Although both upcoming game consoles Xbox One and PlayStation 4 are based on AMD hardware, only PlayStation 4 incorporates hUMA [Heterogeneous Uniform Memory Access] for supporting a shared memory space. This was explained by AMD's Senior Product Marketing Manager Marc Diana to c't [big German IT magazine] at gamescom. This should put the 3D-performance of PlayStation 4 much farther ahead of Xbox One than many have expected so far. AMD sees hUMA as a key element for drastic performance improvements in combined processors. AMD's upcoming Kaveri desktop processors support hUMA as well.

Behind the scenes, c't could hear from developers that the 3D-performance of PlayStation 4 is very far ahead of Xbox One.

 Back in April, AMD manager Phil Rogers explained to c't that hUMA improves 3D-performance in particular. "Game developers have been eager to use very large textures for years. Until now they had to resort to tricks in order to package parts of larger textures into smaller textures. That is because today a texture has to be located in a special place of physical memory before the GPU can process it. With hUMA, applications can work with textures much more efficiently". AMD will give more details on hUMA at its upcoming developer conference in November.

http://www.heise.de/newsticker/meldung/Gamescom-Playstation-4-bietet-Unified-Memory-Xbox-One-nicht-1939716.html

Even with the integration of GPUs and CPUs into the same chip, GPGPU is quite awkward for software developers. The CPU and GPU have their own pools of memory. Physically, these might use the same chips on the motherboard (as most integrated GPUs carve off a portion of system memory for their own purposes). From a software perspective, however, these are completely separate.

This means that whenever a CPU program wants to do some computation on the GPU, it has to copy all the data from the CPU's memory into the GPU's memory. When the GPU computation is finished, all the data has to be copied back. This need to copy back and forth wastes time and makes it difficult to mix and match code that runs on the CPU and code that runs on the GPU.

The need to copy data also means that the GPU can't use the same data structures that the CPU is using. While the exact terminology varies from programming language to programming language, CPU data structures make extensive use of pointers: essentially, memory addresses that refer (or, indeed, point) to other pieces of data. These structures can't simply be copied into GPU memory, because CPU pointers refer to locations in CPU memory. Since GPU memory is separate, these locations would be all wrong when copied.

hUMA is the way AMD proposes to solve this problem. With hUMA, the CPU and GPU share a single memory space. The GPU can directly access CPU memory addresses, allowing it to both read and write data that the CPU is also reading and writing.

hUMA is a cache coherent system, meaning that the CPU and GPU will always see a consistent view of data in memory. If one processor makes a change then the other processor will see that changed data, even if the old value was being cached.

As well as being useful for GPGPU programming, this may also find use in the GPU's traditional domain: graphics. Normally, 3D programs have to use lots of relatively small textures to apply textures to their 3D models. When the GPU has access to demand paging, it becomes practical to use single large textures—larger than will even fit into the GPU's memory—loading the portions of the texture on an as-needed basis. id Software devised a similar technique using existing hardware for Enemy Territory: Quake Wars and called it MegaTexture. With hUMA, developers will get MegaTexture-like functionality built-in.

http://arstechnica.com/information-technology/2013/04/amds-heterogeneous-uniform-memory-access-coming-this-year-in-kaveri/