Xbox One uses a different memory architecture to deliver next-gen console graphic quality  (Comparison with PS4 is not the goal of this post). Its approach is like multilevel CPU cache (L1, L2, L3, …) or big.LITTLE in ARM CPU but for memory. It has one big slow DDR3 (8GB / 68.3 GBps ) and one small eSRAM (32MB / 109 GBps ).
eSRAM stands for embedded Static RAM. SRAM is known for being used in on-die memory such as CPU registers and caches. Its structure is faster and more energy efficient than DRAM. But, it requires a lot more silicon space to make one bit. This makes it more expensive.
This makes Xbox One energy efficient. Normal needs run at slower rate in DDR3. Slower means consuming lesser. While aggressive needs run at faster rate in eSRAM which has energy saving structure advantage.
Microsoft intends to use eSRAM exclusively for its GPU. It is not shared with CPU, Shared Resource Allocation (SRA) or video output. It is used for memory bandwidth hungry tasks like rendering target, texture, and geometry .
Its high speed and limited resource is used for rendering small groups of rapid changes and high post processing needs in graphic e.g. player models and close proximity items. On the other hand, static like things, such as, sky and buildings, are far from a user’s eyes and less changed. These things can be put on DDR with little or none render penalty.
Moreover, eSRAM can perform asynchronous data movement in/out while rendering. This makes its utilization high all the time if programmers optimize it well.
Microsoft sees the important of energy efficiency (which may come from many overheat issues in previous Xbox 360). eSRAM is there for bandwidth hungry graphic tasks. Serving quality with efficiency. eSRAM alone is fast. It adds another level of memory locality. This adds a learning curve to programmers to master this (kind of) new memory architecture. It is a matter of optimization between proper eSRAM size and programming approach to squeeze out this high bandwidth memory.
- AnandTech – The Xbox One: Hardware Analysis & Comparison to Playstation 4 (accessed July 28, 2014)
- SlideShare – Inside XBox One by Martin Fuller (accessed July 28, 2014)
- VGLeaks – Xbox One (Durango) Memory System Overview (accessed July 28, 2014)