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Citiram Chucka Walbourna, jednog od Direct X programera:

Another factor in the PC memory equation has been growing as well: video memory size. In the early days of Direct3D, the typical video card had 16 or 32 MB of Video RAM (VRAM). High-end video cards now have 512 MB, 640 MB, 768 MB, or more VRAM. When video cards had 16 or 32 MB of Video RAM, this memory was mapped directly into every process that used Direct3D for efficient access by the application and video driver.

As video cards grew larger, this became unsustainable. A 768 MB hole in the 2-GB virtual address space of each process would leave very little space for applications. Similarly, taking 768 MB out of the 4 GB physical address space would be too constraining. This problem is exacerbated in dual GPU configurations (SLI®/Crossfire™).

Therefore, video card manufacturers typically implement a 256 MB physical memory window for the video graphics memory, and modern drivers do not create direct process mappings for the entire VRAM size. Process address space is still consumed for working with the AGP aperture (64 MB, 128 MB, or more typically on modern game systems 256 MB in size). While PCIe uses a dynamic aperture, it too is mapped into each process that uses Direct3D.

Developer Spring Enginea:

Video ram is not mapped into virtual address space, there are a few pci pages mapped to it, but those are needed for communication with the pci-devices, they aren't memory. Still video ram _can_ on request mapped, this way a CPU is able to send vertex & texture data directly to a gpu, but those address spaces are normally freed after usage. So GPU drivers only keep ~128MB for this feature locked (duno if in user or system space).
I.e. the link above mentions an example of 2x GTX280 with 1GB vram each, mapping 512MB (2x256MB).

Dakle, ocito os ne alocira cijeli video RAM i ocito postoji neki nacin da ga aplikacija dinamicki alocira (unutar ogranicenja od 2 GB RAM-a dostupnog aplikacijama).

Situacija, koju je netko spomenuo, da zbog grafe sa 4 GB RAM-a ne mozes uopce bootati Windowse je nemoguca.