Err, the GB is determined by the density of the VRAM chips. The interface determines the number of chips.Originally Posted by MiRai
GDDR5(X) chips have a 32bit interface, so you take you GPU interface, divide by 32 and that is the number of VRAM modules you will have on the PCB.
HBM1 is 1024 bits wide.
It then becomes a simple bit of math. gpu interface / VRAM chip interface * density / 8.
Funnily enough, the top end graphics cards tend to use the highest density chips available at the time, which is currently 8Gb (gigabit, not bytes), but it is not mandatory.
/e: it is possible to put more vram on the board, but this would then require the memory controller to switch between modules, so you would not gain any extra "bandwidth" out of doing so, and would probably then generate extra latency (in fact this is what the GTX 970 does, so it's 256bit interface is just marketing gumpf because it is not a true 256bit pipeline throughout the whole shebang).
/e2: The Titan-X (because someone will bring it up), does the memory controller switch to access the 24 VRAM modules (it has 4Gb modules), the internals of how it does it without impacting significantly on performance, who knows, and why the 970 does it so inefficiently in comparison, who knows. My guess is that the Titan-X does not switch to accessing a single module at a time, it is always addressing 12 modules simultaneously, whereas the 970 goes from addressing 7 modules to 1 module, then back to 7 etc (although only when it really needs that extra 512MB).
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