# ✅ The trigger node's layout — from a matched writer/reader pair

[isl-trigger-queue](../isl-trigger-queue.md) located the *appender* (built-in 25).
The node's contents are now read, because the **reader** was found too and the two
agree field for field.

## The reader: `sub_8226E220`

`sub_8226D740` — the per-frame engine→script drain — calls it as

```
8226D758  addi r26, r29, 272        ; r29 = the phase, so r26 = the TRIGGER CONTAINER
8226D79C  or   r3,  r26, r26
8226D7A0  bl   0x8226E220           ; (container, &out88, &out84, &out80,
                                    ;             &out120, &out92, &out112)
```

and it pops one node:

```
8226E228  lwz  r10, 20(r11)   ; the PENDING COUNT -- return 0 if empty
8226E244  lwz  r10, 16(r11)   ; the list head
8226E250  stw  r10, 24(r11)   ; a cursor
8226E254  lwz  r11,  8(r10)   ; -> the node PAYLOAD
```

## ✅ The layout, confirmed from both ends

| payload | written by built-in 25 | read by `sub_8226E220` | source |
|---|---|---|---|
| **+0** | `stw r26, 0(r30)` | → out88 | `local[4]` — the unit |
| **+4** | `stw r28, 4(r30)` | → out84 | `local[12]` |
| **+8** | `stw r27, 8(r30)` | → out80 | computed |
| **+16** | `stfd f31, 16(r30)` | → out120 (a **double**) | `local[16]` |
| **+24** | `stw r25, 24(r30)` | → out92 | the constant 1 |
| **+28** | `stw r28, 28(r30)` | → out112 | `local[12]` again |
| **+32** | `stw r11, 32(r30)` | → out116 | — |
| +36 | `stw r23, 36(r30)` | *not read here* | — |

Container: **+16** head, **+20** pending count, **+24** cursor. The `+20` matches
the count `isl-builtins.md` watched live at `phase+272+20`, from a completely
different direction.

That the writer and the reader touch the same seven offsets, in the same order,
is the check — neither was derived from the other.

`payload+0` being `local[4]` also agrees with `UNIT_ARG`, which lists built-in 25
and was derived from operand ranges.

## 🔴 Two corrections, both mine

### `[node+112]` does not exist

[isl-coroutine-spawner](../isl-coroutine-spawner.md) recorded the drain's spawns
as taking their offset from `[node+112]`. **`r31 = r1 - 256`** — it is the
**stack frame**, so `+112` is an output slot, not a node field. Withdrawn.

### And the trigger does NOT carry a routine offset

The follow-up hypothesis — that `payload+28` reaches the spawner and so names the
routines nothing else starts — does not survive. `payload+28` is `local[12]`, and
across **all 25 call sites disc-wide** those values are the small integers
**1 … 12**:

| | |
|---|---|
| land on the instruction stream | **1 / 25 = 4.0 %** |
| control, random 4-aligned offsets | 16.0 % |
| are unreached run-starts | **0** |

Below chance. They are indices of some kind, not code offsets.

⚠️ My first version of this test used **n = 2** — Stage 02 only. It happened to
give the same answer, but two samples could not have supported it either way.

## ✅ One correction to the corpus, in the other direction

`isl-builtins.md` withdrew the idea that `sub_8226E458` touches the trigger
queue, reasoning: *"At `0x8226D780` the argument is `lwz r4, 324(r29)` —
`[ScriptPhase+324]`, the unit array, not the trigger container."*

The call is `sub_8226E458(r3 = r26, r4 = [phase+324])`, and **`r26` was set to
`phase + 272` twelve instructions earlier**. The withdrawal read the *second*
argument. `sub_8226D740` does operate on the trigger container; what
`sub_8226E458` does *to* it is still unread, so only the argument is corrected
here, not the conclusion.

## ✅ RESOLVED: there are TWO appenders, and only one carries a routine

The puzzle above — `payload+28` holding a 1–12 index while the drain hands it to
a spawner that wants a code offset — had a simple cause: **built-in 25 is not the
only thing that appends a trigger node.**

Searching for functions that write a **double at +16** and a **word at +28** of
the same register finds exactly two:

| builder | how |
|---|---|
| `sub_8226A0D8` — **built-in 25**, vtable slot 28 | fills the payload inline |
| `sub_8226E160` | a free-list recycling variant, called from `sub_82269ED0` |

`sub_82269ED0` is **vtable slot 68 = built-in 19**, and it calls the builder as

```
8226A034  addi r3, r28, 272      ; the trigger container
8226A040  lwz  r4, 4(r30)        ; local[4]   -> node+0   (the unit)
8226A03C  lwz  r5, 12(r30)       ; local[12]  -> node+4
8226A038  lfd  f1, 24(r30)       ; local[24]  -> node+16  (double)
8226A030  lwz  r8, 32(r30)       ; local[32]  -> node+28
```

### ✅ Built-in 19's `local[32]` IS a code offset

| over all 79 call sites disc-wide | |
|---|---|
| land on the instruction stream | **79 / 79 = 100.0 %** |
| control, random 4-aligned offsets | 27.8 % |
| value range | 12 164 … 111 080, **73 distinct** |

Large, distinct, and every one resolves — against a control that resolves barely
a quarter of the time. So **built-in 19 registers a trigger whose handler is a
routine**, and the drain's `base + payload+28` spawn is exactly that handler.

Built-in 25's `payload+28` really is a small index; the two builders simply put
different things in the same slot. Observed alongside it: built-in 25 writes
**`+24 = 1`** and built-in 19 writes **`+24 = 0`**, which is the shape of a kind
tag — 🟡 but the drain was not shown branching on it, so that is an observation,
not a reading.

## 🟡 Not settled

* **What `local[12]` indexes.** Values 1–12, and `SYM1_SLOTS` lists slot 12 for
  built-in 25 (symbol table 1 — routes, messages, objectives), which is
  consistent with a small index but was not verified here.
* ~~What the drain actually spawns.~~ ✅ Resolved above: built-in 19's nodes carry
  a real routine offset at `+28`.
* **Whether `+24` selects between the two node kinds** — 1 from built-in 25, 0
  from built-in 19 — is unshown; the drain reads it into a stack slot that the
  spawn path does not visibly use.
* **This still does not explain the unreached code.** All 79 of built-in 19's
  handler offsets are already reachable — **0 are unreached run-starts**.
* **`sub_8226E458`'s effect** on the container.
* **What starts the ~15 % of unreached code** — still open, and this was not it.