# Step 5 goal — Hardening + modules (saved, to implement next)

## Scope (Phase 4 of the plan)

- **Finish composites:** open-array parameters (`ARRAY OF T` + `HIGH`),
  whole array/record copy (`copy_block`), value composite
  parameters/returns, composite `=`/`#` (`string_comp` for `CHAR`
  arrays, `213` otherwise), VAR actuals with tails (`a[i]`, `p^`,
  fields — currently `233`), `CHAR` slot-packing cleanup (currently
  8x over-allocated).
- **Checking:** opt-in range checks (`DA`/`DB`/`DC`, unchecked stays
  default), `NIL` dereference VM trap (currently reads 0).
- **Modules:** `DEFINITION`/`IMPLEMENTATION` split, `IMPORT`/`EXPORT`,
  qualified identifiers (`M.x` — currently `QualIdent` kills the
  tail), separate compilation to one `.MC4` (loader already handles
  `depCount`, currently rejected).
- **Explicitly deferred:** `LONGINT`/`LONGCARD` 64-bit (`0C5–CC`,
  `0x40 14–1C`), `LONGREAL` quads (`0x12`), procedure
  types/variables, `cones`.

## Design deltas (vs step 4)

- SymTab: open-array descriptors (isOpen + elem), module
  scopes/symbols, export tables.
- MGen: open-array hidden length temps, `copy_block` for value
  params/returns + whole assignment, `string_comp` (`0C4`) for
  `CHAR`-array comparison.
- Policy to fix at start: one-file vs separate-`.MC4` linkage;
  `SYSTEM` services vs language I/O (replace the `ExitCode`
  convention?).

## Acceptance (same bar)

~15 run + ~10 rejection tests, all green, no cascades; full
74-test regression still passing (→ ~99 total).

## Suggested first tests

Open-array sum/`HIGH` (value + VAR); whole-array copy; string
`=`/`#`; multi-module import (exported procedure + variable);
`NIL` trap; `HIGH` on a fixed array (must fail cleanly).