Date: Thu, 10 May 2007 18:51:31 -0700 (PDT) From: youshi10@u.washington.edu To: freebsd-questions@freebsd.org Subject: Re: WOW! {Or Holy whatever} Message-ID: <Pine.LNX.4.43.0705101851310.2113@hymn03.u.washington.edu> In-Reply-To: <1178814197.1231.48.camel@zoot.mintel.co.uk>
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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. >> >> -Garrett > > 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. > > Tom Assumption: ( 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. -Garrett
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