system RAM, because the RAM is shared with the system RAM,
which means you have less overall system RAM to use for apps.


Er? Whilst I agree with the sentiment (low end graphics cards with 512MB
of RAM are solely there to rip off the unwary), that is complete tosh.

Some cards dont have much/any onboard dedicated RAM; instead they use
system memory. Examples of these are nvidia cards labelled 'TC' (Turbo
Cache), most (all?) integrated intel video chipsets.

The other issue is on i386. 32-bit systems have 4GB of address space to
use. Since you want to be able to address the graphics cards memory,
some of this address space is allocated so the OS can address the
memory. This means that if system RAM + video RAM > 4 GB, some of the
system RAM is unaddressable. That itself is a bit simplistic (its not 4
GB, its ~3.5 GB, for various reasons.)

The main point is that if you have a system with 1 GB of system RAM and
put in a graphics card with 640 MB of video RAM, you still have 1 GB of
system RAM to play with, even though you have gone over 1/4 of the
system RAM.



( System memory < 2 GB ) & ( Card is AGP ).

I think Robert is correct because AGP needs DMA whereas (and I'm not 100% on 
this point) PCI-e doesn't.

Most of the statement above was based on DMA and preallocated memory provided 
to the video card.


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