# PIM4 vs GNU Modula-2 vs V3 — feature comparison

**Date:** 2026-10-22 · **V3 state:** `v3-proc-types` (suite 255/255,
self-hosting fixpoint OK 3,935,235 bytes, corpus 503/612, 0 crashes).

**Scope.** **PIM4** is Wirth's *Programming in Modula-2*, 4th edition
(the language specification, not a compiler).  **GNU Modula-2 (gm2)** is
the GCC front end: its PIM and ISO modes plus the extensions common in
its testsuite.  **V3** is this compiler: a PIM/ISO subset plus the gm2
compatibility it deliberately chose.

**Legend:** ✅ supported / standard · 🔄 partial or caveated · ❌ absent ·
⚠️ non-standard extension · n/a not applicable.

The tables describe the common/default behaviour; the exact flags and
modes of gm2 can change individual cells.  V3's deliberate deviations
are listed at the end.

## Standards & dialect

| Feature | PIM4 | GNU Modula-2 | V3 |
|---|---|---|---|
| PIM base language | ✅ (definition) | ✅ (PIM 2/3/4 modes) | ✅ (PIM/ISO subset) |
| ISO 10514-1 base | ❌ (predates ISO) | ✅ | 🔄 mostly; see missing ISO rows below |
| ISO 10514-2 generics | ❌ | ✅ | ✅ (refinement form) |
| ISO 10514-3 OO | ❌ | ❌ | ❌ (`CLASS` is Clarion-form, not ISO OO) |
| Declaration before use | ✅ strict | 🔄 multi-pass, accepts forward refs | ✅ strict for data; 🔄 forward **procedure** calls allowed |
| `FORWARD` declarations | ✅ | ✅ | ✅ |
| `EXPORT QUALIFIED` / `UNQUALIFIED` | ❌ (PIM export list only) | ✅ | ✅ |
| Foreign modules (`DEFINITION MODULE FOR "C"`) | ❌ | ✅ | ✅ |

## Lexical

| Feature | PIM4 | GNU Modula-2 | V3 |
|---|---|---|---|
| Identifiers with `_` | ❌ (classic names) | ✅ | ✅ |
| Numeric bases | ✅ octal `B`, hex `H` | ✅ `B`/`H` (and more) | ❌ (`0x`/`0X` only; `012H`/`666B` rejected) |
| Nested `(* *)` comments | ✅ | ✅ | ✅ |
| `'…'` and `"…"` strings | ✅ | ✅ | ✅ |
| Unicode `UCHAR`/`UString` | ❌ | ❌ | ✅ (extension: `U'a'`, `U"…"`, `UCHR`, `UORD`) |

## Types

| Feature | PIM4 | GNU Modula-2 | V3 |
|---|---|---|---|
| `INTEGER`/`CARDINAL`/`SHORTINT`/`LONGINT`, `REAL`/`LONGREAL`, `BOOLEAN`, `CHAR` | ✅ | ✅ | ✅ |
| Subranges | ✅ | ✅ | ✅ (typed CHAR/enum subranges since 2026-10-22) |
| Enumerations | ✅ | ✅ | ✅ |
| Sets (`SET OF`) | ✅ any finite ordinal (impl. limit) | ✅ | 🔄 bool/char/enum/small-int subrange; span ≤ 65536; no `SET OF CARDINAL` |
| `PACKEDSET` | ❌ (ISO) | ✅ | ❌ |
| Fixed arrays, multi-dimensional | ✅ `ARRAY [a], [b] OF` | ✅ | ✅ both `[a], [b]` and `[a, b]` |
| Open arrays (1-D) | ✅ | ✅ | ✅ |
| Multi-dimensional open arrays | ❌ (ISO addition) | ✅ | ❌ (V3 descriptor-per-level representation; `unbounded*` gaps) |
| Records, variant records | ✅ | ✅ | ✅ (plus empty arms/`ELSE` extensions) |
| Pointers, `NIL`, opaque types | ✅ | ✅ | ✅ |
| Procedure types | ✅ (`PROCEDURE`) | ✅ (`PROCEDURE`, `PROC`) | ✅ (`PROCEDURE`, `PROC`; comma/unnamed forms) |
| Complex types (`COMPLEX`, `LONGCOMPLEX`) | ❌ | ✅ (ISO) | ❌ (`COMPLEX64` is a `REAL` placeholder, no arithmetic) |
| SYSTEM sized types (`WORD32`, `CARDINAL8`, `BITSET32`, …) | ❌ | ✅ | ✅ names, but mapped to V3's 32-bit/64-bit model |
| `BYTE` granularity | ✅ 8-bit | ✅ 8-bit | 🔄 `BYTE` = 32-bit `CARDINAL`; `ARRAY OF BYTE` is not byte-granular |
| `SHORTREAL` (32-bit) | ❌ | ✅ | ❌ (all reals are 8 bytes) |
| `ADDRESS` arithmetic (`ADDADR`/`SUBADR`/`DIFADR`) | ❌ | ✅ | ❌ |
| Absolute variables `VAR [addr]: T` | ❌ | ✅ (extension) | ❌ |
| Set ordering (`<`, `<=`, `>`, `>=`) | ❌ (`=`/`#`/`IN` only) | ✅ (extension) | ❌ |

