Resizable BAR
A PCIe feature that lets the system change the size of the "Base Address Register" window so the CPU can access all of the graphics memory at once, instead of the old 256 MB window. AMD's SAM and the equivalents from Nvidia/Intel all build on this.
Mechanic: the BAR (Base Address Register) defines how much of a PCIe device's memory is mapped into the CPU's address space. The default of 256 MB forced inefficient piecewise access to VRAM. Resizable BAR lets the BAR grow to the full VRAM size → fewer, larger transfers and better GPU feeding. Requirements: UEFI/CSM configuration (you must run pure UEFI, often enabled manually in the BIOS), support in the CPU, motherboard, and GPU. The performance gain is game-dependent (usually a few percent). AMD named its implementation Smart Access Memory; the technology itself is an open PCIe standard. Related to Smart Access Memory and PCIe bifurcation.