Xbox 360/Hardware information/Xenos (GPU)

A repository for ROMhacking relevant info for Xenos.

Basic Specifications

• ATI Xenos @ 500MHz
  • World's first unified shading GPU
  • TeraScale uarch
  • 3 "SIMD cores" (predecessor to CUs)
  • 8 ROPS
  • 16 TMUs
  • 240 shaders
  • Direct3D Feature Level 9_0c, with some things from higher feature levels (mainly FL 10)
  • MEMEXPORT allows for GPGPU compute
  • Connected to 10MB of eDRAM framebuffer
    • eDRAM has extremely high bandwidth and PIM for free MSAA at 720p and low-cost at 1080p

Things to note: If using data from MEMEXPORT on CPU, you must enable both d3d12_readback_memexport and d3d12_readback_resolve in Xenia's options or use HW!

ucode Programming

Xenos, like other Xbox console GPUs, is able to directly ingest and execute ucode without having to be compiled by the driver first, allowing programmers to squeeze out more performance by relying on the fixed HW spec.

To do: Fix this. Xenos ucodes use the 10 MSBs of the word to select the instruction, so hex values are not ideal for grouping them

Xenos ucode

Instruction	Hex	Description
exec	(Unknown)	Start executing code
cnop?	(Unknown)	NOP, end of program
add	(Unknown)	Adds op2 and op3 and stores the result in op1
cndeq	(Unknown)	If op2 = 0.0f, op1 = op3, else op1 = op4

Texture Formats

Xenos Supported Texture Formats

Format	Description	Xenos Specific	Other
Uncompressed	Bitmap	No	Not recommended due to high VRAM use
DXT1/BC1	4x4 pixels packed into 64 bits c0 16-bit color (ARRRRRGGGGGGBBBBB) c1 16-bit color (ARRRRRGGGGGGBBBBB) pixelIndices[16] 2-bit indices, c0 = 00, c1 = 01, c2 = 10, c3 = 11 c0 > c1: c2 and c3 linearly interpolate between c0 and c1 c0 < c1: c3 rather than representing a middle point between c0/c1, represents full transparency	No
DXT2/BC2	4x4 pixels pack into 128 bits pixelAlphaTable[16] 4-bit alpha data c0 16-bit color (-RRRRRGGGGGGBBBBB) c1 16-bit color (-RRRRRGGGGGGBBBBB) pixelIndices[16] 2-bit indices, c0 = 00, c1 = 01, c2 = 10, c3 = 11 c2 and c3 linearly interpolate between c0 and c1. Data is considered pre-multiplied by alpha	No
DXT3/BC2	pixelAlphaTable[16] 4-bit alpha data c0 16-bit color (-RRRRRGGGGGGBBBBB) c1 16-bit color (-RRRRRGGGGGGBBBBB) pixelIndices[16] 2-bit indices, c0 = 00, c1 = 01, c2 = 10, c3 = 11 c2 and c3 linearly interpolate between c0 and c1. Data is considered NOT pre-multiplied by alpha	No
DXT3A	4x4 pixels packed into 64 bits pixelScalars[16] 4-bit scalar value	Yes
DXT3A as 1111	Same as DXT3A, but each bit of scalar data represents mask for respective channel	Yes
DXT4/BC3	4x4 pixels packed into 128 bits a0 8-bit alpha value a1 8-bit alpha value pixelAlphaIndices[16] 3-bit index c0 16-bit color (-RRRRRGGGGGGBBBBB) c1 16-bit color (-RRRRRGGGGGGBBBBB) If a0 > a1, all alpha indices are linearly interpolated, else only a2-a5 are linearly interpolated while a6 and a7 are 0 and 255 respectively. Data is considered pre-multiplied by alpha	No
DXT5/BC3	4x4 pixels packed into 128 bits a0 8-bit alpha value a1 8-bit alpha value pixelAlphaIndices[16] 3-bit index c0 16-bit color (-RRRRRGGGGGGBBBBB) c1 16-bit color (-RRRRRGGGGGGBBBBB) If a0 > a1, all alpha indices are linearly interpolated, else only a2-a5 are linearly interpolated while a6 and a7 are 0 and 255 respectively. Data is considered NOT pre-multiplied by alpha	No
DXT5A	4x4 pixels packed into 64 bits s0 8-bit scalar s1 8-bit scalar pixelScalarIndices[16] 3-bit indices Scalars are linearly interpolated	Yes
DXN	4x4 pixels packed into 128 bits Same as DXT5A but with two channels of scalar	Yes
CTX1	4x4 pixels packed into 64 bits s0 8-bit scalar channel 0 s1 8-bit scalar channel 1 s2 8-bit scalar channel 2 s3 8-bit scalar channel 3 pixelScalarIndices[16] 2-bit indices, shared between both channels	Yes

Useful Links

To do: Organize these better

https://www.techpowerup.com/gpu-specs/xbox-360-gpu-90nm.c1919

https://learn.microsoft.com/en-us/windows/win32/direct3d9/dx9-graphics-reference

https://learn.microsoft.com/en-us/windows/win32/direct3d9/dx9-graphics-programming-guide

https://learn.microsoft.com/en-us/windows/win32/direct3d10/d3d10-graphics-reference

https://learn.microsoft.com/en-us/windows/win32/direct3d10/d3d10-graphics-programming-guide

https://xenia.jp/updates/2021/04/27/leaving-no-pixel-behind-new-render-target-cache-3x3-resolution-scaling.html

https://en.wikipedia.org/wiki/Unified_shader_model

https://fileadmin.cs.lth.se/cs/Personal/Michael_Doggett/talks/unc-xenos-doggett.pdf

http://www.students.science.uu.nl/~3220516/advancedgraphics/papers/inferred_lighting.pdf

https://www.amd.com/content/dam/amd/en/documents/radeon-tech-docs/instruction-set-architectures/R600_Instruction_Set_Architecture.pdf

https://en.wikipedia.org/wiki/S3_Texture_Compression

http://web.archive.org/web/20100423054747/http://msdn.microsoft.com:80/en-us/library/bb313877.aspx