- last post: 01.01.0001 12:00 AM PDT
Specifications of the graphics engine the Xbox 2 console is reported to have impress much: the chip seems to have 10 times higher geometry and 4 times higher pixel performance compared to the RADEON X800 XT.
The Xbox 2’s central processing unit is a custom processor based on PowerPC technology. The CPU includes three independent processors (cores) on a single die. Each core runs at 3.50GHz speed of faster. The Xbox 2 microprocessor can issue two instructions per clock cycle per core. At peak performance, Xenon can issue 21 billion instructions per second.
The chip for Microsoft’s future console was designed by IBM in close consultation with the Xbox team, leading to a number of revolutionary additions, including a dot product instruction for extremely fast vector math and custom security features built directly into the silicon to prevent piracy and hacking.
Each core has two symmetric hardware threads (SMT), for a total of six hardware threads available to games. Not only does the “Xenon CPU” include the standard set of PowerPC integer and floating-point registers (one set per hardware thread), the microprocessor also includes 128 vector (VMX) registers per hardware thread. This astounding number of registers can drastically improve the speed of common mathematical operations, according to the document.
Each of the three cores includes a 32KB L1 instruction cache, a 32KB L1 data cache and share a 1MB L2 cache. The L2 cache can be locked down in segments to improve performance. The L2 cache also has the very unusual feature of being directly readable from the GPU, which allows the GPU to consume geometry and texture data from L2 and main memory simultaneously.
Microsoft claims that instructions of the next-generation console are exposed to games through compiler intrinsic, allowing developers to access the power of the chip using C language notation.
Xbox 2 Graphics Processor to Use Shader Model 3.0
The graphics processor designed for the Xbox 2 console is a custom 500MHz chip from ATI Technologies.
The shader core has 48 Arithmetic Logic Units (ALUs) that can execute 64 simultaneous threads on groups of 64 vertices or pixels. ALUs are automatically and dynamically assigned to either pixel or vertex processing depending on load. The ALUs can each perform one vector and one scalar operation per clock cycle, for a total of 96 shader operations per clock cycle. Texture loads can be done in parallel to ALU operations. At peak performance, the GPU can issue 48 billion shader operations per second.
The GPU has a peak pixel fillrate of 4 or more gigapixels/sec (16 gigasamples/sec with 4x antialiasing). The peak vertex rate is 500 or more million vertices/sec. The peak triangle rate is 500 or more million triangles/sec. Microsoft reportedly states that the figures are attainable with non-trivial shaders.
Microsoft’s future console is designed for HDTV output. In order to fit 720p frame-buffer inside the chip, a special 10MB or larger on-die embedded dynamic RAM (EDRAM) buffer will be incorporated. Larger frame-buffers are also possible because of hardware-accelerated partitioning and predicated rendering that has little cost other than additional vertex processing. Along with the extremely fast EDRAM, the GPU also includes hardware instructions for alpha blending, z-test, and antialiasing.
The Xbox 2 graphics architecture is a unique design that implements a superset of Direct3D version 9.0. It includes a number of important extensions, including additional compressed texture formats and a flexible tessellation engine. Xenon not only supports high-level shading language (HLSL) model 3.0 for vertex and pixel shaders but also includes advanced shader features well beyond model 3.0, Microsoft claims. For instance, shaders use 32-bit IEEE floating-point math throughout. Vertex shaders can fetch from textures, and pixel shaders can fetch from vertex streams. Xenon shaders also have the unique ability to directly access main memory, allowing techniques that have never before been possible.
As with Xbox, Xenon will support precompiled push buffers (“command buffers” in Xenon terminology), but to a much greater extent than the Xbox console does. The Xbox team is exposing and documenting the command buffer format so that games are able to harness the GPU much more effectively.
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