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re: the menu's music is BGM_103 -- found by accident while chasing Q6,

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confirmed three ways

Chasing where the event code comes from, sub_821C5580 turned out to do
two things worth having.

For Q6 it arms the outer gate: li r11,3 ; stw r11,16(r28) is exactly the
value sub_821C7850 tests before dispatching, which answers the "what does
this+16 == 3 gate on" question I left open last iteration. It then passes
r5 = [r27+4], an event read out of a structure, so the vocabulary is
still not enumerable. Of the four callers of sub_821C7850, three pass
constants -- 0, 0 and 5 -- and this one does not.

The accident is a few instructions earlier: li r5, 1103 into a sound-play
call. 1103 is a BGM cue id, BGM_103. That closes a residual I had written
off as undecodable: which bank the menu plays. The cue table genuinely
cannot say, since its BGM entries are numeric -- but the code can, and it
checks out three independent ways. The census says BGM_103.slb is two
waves of 3876864 and 3930112 bytes; the XMA probe at the main menu saw
two stereo streams of 3876864 and 3930112 bytes. Byte for byte on both.

That also corrects me. bgm-two-stems.md said those observed sizes matched
no bank's declared waves and inferred the decoder gets a window rather
than a whole wave. Wrong on both counts -- I had checked only the BGM_0xx
rows of the census because that was the block on screen. METHOD gets it:
check a measurement against the whole reference set, not the part you
happened to be reading.
This commit is contained in:
Sylpheed RE agent
2026-08-28 20:10:41 +00:00
parent a98e96b0f0
commit 86e52b01d0
6 changed files with 97 additions and 13 deletions
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docs/re/METHOD.md
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@@ -220,3 +220,9 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
into place later (`addi r27, r11, X` … `mr r5, r27`). A backward search for the
destination register alone found 7 of 48 sites and missed the two known-good
ones. Simulate forward over a window instead, tracking `lis`/`addi`/`mr`.
* **Check a measurement against the WHOLE reference set, not the part you were
reading.** Two observed BGM stream sizes were recorded as matching "no bank's
declared waves", which spawned a wrong theory that the decoder gets a window
rather than a whole wave. They match `BGM_103` exactly — the check had covered
only the `BGM_0xx` rows of the census, because that is the block that had been
on screen.