# ✅ Which built-ins take a unit — read from the implementations, and the old set was short

`isl.py`'s `UNIT_ARG` decides whether a built-in's slot-4 operand is printed as
a **unit name** or as a raw number. It was built **statistically**, from operand
ranges, and its own comment says slots were listed *"only when the ratio stayed
below 1.0"* — i.e. only when every observed value resolved. That is
conservative, and it was.

## The direct method

[`isl-builtin-dispatch`](isl-builtin-dispatch.md) makes this a lookup: a
built-in's stub tail-calls a fixed slot of the `ScriptPhase` vtable at
`0x820A84BC`, so the implementation can simply be read. Every unit-taking
built-in opens the same way — this is `unit_state`, `builtin80`, `builtin117`
and `builtin136`, character for character:

```
lwz    r10, 324(r31)        ; the unit array
lwz    r11, 4(r30)          ; r30 = the local[] base, so this is local[4]
rlwinm r11, r11, 2, 0, 29   ; x4
lwzx   r11, r11, r10        ; -> the record
lwz    r11, 4(r11)          ; the handle
cmpli  cr6, 0, r11, 0x0     ; "is this squadron gone?"
```

## ✅ Result: 31 → 55

| | |
|---|---|
| built-ins whose implementation indexes `[phase+324]` by an argument | **55** |
| of the statistical set's 31, confirmed | **31 — all of them** |
| **UNIT_ARG claims a unit, the implementation does not** | **0** |
| **implementation says unit, UNIT_ARG missed it** | **24** |

The 24: `21, 22, 23, 32, 42, 44, 46 (squadron_trace), 49, 50, 51, 55, 60, 61,
72 (group_ratio_pct), 80, 83, 94 (is_engaged), 101, 109 (set_unit_flags), 117,
136, 137 (wait_units_ready), 141, 142 (deploy_and_wait)`.

Zero false positives is worth stating on its own: the statistical method was
**right about everything it claimed** and only too cautious about what it
omitted.

## ⚠️ The first control I chose was worthless — recorded because it nearly passed

I first checked whether the additions' slot-4 operands resolve to a symbol-table-2
index. They did, **100.0 %** — and it means nothing:

| set | operands resolving to a symtab-2 unit |
|---|---|
| the 31 baseline | 100.0 % |
| the 24 additions | 100.0 % |
| **the 92 built-ins in neither set** | **99.3 %** |

Symbol table 2 is dense enough that almost any small integer lands in it, so the
test does not discriminate. A control that the negative class also passes is not
evidence, and this one was one careless glance from being written up as proof.

## ✅ The control that does discriminate

`isl-builtins.md` documents that a symbol operand is a **two-word pair** — a tag
holding the constant 1, then the index. So slot 0 should be 1 exactly when
slot 4 is a unit:

| set | calls with `slot0 == 1` |
|---|---|
| the 31 baseline | **100.0 %** (13 677 calls) |
| the 24 additions | **100.0 %** (140 calls) |
| the 92 in neither set | **2.5 %** (2 903 calls) |

A 40× separation, and the additions sit exactly on the positive class.

## Effect on the artefacts

`data/isl-stage02.txt` and `-phase-ends.txt` regenerate **byte-identical**;
`-conditions.txt` changes on 28 sites, every diff line pairing, all of them a
raw number becoming a unit name:

```
- 0x01B6C0  if builtin80(1, 117) == 0   ->  + 0x01B6C0  if builtin80(TCT206) == 0
```

⚠️ **That example was itself a bogus site** — see
[isl-conditions](isl-conditions.md#-the-correction--i-fixed-the-instance-not-the-class).
`0x1B6C0` is reached only by a branch, so attributing its comparison to
`builtin80` was wrong, and `builtin80` is a command rather than a predicate. The
UNIT_ARG result is unaffected — it was derived by reading implementations, not
from this listing — but the illustration was picked from bad output.

## 🟡 Not settled

* **The 24 are still unnamed.** Knowing an argument is a unit is not knowing what
  the built-in does. `builtin80` (69 operands disc-wide) is tested against
  0, 1, 2, 3, 4 in a switch chain, so it returns a small enumeration — but I am
  not naming it from that, and its body past the liveness check is unread.
* `builtin103` (`0x8226BFA8`) is a predicate over **`[phase+10152]` and
  `[phase+10156]`**, neighbours of a value read out at `+10160`; it takes no unit.
  The fields have 9, 7 and 1 writers respectively, none of them read.
* `builtin105` (`0x8226BFF0`) tests a unit record's **`+16` against 4**. What
  `rec+16` holds is not established — `isl-builtins.md` only rules out its being
  what `unit_state` reads.