SCJUMP — progression / scene-sequencing logic

SCJUMP.BIN (29,790 instructions) is the game's progression state machine: it decides "what comes next" in the story. tools/scjump_decode.py decodes its decision logic faithfully into a readable, VM-verified table.

Mechanism

Top switch on 0x3234 (chapter_mode, 1..9) dispatches to a per-chapter block (1→0x81, 2→0x9f, 3→0x1ded, 4→0x1e6d, 5→0x54c1, 6→0x93c4, 7→0x1e868, 8→0x2aa42, 9→0x2b8a3).
Each chapter block is an acyclic tree (0 back-edges) of comparisons on progress counters (scjump_progress_a/b/c/d = 0x4dfbc/0x2052e/0x152618/0xe6c5d), story flags, and a native computed value (see below).
At each of 1755 decision sites it writes 0x0 = 1 (decision-valid) and 0x62ccf = <decision value> (847 distinct values, 0..1693).

The decode

scjump_decode.py walks the CFG with a guarded DFS (a guard stack + a local→symbolic-value map, so mov local, global ; cmp local, imm resolves back to a real global condition) and emits, per site, (chapter, path-condition guards) → decision. Output (generated, disposable): build/scjump-decisions.json (machine) and build/scjump-decisions.md (human, globals rendered by their vm-map/globals.toml registry names). Regenerate: py -3.11 -X utf8 tools/scjump_decode.py.

Native-gated decisions (a key finding)

1732 of 1755 decisions have at least one guard on a native computed value — op 0x60 (u0041A270(local, 0x64), e.g. local = f(100), then local < 25). This is resolved at runtime by the engine (it looks like a random/derived roll: ≈ rand()%100 < N), so it is not recoverable statically and is honestly marked opaque in the guards. The decode still captures every global condition (chapter, counters, story flags) on each path; only the native term is opaque. Takeaway: SCJUMP mixes deterministic story-state gating with native/random gating — many "what comes next" choices are not pure story flags.

Verification

--verify cross-checks the decode against the executing VM (vm0.py) two ways:

static-witness — for the fully-static decisions (no native/opaque guard), synthesize a state from the guards and confirm the VM emits that decision (3/3 exact).
execution-driven — seed thousands of random states, run SCJUMP (the VM executes the native op too), and for every realized decision assert the table has that site with the right value and no static guard is violated by the realizing state (279/279 consistent over 2000 seeds, 0 failures). This is the oracle that covers native-gated paths. Run: py -3.11 -X utf8 tools/scjump_decode.py --verify.

The decision→scene boundary (resolved 2026-07-20)

A FIELD snippet does lookup-array(ptr, 0x5f0ed, 0x62ccf) then u00428010(ptr), which the spec guessed was the scene resolver. Correction (resolved 2026-07-20, via Ghidra): u00428010 (op 0x1a2) snapshots that selected global integer array cell into the shared SAVE.DAT profile table, keyed by its resolved global-bank index. Paired op 0x1a3 restores a selected cell or zero. Its write of 3 at ctx+0x53d88 is only the instruction length. So this is profile persistence, not decision→scene dispatch. The separate boundary is now located in SYSTEM4.BIN: on the normal path it indexes the global resource-id table rooted at G[0x87a57] by scjump_decision_out (G[0x62ccf]), copies the selected raw SYS4INI id into G[0x699], substitutes 0x22 (SC0000.BIN) when the result is zero, and executes call-script G[0x699] at SYSTEM4 offset 0x477. The called scene returns to the still-live SYSTEM4 frame.

The captured initial New Game path sets decision zero in GAMESTART.BIN; SYSTEM4 consequently takes the 0x22 fallback and enters SC0000. Later decisions use the same mapping/call boundary. See docs/engine-re.md §Natural boot and New Game control spine for the native trace and complete boot chain.

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