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I agree that PCIe is mostly shared memory system. But for videocards this sharing is unequal, because their RAM sizes exceeds 32bit address space, and lot of still used mainboards have 32bit PCIe controller, so all PCIe addresses should be inside 4GB address space, and this is seen on windows machines as total installed memory is nor all, but minus approximately 0.5GB, from which 256MB is videoram access window. So in most cases, remain in force rule, that videocard share all it's memory through 256mb window using bank-switching. As for GPU read main system memory, usually this is useless, because vram is magnitudes faster, even if not consider usage of bus bandwidth by other devices, like HDD/SSD. And in most cases, only usage of access GPU to main system memory, is traditional read of textures (for 3D accelerator) from main system memory - for example ALL 3D software using GPU rendering, could only use for this videoram, none use system ram. |