Inside the Xbox GPU

Microsoft has often emphasized the power of the Xbox's Nvidia-designed graphics chip, code-named the NV2A. However, there have been varying reports as to whether the Xbox chip is derived from the Nvidia's most recent PC graphics chip, the GeForce3 (known internally as the NV20), or a future design, the NV25. Microsoft has previously stated that the NV2A is closely related to the NV25, but Nvidia representatives close to the engineering teams have told GameSpot otherwise.

In fact, the NV2A is a faster GeForce3 with one major addition to the hardware: a second vertex shader pipeline. Since the chip design was recently finalized for production, Nvidia could confirm that it will run at the faster speed of 250MHz. This puts the pixel fill rate at 1,000 megapixels and the texture fill rate at 2,000 megatexels--much lower than the Microsoft's original official spec of over 4,000 megapixels and 4,000 megatexels, but fill rate isn't the most important spec. Fill rate is not a determining factor because, as a console, the Xbox is naturally destined to run at the low resolution of consumer TVs. Also, most of the Xbox's special capabilities, such as vertex shaders or full-scene antialiasing, occur at different points in the pipeline and are not at all dependent on fill rate. More important is the NV2A's bandwidth to the 64MB of shared memory. The memory in the console is 200MHz double-data rate (DDR) SDRAM, which provides a maximum of 6.4GB per second of throughput to both the CPU and graphics chip. The custom CPU uses up to 1GB per second of this shared bandwidth. This leaves less headroom than the GeForce3 has with its 7.36GB per second bandwidth, but again television's lower display resolution means this should be plenty. The shared memory design also avoids the bottlenecks PC developers see when sending a lot of data from a PC's main memory over the AGP bus to the graphics card.

Even more interesting than the speed boost is the second vertex pipeline. This part of the chip allows game developers to create amazing real-time lighting and animation effects, and it is highly programmable, meaning that more and more effects will become available over time. While such vertex effects are similar to techniques used in prerendered computer animation, it's likely that it will take developers some time to fully take advantage of these features. The Xbox hardware has to stay constant for several years, so it's definitely a good thing to have extra power to grow into. One reason we won't see Nvidia's PC products match this capability in the near term is simply because DirectX 8 can't address the additional vertex pipeline. But Microsoft has modified the Xbox's version of DirectX to give developers access to the hardware's full capabilities.

The Xbox will have numerous additional graphical capabilities thanks to the custom version of DirectX 8 that the Xbox uses. This version, called Xware, provides access to features that Nvidia also included in the GeForce3 but aren't currently available in DirectX 8. This disparity is due in part to how Microsoft sets this PC standard for multiple graphics companies. It can't include all submitted features. As a result, the Xbox adds the small but significant ability to make point sprites volumetric and support for shadow buffers, among other things. However, one consequence of this is that games programmed for the Xbox will have to have these features removed before being ported over to the PC.

Finally, the preproduction demo consoles Microsoft used to show off Xbox games at E3 didn't include the final audio hardware--Nvidia's MCP chip--to accelerate game sound. Some Xbox developers on the floor explained that the occasional performance slowdowns seen in the games were mostly due to the fact that all audio processing is temporarily being handled by the 733MHz CPU. In the final Xbox, the MCP will handle all of the console's audio processing, networking, and I/O. It's the first chip to take advantage of Dolby's new real-time Dolby Digital encoding algorithm, which will produce true 5.1 game sound for home theater systems.