# Step 2 — Scalars + MC64 backend (tag: `step2`)

Commit `eee1504` — "step2: scalars + MC64 backend — SymTab, MGen
emitter, 25/25 tests green; fix bit_in operand order"
(88 files, +4980/−132).

## Goal

Full scalar subset of Modula-2 compiling to runnable MC64 images:
`CONST`/`TYPE`/`VAR`, complete expression hierarchy, all structured
statements, static type checking, exact error codes.

## What was built

- `m2c/SymTab.def/.mod` — symbol table + type checker, ported from
  the proven Test1 design: scopes, aliases/subranges/enums/arrays/
  records/sets/pointers descriptors, `InvalidType` cascade
  suppression; codes 200/201/202, 210–224; plus `PendCount`/
  `PendName` for VAR-list emission.
- `m2c/MGen.def/.mod` — MC64 image emitter: one 8-byte slot per
  global, literal `CONST`s as startup stores, stack-machine
  expressions (`A6/A7/A8/B8/A9/AA/B9`, signed `B2–B5` compares,
  `D5–D9` reals, `E6/E8/B6` logic), `E0/E1` jumps with fixups,
  `LOOP/EXIT` stack, `CASE` as compare-chains, `FOR` with hidden
  temp global, `SET`s via `power2`/`build_field_mask`/`bit_in`/`A0`,
  embedded decimal-print helper. Test convention: a global
  `VAR ExitCode : INTEGER` is printed as decimal + CRLF.
- `m2c/M2c.atg` — full scalar grammar (~900 lines): type checking
  identical to the reference rules; backend lowers scalars + sets,
  everything else parses but gets error 230.
- `m2c/run_tests.sh` — extended: `expect_run` compiles, runs the
  `.MC4` (name derived from the `MODULE` line, not `ls -t`) under
  `mcint`, and compares the printed value.

## Tests — 25/25

14 run tests (7, 15, 42, 20, 103, 55, 55, 7, 85, 31, 97, 1, 74,
sets 15) + 6 rejections (one 210, rest 230-only, no cascades) +
4 phase-0 + 1 silent-minimal. All negatives produce exactly one
error each.

## Bugs found and fixed

1. Image writer skipped the 64-byte file header (shifted image).
2. `ParseCard` used `10H`/`16H` (hex 16/22!) for decimal bases —
   multi-digit literals wrong, single digits accidentally right.
3. `ParseReal` overlaid 8 bytes of gm2's 80-bit-extended `LONGREAL`
   — switched to 8-byte `REAL` like the VM's own `Stack.mod`.
4. **`bit_in` (`0E7H`) operands swapped in `mc64/Interpreter.mod`**
   vs both the spec prose and the original Turbo MCode
   (`op := Pop(); Push(Pop() IN BITSET(op))`) — fixed (2 lines),
   MC64 suite re-verified 24/24.
5. Runner used same-second `ls -t` ties + non-portable `$'\r'`
   stripping — deterministic names + `tr -d '\r'`.

## Known limits (in `M2c.atg` header)

Truncating DIV/MOD, signed compares for CARDINAL, eager AND/OR,
literal-only CONST/BY, REAL as binary64, no procedures yet.