WITH MICROSOFT'S OFFICIAL announcement of the next-generation Xbox 360 console this week, ATI has decided to disclose some of the architectural details of the graphics processor that it created for the system. I had a brief but enlightening conversation with Bob Feldstein, Vice President of Engineering at ATI, who helped oversee the Xbox 360 GPU project. He spelled out some of the GPU's details for me, and they're definitely intriguing.
Feldstein said that ATI and Microsoft developed this chip together in the span of two years, and that they worked "from the ground up" to do a console product. He said that Microsoft was a very good partner with some good chip engineers who understood the problems of doing a non-PC system design. Also, because the part was custom created for a game console, it could be designed specifically for delivering a good gaming experience as part of the Xbox 360 system.

Unified shaders
Feldstein cited several major areas of innovation where the Xbox 360 GPU breaks new ground. The first of those is the chip's unified shader array, which does away with separate vertex and pixel shaders in favor of 48 parallel shaders capable of operating on data for both pixels and vertices. The GPU can dynamically allocate shader resources as necessary in order to best address a computational constraint, whether that constraint is vertex- or pixel-related.

This sort of graphics architecture has been rumored as a future possibility for some time, but ATI worried that using unified shaders might cause some efficiency loss. To keep all of the shader units utilized as fully as possible, the design team created a complex system of hardware threading inside the chip itself. In this case, each thread is a program associated with the shader arrays. The Xbox 360 GPU can manage and maintain state information on 64 separate threads in hardware. There's a thread buffer inside the chip, and the GPU can switch between threads instantaneously in order to keep the shader arrays busy at all times.

This internal complexity allows for efficient use of the GPU's computational resources, but it's also completely hidden from software developers, who need only to write their shader programs without worrying about the details of the chip's internal thread scheduling.

 
A block diagram of the Xbox 360 GPU. Source: ATI.