Does memory bus on a crossfire/sli setup stack?

[-Slacker-]

So if, for example, I pair two HD5770's in crossfire, will they run on their individual memory bus of 128bit or will I get a combined 256bit bus from them?

Thanks in advance.

 

[dguy6789]

You'll get the combined 256 bit bus from them both. You won't get the combined total memory though.

 

[Daedalus685]

You'll get the combined 256 bit bus from them both. You won't get the combined total memory though.

That is not quite true.

It will function very much like a raid 1 array. The same data has to be mirrored to both cards. In theory write performance (loading textures) will be the same as a single card but all reads would be up to twice as fast (double bus).

I'm not sure what the break down would be, likely more reads though (much of the same starting data would be used for processing).