Opening the Lid on Xbox 360 by UtahSaint
 
At Hot Chips, Microsoft unveiled architectural details of the key chips that have been developed for its Xbox. More recently, Infineon released details of three additional chips it has developed with Microsoft for the Xbox 360. Also at Hot Chips,Toshiba researchers described a critical audio/video I/O chip expected to surface in Sony's Playstation 3 when it debuts sometime next spring. The SCC, a "super companion chip" with A/V interface, works with the so-called Cell Processor. Unveiled at ISSCC earlier this year, the Cell Processor is a highly parallel compute engine, developed jointly by IBM Corp., Sony, and Toshiba.
 
Keys to the success of both systems are: breadth of functionality, quality of video and audio, and ease of programmability. Both the Xbox and PS3 promise to deliver the most realistic and highest-performance game graphics to date, rivaling the best graphics workstations but at consumer costs.
 
"While the (Xbox 360) has the muscle to power awe-inspiring graphics, audio, and online play thanks to its 720p/1080i (HDTV) output, a 16x9 cinematic aspect ratio, anti-aliasing for smooth textures, and full surround sound, it also has the intelligence to serve as an all-in-one entertainment device that plays CDs, DVDs, MP3s, and digital content from an array of devices, such as portable music players and digital cameras," said Robbie Bach, Microsoft's chief Xbox officer.
 
Xbox product group VP, Todd Holmdahl, added that Xbox 360 designers focused on achieving a balance among system components, rather than aiming for great theoretical performance in any one component. The latter approach could mean lots of performance left on the table as a result of system bottlenecks.
 
"Right from the start, we built the system as the hardware instantiation of our software vision-that systems should be extremely powerful when running the type of software that game developers build," said Holmdahl. "This meant that we had to look very carefully at where bottlenecks happen when running game systems, and project what this would look like in the next generation. We analyzed hundreds of Xbox 1 games in detail, and we have great insight into the future due to the innovations that happen on our Windows platform."
 
The Xbox 360 CPU chip consists of three symmetric Power PC cores running at 3.2 GHz. Each core includes data and instruction caches of 32 kbytes each, and the chip has a shared 1-Mbyte L2 cache that is eight-way set associative. Additionally, each core includes a customized vector floating-point math unit (the VMX128), and can run two program threads. Holmdahl noted that the key here is the six hardware threads available. "This matches where we predict game engine utilization will peak," said Holmdahl. The symmetric nature of the cores and the shared cache makes it easier to move code between the cores, as well as implement communication and synchronization. "Multi-threaded simulation code is hard to write," he said, "and using elaborate DMA controller and DSP-style programming, as in previous generations, complicates developers' lives and is wasteful of their time."
 
The CPUs can provide high-bandwidth data streaming support with minimal cache thrashing by using a 128-byte cache line size. A tight data streaming capability between the CPU and the graphics processor unit (GPU) is also available. The GPU can read 128 bytes at a time from the L2 cache and it provides low-latency cacheable writebacks to the CPU. The GPU also shares Direct3D (D3D) compressed data formats with the CPU to at least double bus bandwidth for graphics data.
 
A 5.4-Gbit/s front-side bus offers 10.8-Gbyte/s read and 10.8-Gbyte/s write speeds. Supporting the custom CPU is a 500-MHz GPU developed in conjunction with ATI Technologies. The GPU includes 48 parallel unified shaders and 10 Mbytes of embedded DRAM (a custom memory chip co-located in the same package as the GPU) for 256-Gbyte/s frame buffering. Rounding out the system hardware support are 512 Mbytes of 700-MHz GDDR3 memory with a 22.4-Gbyte/s memory interface bus bandwidth; a 12X, dual-layer DVD; and a 20-Gbyte hard drive that's standard on the premium model and an option for the core version. The system employs a unified memory architecture that allows the 512 Mbytes to be shared between the CPUs and the GPU with no partitioning headaches for the programmers.
 
Support from Infineon includes a removable solid-state memory unit, a single-chip wireless game-pad controller, and an advanced security chip. In the memory unit, Infineon crafted a custom memory controller and developed the embedded software that runs on the chip. For the security chip, the company created a custom solution based on its proven authentication technology. The game-pad controller eliminates the cables needed for game play, thus freeing the players from the tethers of wired controllers.
 
According to Holmdahl, most game/entertainment systems have a significant limiting factor, such as a lack of sufficient memory to feed the CPU and GPU, not enough bandwidth between system components, or an inability to apply processing resources automatically to a changing load profile. "Those and other limitations and restrictions have been dealt with in the design of Xbox 360," Holmdahl asserted. "The Xbox 360 was designed from the ground up to map to exactly what game developers told us they needed."
 
Holmdahl cited the GPU as one example, with its improved raw shader power over earlier Xbox systems. The 48 parallel shader ALU pipelines are dynamically scheduled and up to 24 billion shader instructions/s can be executed by the pipeline. "This means that the GPU dynamically adjusts to the incoming stream of work that the gaming program is sending it. It also allows the GPU to dynamically schedule around events, such as waiting for data from memory, that would normally block ALUs from processing. GPU resources are kept as active as possible without programmer intervention." The pixel fill rate can hit 4 billion pixels/s (eight per cycle) and the geometry engines can deliver up to 500 million triangles/s. Texture fills can be done at up to 8 billion bilinear filter samples/s.
 
Moreover, the GPU can optimize vertex or pixel shading automatically for each game without the developer having to write extra code. The embedded DRAM and high-speed frame buffering effectively provides free anti-aliasing to eliminate the jagged edges characteristic of earlier game systems. "The embedded DRAM is unique to Xbox 360 and can process information about 40 times faster than Xbox 1, while allowing for roughly ten times more bandwidth than the competition," Holmdahl asserted. "With the special logic on the memory die we can process the information there without having to send it back to the core GPU. This means no signal aliasing, as well as the ability to process a great deal of information quickly, resulting in more pixels, faster, and the delivery of crisp, realistic images. The GPU also has a tesselator to provide smooth, round silhouettes."
 
"A traditional memory architecture will get 'fill-bound' as it tries to read and write to memory to build the scene," Holmdahl explained. "This is especially true of next-generation visuals that require high scene complexity, high dynamic range, HD resolutions and anti-aliasing."