6990 effective RAM

[chrisNWD]

"The card comes with 4GB of RAM, which due to the internal CrossFire setup of the card reduces the effective RAM capacity to 2GB, the same as AMD’s existing 6900 cards." - Anandtech's review of the AMD HD6990.

If the above is true, this begs the question: Why equip the cards with 4GB of RAM in the first place when only 2GB would be usable? I understand that this is how CrossFire normally works, but surely AMD could have developed the card to either take advantage of all the available RAM or engineered the board in such a way that they don't have to include redundant memory chips.

Was this the most cost and/or time effective approach? Or am I just a n00b and don't understand CrossFire properly?

Also, presuming that only 2GB is usable, if they advertise the card as 4GB is that not at the very least misleading advertising?

 

[Rroff]

Each core needs a mirror copy of the data to function, the latency involved with sharing the memory would kill potential performance - longer traces and having to schedule read/writes between cores (and each core would need some memory that was exclusive to that core even tho ~80% of the data could potentially be shared which would make it very complex i.e. they each need their own framebuffer and other rendering buffers).

I'm not a fan of the way its advertised either.

 

[chrisNWD]

Thanks for the explanation. Makes sense now. The advertising is misleading, but technically correct I guess.

Not that it matters anyway, RAM is to video cards what GHz is to processors nowadays (i.e. a totally unimportant number out of context that is thrown around by marketing types).

 

[Ezyryder]

Not really, for high resolution monitors and multi-monitor set ups a large amount of ram is necessary for smoothness and framerate.