Hardware Oddity XF State and STPOS

XF context writes and STPOS

Evidence: generation-tagged NVIDIA-family reverse engineering, not yet an Xbox measurement. The envytools XF overview distinguishes ordinary vertex programs, side-effecting vertex programs, and vertex state programs. In the documented NV20-era modes, the latter can update XFCTX, which also supplies parameters; state programs do not emit an ordinary transformed vertex. The research also describes STPOS, a readable shadow of position output used in the Kelvin viewport transformation.

Do not treat all “constants” as immutable or position output as necessarily terminal. Equally, do not import the eight-entry call stack or instruction timeout from the XF ISA page: those features are marked NV30 and later.

Proposed test: write a context value in one program and consume it later; separately test partial position writes followed by STPOS reads. Record the exact NV2A mode/method setup and compare outputs on hardware.

See also NV2A Graphics Architecture.