The CPU was clearly an important part of the Dreamcast specification, and selection of the device was a lengthy and carefully considered process. Factors considered included performance, cost, power requirements, and delivery schedule. There wasn't an off-the-shelf processor that could meet all requirements, but
Hitachi's SH-4 processor, which was still in development, could adapt to deliver the 3D geometry calculation performance necessary. The final form has an internal floating-point unit of 1.4 Gflops, which can calculate the geometry and lighting of more than 10 million polygons per second. Among the features of the SH-4 CPU is the store queue mechanism that helps send polygon data to the rendering engine at close to maximum bus bandwidth.1 The final device is implemented using a 0.25-micron, five-layer-metal process.
The system ASIC combines a PowerVR rendering core with a system bus controller, implemented using a 0.25-micron, five-layer-metal process. Imagination Technologies (formerly VideoLogic) provided the core logical design and Sega supplied the system bus. NEC provided the ASIC design technologies and chip layout, including qualification for 100-MHz operation. Fill rates are a maximum of 3.2 Gpixels per second for scenes comprising purely opaque polygons, falling to 100 million pixels per second when transparent polygons are used at the maximum hardware sort depth of 60.
Overall rendering engine throughput is 7 million polygons per second, but in Dreamcast, geometry data storage becomes the limiting factor before pixel engine throughput.
You're only as fast as your slowest component, so the DC is rated at 7 million polygons per second maximum sustainable rate, and in a game situation, would most likely be rated around 5 to 6 million polygons per second depending on how good a top developer would be at squeezing performance out of the system. I consider a rate lower than 7 mpps, simply because other game code has an effect on the polygon rate. The more complex the game AI is, the lower the polygon rate that the machine can achieve.
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