Xenos (graphics chip)

The Xenos is a custom graphics processing unit (GPU) designed by ATI (now taken over by AMD), used in the Xbox 360 video game console developed and produced for Microsoft. Developed under the codename "C1", it is in many ways related to the R520 architecture and therefore very similar to an ATI Radeon X1900 series of PC graphics cards as far as features and performance are concerned. However, the Xenos introduced new design ideas that were later adopted in the TeraScale microarchitecture, such as the unified shader architecture. The package contains two separate dies, the GPU and an eDRAM, featuring a total of 337 million transistors.

The chip is based on an ATI Radeon X1800 XL, the shader units are organized in three SIMD groups with 16 processors per group, for a total of 48 processors. Each of these processors is composed of a 5-wide vector unit (total 5 FP32 ALUs) that can serially execute up to two instructions per cycle (a multiply and an addition). Thus each of the 48 processors can perform 10 floating-point ops per cycle. All processors in a SIMD group execute the same instruction, so in total up to three instruction threads can be simultaneously under execution.

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