Set the game's x87 control word before each emulated run

What to know

2026-10-24, Hardwar's audio_truth.py.

Hardwar runs the x87 with control word 0x0c7f (24-bit precision, truncation), set once at start-up by fpu_setup (0x4af9a0, maths.md). A routine emulated on its own never runs that set-up, so Unicorn's FPU stays in its default mode (64-bit precision, round to nearest), and every fist, fstp and division rounds differently from the game.

Symptom: the sound routing matched the port in 19,294 of 20,000 cases; the rest differed by one unit in a pan word, which comes from vec3_unit_from_i32 (0x4b2784, an fistp of a scaled float). Nothing else differed, so it looked like an off-by-one in the port.

Fix: write the control word before each run, emu.uc.reg_write(UC_X86_REG_FPCW, 0x0c7f). With it every case matched. Any routine that reaches the x87 (lengths, normalising, the sky and the light models) needs the same; the alternative is to run fpu_setup once, which needs .bss zeroed first (emulator-bss.md).

For GameFrame: a pe32_emu option for a start-up FPU control word, and a pitfall entry ("emulated routines miss the program's FPU set-up").