7b445916bc
Read the encoding off the interpreter rather than guessing: instruction is a big-endian u32 whose LOW byte is the opcode (25 of them, table 0x822635FC), byte[2] is the instruction length -- every handler advances the pc by it -- and bytes[0..1] are operand kinds. Op 12 is a jump whose operand is relative to the code base [phase+232], which settles that offsets are code-base-relative for this opcode. Op 19 is the built-in call: id in word@+4, and word@+8 is a monotonically increasing STATEMENT id (0x245, 0x248, 0x24A, ...). Confirmed by disassembling Stage02.ssb: the stream decodes cleanly from the code base and routines terminate on ret exactly where expected. Scanning the code region on the call encoding: 2846 call sites, 73 of the 147 built-ins used. The phase-control ones are located -- built-in 6 (end phase) at 12 sites, 62 at 3, 39 (mark last phase) at 8 -- so a phase has several exit paths, as a mission with win and lose branches should. New tool tools/re-capture/isl.py with --calls and --to (resync-into-target, needed because instructions are variable-length so you cannot walk backwards). Not settled: the 147 built-ins are uncharacterised, so this is structure without meaning -- we can see THAT a phase ends, not WHAT was tested.
112 lines
4.8 KiB
Markdown
112 lines
4.8 KiB
Markdown
# The ISL script bytecode — instruction encoding decoded
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Status: ✅ the encoding, the 25-opcode table and the call form, read off the
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interpreter and confirmed by disassembling `Stage02.ssb`; 🟡 most opcode
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*semantics* are named only by their handler; ❔ the 147 built-ins are not yet
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characterised.
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Follows [mission-script-ssb](mission-script-ssb.md) (where the scripts live) and
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[mission-phase-advance](../mission-phase-advance.md) (why they matter).
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Tool: `tools/re-capture/isl.py`.
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## ✅ Encoding
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`ScriptPhase::Update` (`sub_82263408`) fetches one **big-endian u32** per
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instruction:
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```
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0x822635D4 lwz r11,0(r31) ; the instruction word
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0x822635D8 clrlwi r4,r11,24 ; OPCODE = the LOW byte
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0x822635DC cmplwi 0x18 ; 25 opcodes
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0x822635FC jump table, 25 absolute VAs
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```
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Every handler advances the pc with `lbz r11,2(r31); add r31,r11,r31`, so:
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| byte | 0 | 1 | 2 | 3 |
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|---|---|---|---|---|
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| meaning | operand kind A | operand kind B | **instruction length in bytes** | **opcode** |
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Operand words follow. The common form is 12 bytes (opcode word + two operands).
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Operand kinds go through resolvers with their own 4-entry table
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(`sub_82271D40` for integers, `cmplwi 0x3`), so there are **4 operand kinds**.
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## ✅ The opcode table
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| op | handler | what the handler does |
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|---|---|---|
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| 0 | `82263660` | integer assign — resolve rvalue (`82271D40`, kind byte[0], word@+8), resolve lvalue (`82272030`, kind byte[1], word@+4), `stw` |
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| 1 | `8226369C` | float assign — same shape with `82271F10`/`82272120` and `stfd` |
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| 2,4,6,8 | `822636D0` | → `822713E8` (a compare/branch family; four opcodes share one handler) |
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| 3,5,7,9 | `822636E4` | → `822714D0` (the sibling family) |
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| 10 | `822636F8` | → `82271598` |
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| 11 | `8226370C` | → `822716E0` |
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| **12** | `82263720` | **JUMP** — `r31 = [phase+232] + word@+4` |
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| 13–18 | `82263738`… | → `82271830`, `822718C8`, `82271960`, `822719F8`, `82271AC8`, `82271B60` |
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| **19** | `822637B0` | **CALL BUILT-IN** → `sub_82272220` |
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| 20 | `82263874` | `li r29,1` then the suspend path — **yield / return** |
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| 21 | `822637C4` | `sub_82175C20(phase+44, phase+168)` |
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| 22 | `822637E4` | `sub_82274BA0(phase+64, phase+184)` |
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| 23,24 | `82263804`… | → `82271C30`, `82271CB8` |
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Handler return codes drive the outer loop at `0x82263828`: **0** continue,
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**1** suspend, **2**/**3** other exits.
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### ✅ Jump operands are code-base-relative
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Op 12 adds its operand to `[phase+232]`, the code base — i.e. the `.ssb`
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header's code offset (`0x24` in every file). That settles, for this opcode, the
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question `mission-script-ssb.md` left open about whether offsets are file- or
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code-base-relative.
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### ✅ The call form, and a statement counter
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`sub_82272220` reads the **built-in id from word@+4** (`cmplwi 0x92` → 147
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built-ins, table `0x8227226C`) and stores **word@+8** into `[phase+200]`.
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That second word turns out to be a **monotonically increasing statement id** —
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`0x245, 0x248, 0x24A, 0x24B, 0x24C, 0x24D, 0x24E, 0x252…` along a routine. It is
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a source-position counter, presumably for the script's own error traces.
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## ✅ It decodes — Stage 02
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Disassembling from the code base runs cleanly, and routines terminate on `ret`
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(op 20) exactly where expected. Data in `data/isl-stage02.txt`.
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Scanning the whole code region for the call encoding:
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```
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2846 call sites, 73 distinct built-ins used (of 147)
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most used: 11 (×372), 69 (×255), 1 (×216), 64 (×213), 30 (×179), 20 (×167)
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```
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**The phase-control built-ins, located:**
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| built-in | meaning | sites in Stage02 |
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|---|---|---|
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| **6** | end phase (`[ScriptPhase+196] = 1`) | **12** |
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| **62** | force-end, skipping the end event | **3** |
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| **39** | mark last phase (`[phase+300] = 2`) | **8** |
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| 40 | `[phase+300] = 1` | 4 |
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Twelve end-phase sites across three phases — so a phase has several exit paths,
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which is what a mission with win *and* lose branches should look like.
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Argument passing is visible in the disassembly: pairs of
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`set.i k=01,02 <0> <value>` / `set.i k=02,03 <slot> <0>` stage arguments into
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slots, then `call`. Floats are staged the same way — e.g. `40080000` = 3.0
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immediately before several calls.
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## ❔ What this does not settle
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* **The 147 built-ins are uncharacterised.** Without them the disassembly is
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structure without meaning: we can see *that* a phase ends here, not *what was
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tested*. That is the remaining step to per-phase clear conditions.
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* Opcodes 2–11 and 13–18 are named only by handler address. The four-way sharing
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(2/4/6/8 and 3/5/7/9) suggests the handler re-reads the opcode to pick a
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comparison or a type, but that is not yet read.
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* Operand *kinds* (4 of them) are not decoded — the `k=01,02` / `k=02,03` pairs
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are recorded literally.
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* The mission-level stream at `+0x24` of a `.ssb` — as opposed to this ISL
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stream — is still only partly read.
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