# Step 3 — Procedures (tag: `step3`)

Commit `c0f1924` — "step3: procedures — nested, params, recursion,
FORWARD, RETURN; 49/49 tests green" (91 files, +2730/−548).

## Goal

Procedures (Phase 2 of the plan): declarations incl. nesting, value
and VAR parameters, functions, `RETURN`, recursion, forward
declarations; frames per spec §5.2; calls via ED/EC/EE.

## What was built

- `SymTab`: frame slots (`SymSlot`), proc depths
  (`SymDepth`/`CurDepth`), proc records numbered 1..64, forward +
  define signatures with `VerifyProc`, return-type stack
  (`CurRet`/`InFunction`), new kinds `Proc/Param/VarPar`.
- `MGen`: N-entry proc table (print helper moved to last slot),
  `ProcEntry`/`Leave`, calls via `ED` (global), `EC` (directly
  nested), `EE` + display walk (`11H`); actuals evaluated
  left-to-right into temps, pushed reversed (nested-call-safe
  frame stack in `ActBegin/Value/End`, which also own arity/type/
  VAR-actual checks); frame-aware loads/stores/addresses.
- `M2c.atg`: `ProcedureDecl` (nested, `FORWARD;` form verified
  against gm2 — caret form rejected), `Design` split into
  `Head`+`Tail` with `AssignOrCall`, `CallTail`, `ReturnStat`,
  `v/vn` attributes threading "is plain variable" for VAR-actual
  checks.
- Codes 231 (forward mismatch/missing body), 232 (bad RETURN),
  233 (invalid call); proc `END`-name reuse 202 (text widened to
  "module/procedure name mismatch").

## Tests — 49/49 (25 step-2 regressions + 24 new)

14 run: proper/value-params (incl. 6 params)/functions/factorial/
fib/VAR-swap/nested display/forward mutual recursion/early RETURN/
shadowing/nested calls/dropped results/VAR chains/empty bodies/
REAL functions. 10 rejections, each exactly one error: non-proc
call, arity, actual type, proper-in-expression, 3× RETURN misuse,
forward mismatch, missing body, undeclared callee.

## Bugs found and fixed

1. `FORWARD;` double-semicolon — the branch must not consume `;`
   (it belongs to `Declaration`).
2. Bare `P;` calls need no parens — empty-call branch in statements
   (expressions still require parens).
3. `2EH` pops an address off the stack — same-frame indirect access
   must go through `2CH`+`60H`; deleted `LoadStk`.
4. Dead callee value on expression calls
   (`Double(10)+Double(11)` → 22) — `CallTail` drops it on entry.

## Research notes

- Forward syntax confirmed empirically (`PROCEDURE P(...);
  FORWARD;` compiles under gm2; `^` and heading-alone do not).
- Decoder-verified before use: `2E/3E` unsigned (outer locals need
  the `41H/51H` path), `11H np=0` valid, `EC/EE/ED` semantics.
- Limits: 64 procedures, 64 actuals/call, 16 names per FP-section,
  8-deep nested calls; truncating DIV/MOD and eager AND/OR carry
  over from step 2.