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+# m2comp sessions, steps 1–4 — day wrap-up
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+
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+Date: 2026-09-16. Built in one day, with E. directing grammar
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+choices against Kowarsch (arXiv:1809.07080) and the Blaise pipeline
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+as process reference.
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+
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+## Where we stand (all committed and tagged)
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+
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+| Step | Tag | Content | Suite |
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+| ---- | --- | ------- | ----- |
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+| 1 | m2comp-step1 | Coco/R lexer+parser, `gm2 -fiso`, LL(1)-clean | 3/3 |
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+| 1.1 | m2comp-step1.1 | Bare subrange type, syntax showcase | 4/4 |
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+| 2 | m2comp-step2 | SymTab semantic analysis (200–224, 230/232/233) | 19/19 |
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+| 3 | m2comp-step3 | AST layer (Mössenböck recipe) + dump | 25/25 |
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+| 4 | m2comp-step4 | MC64 tree-walking backend, `.MC4` under `mcint` | 41/41 |
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+
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+Layout: all sources in `m2comp/src/` (grammar, frames, FileIO,
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+SymTab, AST, MGen, generated scanner/parser/driver), binary at
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+`m2comp/M2comp`, tests in `m2comp/tests/` (check + reject + AST
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+dump + execution with ExitCode values), docs in `m2comp/docs/`
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+(per-step summaries, Kowarsch text, `AST.pdf`, `oberon-master/`
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+reference, this note).
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+
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+## Hard-won facts (don't rediscover)
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+
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+- gm2 is multipass: `FORWARD` is rejected ("too many errors in
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+ pass 3"); CR's `-m` flag comments forwards out; m2c has none.
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+- Op-code ranges must be globally unique for tree-walk dispatch
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+ (Add 0–2, Mul 10–14, Rel 20–27); overlap silently misroutes.
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+- `51H`/`70H` store orders differ; use `70H` (`[addr, value]`).
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+- Bodies emitted inline need jump-over (`Jmp/DefLabel`).
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+- `| head` masks VM hangs (rc is head's); harness uses `timeout`
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+ + rc checks; diverging programs fail fast.
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+- `showcase.mod` diverges by construction (infinite regress);
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+ check-only. `ok_proc` fixed to terminate.
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+- Negative-vs-zero `D5` quirk reproduces byte-identically under
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+ m2c — VM domain, avoided in tests.
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+
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+## Tomorrow: step 5 candidates (in priority order)
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+
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+1. `DEFINITION`/`IMPLEMENTATION` separate compilation
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+ (multi-`.MC4`, `depCount` loader path) — biggest language gap.
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+2. Opaque types + procedure types.
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+3. `WITH` / `CASE` (grammar + SymTab already shaped for them).
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+4. `LONGINT`/`LONGREAL` quads (`0C5–CC`, `0x12`).
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+5. Debug listings / OPDF-style debug info.
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+
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+`SymTab` (`ByNum`, exports, bounds), `AST` (pnum/typ/aux/tag),
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+`MGen` (scope stack, image assembly) are ready for all of these.
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+Start with `./build.sh && ./run_tests.sh` (41/41, ~2 min).
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