# The ISL script bytecode — instruction encoding decoded

Status: ✅ the encoding, the 25-opcode table and the call form, read off the
interpreter and confirmed by disassembling `Stage02.ssb`; 🟡 most opcode
*semantics* are named only by their handler; ❔ the 147 built-ins are not yet
characterised.

Follows [mission-script-ssb](mission-script-ssb.md) (where the scripts live) and
[mission-phase-advance](../mission-phase-advance.md) (why they matter).
Tool: `tools/re-capture/isl.py`.

## ✅ Encoding

`ScriptPhase::Update` (`sub_82263408`) fetches one **big-endian u32** per
instruction:

```
0x822635D4  lwz    r11,0(r31)     ; the instruction word
0x822635D8  clrlwi r4,r11,24      ; OPCODE = the LOW byte
0x822635DC  cmplwi 0x18           ; 25 opcodes
0x822635FC  jump table, 25 absolute VAs
```

Every handler advances the pc with `lbz r11,2(r31); add r31,r11,r31`, so:

| byte | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
| meaning | operand kind A | operand kind B | **instruction length in bytes** | **opcode** |

Operand words follow. The common form is 12 bytes (opcode word + two operands).
Operand kinds go through resolvers with their own 4-entry table
(`sub_82271D40` for integers, `cmplwi 0x3`), so there are **4 operand kinds**.

## ✅ The opcode table

| op | handler | what the handler does |
|---|---|---|
| 0 | `82263660` | integer assign — resolve rvalue (`82271D40`, kind byte[0], word@+8), resolve lvalue (`82272030`, kind byte[1], word@+4), `stw` |
| 1 | `8226369C` | float assign — same shape with `82271F10`/`82272120` and `stfd` |
| 2,4,6,8 | `822636D0` | → `822713E8` (a compare/branch family; four opcodes share one handler) |
| 3,5,7,9 | `822636E4` | → `822714D0` (the sibling family) |
| 10 | `822636F8` | → `82271598` |
| 11 | `8226370C` | → `822716E0` |
| **12** | `82263720` | **JUMP** — `r31 = [phase+232] + word@+4` |
| 13–18 | `82263738`… | → `82271830`, `822718C8`, `82271960`, `822719F8`, `82271AC8`, `82271B60` |
| **19** | `822637B0` | **CALL BUILT-IN** → `sub_82272220` |
| 20 | `82263874` | `li r29,1` then the suspend path — **yield / return** |
| 21 | `822637C4` | `sub_82175C20(phase+44, phase+168)` |
| 22 | `822637E4` | `sub_82274BA0(phase+64, phase+184)` |
| 23,24 | `82263804`… | → `82271C30`, `82271CB8` |

Handler return codes drive the outer loop at `0x82263828`: **0** continue,
**1** suspend, **2**/**3** other exits.

### ✅ Jump operands are code-base-relative

Op 12 adds its operand to `[phase+232]`, the code base — i.e. the `.ssb`
header's code offset (`0x24` in every file). That settles, for this opcode, the
question `mission-script-ssb.md` left open about whether offsets are file- or
code-base-relative.

### ✅ The call form, and a statement counter

`sub_82272220` reads the **built-in id from word@+4** (`cmplwi 0x92` → 147
built-ins, table `0x8227226C`) and stores **word@+8** into `[phase+200]`.

That second word turns out to be a **monotonically increasing statement id** —
`0x245, 0x248, 0x24A, 0x24B, 0x24C, 0x24D, 0x24E, 0x252…` along a routine. It is
a source-position counter, presumably for the script's own error traces.

## ✅ It decodes — Stage 02

Disassembling from the code base runs cleanly, and routines terminate on `ret`
(op 20) exactly where expected. Data in `data/isl-stage02.txt`.

Scanning the whole code region for the call encoding:

```
2846 call sites, 73 distinct built-ins used (of 147)
most used: 11 (×372), 69 (×255), 1 (×216), 64 (×213), 30 (×179), 20 (×167)
```

**The phase-control built-ins, located:**

| built-in | meaning | sites in Stage02 |
|---|---|---|
| **6** | end phase (`[ScriptPhase+196] = 1`) | **12** |
| **62** | force-end, skipping the end event | **3** |
| **39** | mark last phase (`[phase+300] = 2`) | **8** |
| 40 | `[phase+300] = 1` | 4 |

Twelve end-phase sites across three phases — so a phase has several exit paths,
which is what a mission with win *and* lose branches should look like.

Argument passing is visible in the disassembly: pairs of
`set.i k=01,02 <0> <value>` / `set.i k=02,03 <slot> <0>` stage arguments into
slots, then `call`. Floats are staged the same way — e.g. `40080000` = 3.0
immediately before several calls.

## ✅ The four operand kinds, and how arguments are passed

Resolver table `0x82271D74`, four entries:

| kind | code | meaning |
|---|---|---|
| 0 | `lis 0x828E` / `bl 82454A40` / `lwzx` | **global[i]** — indexed global array |
| 1 | `mr r3,r31` | **immediate** — the operand word itself |
| 2 | `[phase+164]` if `i==0` else `[phase+168]` | **special[i]** — two scratch registers |
| 3 | `addi r3,r3,20` / `lwzx` | **local[i]** — `[phase+20 + i]` |

Byte[0] is the rvalue's kind (operand word@+8) and byte[1] the lvalue's
(word@+4). That turns the recurring pair into something readable:

```
set.i k=01,02  <A> <V>     special[A] = V          (immediate -> special)
set.i k=02,03  <B> <0>     local[B]   = special[0]
```

— i.e. **argument staging**. Values land in `local[]` at byte offsets
0, 4, 8, 0xC…, and the following `call` consumes them; a built-in's arguments
are not in its own instruction. `isl.py` now tracks the staging and prints them.

⚠️ **Immediates in `set.f` are DOUBLES**, carried as two words — op 1 stores with
`stfd`. Reading only the high word as a *float* gives `2.125` where the script
means **3.0**, which is exactly the sort of plausible-but-wrong number that would
have been believed. The 16-byte `set.f` form is `high, low`.

With that, the run-up to the first `END PHASE` in Stage 02 reads:

```
0050F4  builtin=64(0x42, 0x2, 0x1, 0x9, 0x1, -1)
005160  builtin=120
005188  builtin=59(3)
0051B0  builtin=85(3)
0051D8  builtin=4(3)
0051E4  builtin=6            <-- end phase
0051F0  builtin=11
```

Three separate built-ins taking `3` immediately before the phase ends — a
plausible "wait 3 seconds" family, **unconfirmed** until the built-in table is
read.

## ❔ What this does not settle

* **The 147 built-ins are uncharacterised.** Without them the disassembly is
  structure without meaning: we can see *that* a phase ends here, not *what was
  tested*. That is the remaining step to per-phase clear conditions.
* Opcodes 2–11 and 13–18 are named only by handler address. The four-way sharing
  (2/4/6/8 and 3/5/7/9) suggests the handler re-reads the opcode to pick a
  comparison or a type, but that is not yet read.
* The four-way opcode sharing (2/4/6/8 and 3/5/7/9) suggests the handler
  re-reads the opcode to pick a comparison or a type; not yet read.
* The mission-level stream at `+0x24` of a `.ssb` — as opposed to this ISL
  stream — is still only partly read.