Yawn... Buzzwords you don't understand...
Virtual Addressing
All GPU memory accesses on Durango use virtual addresses, and therefore pass through a translation table before being resolved to physical addresses. This layer of indirection solves the problem of resource memory fragmentation in hardwarea single resource can now occupy several noncontiguous pages of physical memory without penalty.
Virtual addresses can target pages in main RAM or ESRAM, or can be unmapped. Shader reads and writes to unmapped pages return well-defined results, including optional error codes, rather than crashing the GPU. This facility is important for support of tiled resources, which are only partially resident in physical memory
Durango has no video memory (VRAM) in the traditional sense, but the GPU does contain 32 MB of fast embedded SRAM (ESRAM). ESRAM on Durango is free from many of the restrictions that affect EDRAM on Xbox 360. Durango supports the following scenarios:
Texturing from ESRAM
Rendering to surfaces in main RAM
Read back from render targets without performing a resolve (in certain cases)
The difference in throughput between ESRAM and main RAM is moderate: 102.4 GB/sec versus 68 GB/sec. The advantages of ESRAM are lower latency and lack of contention from other memory clientsfor instance the CPU, I/O, and display output. Low latency is particularly important for sustaining peak performance of the color blocks (CBs) and depth blocks (DBs).