Question. As far as I understand it my graphics card uses a different type of RAM from my cpu. So does this new integrated RAM work more like DDR4 or what?
Part of the reason the memory on your GPU is a different type is because it is a separate, "private" pool of memory
not shared with the rest of the computer. The video card drivers use that space to keep things the GPU would rather not spend time fetching from main memory (or disk) all the time, such as shader code, texture data, the images being built and sent to the screen, etc. Likewise, when the GPU is used for calculation rather than rendering (Bitcoin mining, CUDA/OpenCL/Stream/DirectCompute, etc), the results have to be exported back to the main memory. Basically, time gets wasted because information has to constantly be sent/fetched back and forth over the PCI/AGP/PCIe bus between main memory and this private GPU VRAM pool. This swapping intensifies even more if the GPU does not have enough onboard VRAM to do whatever it wants to do, much like a computer swapping to disk when it runs out of RAM. What
really suffers from this arrangement are integrated GPUs, because since they have no private VRAM of their own, the drivers have to rope off a section of main system memory to act as that private pool, meaning that the computer ends up wasting time copying things from one section of its system memory to another just because the GPU will refuse to work with anything that isn't inside its fenced-in area.
When Apple says they are using a "unified memory pool," this does away with all the copying by letting the GPU and CPU share access to the same pool of memory. This increases complexity from the drivers' point of view ("Whose turn is it?"), but it
drastically reduces the amount of time lost to copying, since this copying is no longer even needed. It should also reduce latency and input lag, since there is no longer any reason to wait for all this copying to get done. The amount of overall memory necessary to do all this stuff is also reduced because there is no longer a need to reserve an additional portion of memory as a sort of shipping and receiving area to hold the stuff that's being packaged and sent back and forth across a traditional computer's bus.
As for
how it works, it works like whatever kind of memory it is...DDR3, DDR4, whatever. The
type of memory isn't the revolutionary part, it's the how-it's-accessed-and-used part that is the new thing.
--Patrick