question about crossfire

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shiftyape

shiftyape

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Feb 23, 2013
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so if i had a card that was 1gb, and a card that was 2gb, would i be able to use 3gb of vram? some people have told me that it can only use the memory of the smaller card, for a total of 1gb for both cards to run on. others have told me that if i had a 2gb card and a 1gb card, that both cards would get to use 1gb, so 2gb total. which one of these is true?
 
Solution
renz496
The basic principle is each gpu have their own memory since they can't share the memory between them. But in CF (or SLI for nvidia cards) when pairing both gpu it is best to pair the same gpu with same spec from clock down to memory amount. If not the card with faster clock will have to downclock itself to match the slower card. Same case with useable VRAM. If you were combining 1GB and 2GB card the card with 2GB VRAM will beonly to use 1GB of it's VRAM to match the other card.

And with CF there were less restriction when pairing cards unlike SLI. You can pair a card that has same chip inside like 7970 and 7950. But the performance will still the same as if you were cross fire 2 7950 together. You will not getting the performance...
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shiftyape

shiftyape

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so a total of 2gb of vram right? sorry i dont understand that much
 
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shiftyape

shiftyape

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so what exactly does that mean? will it act like a single card that has 2gb of vram? will the memory performance be increased at all?
 
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renz496

renz496

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The basic principle is each gpu have their own memory since they can't share the memory between them. But in CF (or SLI for nvidia cards) when pairing both gpu it is best to pair the same gpu with same spec from clock down to memory amount. If not the card with faster clock will have to downclock itself to match the slower card. Same case with useable VRAM. If you were combining 1GB and 2GB card the card with 2GB VRAM will beonly to use 1GB of it's VRAM to match the other card.

And with CF there were less restriction when pairing cards unlike SLI. You can pair a card that has same chip inside like 7970 and 7950. But the performance will still the same as if you were cross fire 2 7950 together. You will not getting the performance between 7970CF and 7950CF just because the other card was 7970.
 
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shiftyape

shiftyape

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thanks for your answer. however, i still dont understand if that is the same as having 1 card that has 2gb of vram? if not, does it improve memory performance in any way?
 
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renz496

renz496

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Did you mean about each card have 1gb and total useable is 2 gb since there were two cards?

Sometimes this stuff can be confusing espcially on dual gpu like 690 and 7990. Usually on the box gpu vendor (such as asus, gigabyte etc) mentioning the total amount of vram available on the board (well maybe for marketing purpose since moar are better :D ) but what they usuallydid not mention was useable VRAM per gpu. For example with 690 they will advertise on the box the card have 4GB or VRAM. But in reality it was 2 GB per gpu since 690 have two 680 cores onboard. So for non dual gpu case whereyou were combining 1gb and 2 gb cards the useable VRAM is 1GB per gpu. But total VRAM physically useable to you is 2GB. 1GB for the first card and another 1GB on the second card.

And because of CF/SLI technology limitation the another 1GB of the VRAM on the 2GB card will become totally useless hence it will not help to increase the performance even if the game need more than 1GB VRAM
 
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ss202sl

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May 23, 2012
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NO. The total usable is 1GB. When is in crossfire the (1GB) ram is duplicated to each card. each card has access to the same 1GB of data. The data is duplicated to each card so the GPU on each card has fast access to ALL the RAM (1GB in your case), but it is only 1GB of data,
 
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shiftyape

shiftyape

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ok i get it now. so there is no way to extend video memory without getting a new card and replace the old one. it doesnt even improve the bandwidth of the cards. well... thanks for your answers
 
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