## Statements & procedures

| Feature | PIM4 | GNU Modula-2 | V3 |
|---|---|---|---|
| `IF`/`CASE`/`WHILE`/`REPEAT`/`LOOP`/`EXIT`/`WITH`, `RETURN`, `HALT` | ✅ | ✅ | ✅ |
| `FOR` over any ordinal (CHAR/enum) | ✅ | ✅ | ✅ (since 2026-10-22) |
| `GOTO` / labels | ✅ | ✅ | ✅ |
| `INC`/`DEC` on enumerations | ❌ (integer) | ✅ | ❌ (integer only) |
| Empty string `""`/`''` as `CHAR` | ❌/unclear (ISO rule) | ✅ | ✅ (ISO rule, since 2026-10-22) |
| `LENGTH` | ❌ (ISO adds it) | ✅ | ✅ |
| `HIGH` | ✅ | ✅ | ✅ / 🔄 (`HIGH` of a string `CONST` still missing) |
| `TSIZE`/`SIZE`/`ADR` | ✅ | ✅ | ✅ |
| `SHIFT`/`ROTATE` | ⚠️ implementation SYSTEM extension | ✅ | ✅ |
| `TBITSIZE` | ❌ | ✅ | ❌ |
| ISO exceptions (`EXCEPT`, `RETRY`, `EXCEPTIONS`) | ❌ | ✅ | ❌ |
| Module finalization (`FINALLY`) | ❌ | ✅ | ❌ |
| `__FILE__` / `__LINE__` / `__FUNCTION__` | ❌ | ✅ | ✅ / ✅ / ❌ |
| `__BUILTIN__` / `__ATTRIBUTE__` pragmas | ❌ | ✅ | ✅ (parsed, ignored) |
| Inline `ASM` | ❌ | ✅ | ❌ |

## Modules, compilation, libraries

| Feature | PIM4 | GNU Modula-2 | V3 |
|---|---|---|---|
| Definition / implementation / program modules | ✅ | ✅ | ✅ |
| Local modules | ✅ | ✅ | ✅ |
| Separate compilation | ✅ | ✅ (object files/archives) | 🔄 one invocation compiles all units into one image; no incremental objects |
| Generics | ❌ | ✅ | ✅ |
| Object orientation | ❌ | ❌ | 🔄 Clarion-form `CLASS` (single inheritance, virtual dispatch), not ISO 10514-3 |
| PIM coroutines (`PROCESS`, `NEWPROCESS`, `TRANSFER`, `IOTRANSFER`) | ✅ | ✅ | ✅ (`IOTRANSFER` degrades to a transfer) |
| ISO `COROUTINES` module | ❌ | ✅ | ❌ (PIM interface only) |
| Standard library | ❌ (none defined) | ✅ PIM + ISO libraries | 🔄 subset: ISO/PIM I/O, strings, storage, SysClock; no ISO exceptions/termination libs |
| Self-hosting | n/a | ✅ (part of GCC) | ✅ (byte-identical 3-stage fixpoint) |
| Backend / target | n/a | GCC (many targets) | QBE → C (amd64 SysV today) |

## The practical differences

- **PIM4** is the clean specification: strict declaration order, no ISO
  exceptions/complex numbers/`PACKEDSET`/generics, no standard library,
  no sized SYSTEM types or address arithmetic.
- **gm2** is the superset in practice: PIM and ISO modes, generics,
  exceptions, complex numbers, `PACKEDSET`, multi-dimensional open
  arrays, sized SYSTEM types, address arithmetic, absolute variables,
  `ASM`, pragmas, and many extensions — but deliberately permissive
  (multi-pass resolution accepts forward references).
- **V3** is a PIM/ISO compiler with the opposite bias: it rejects gm2
  leniencies (declaration order), implements the common ISO/PIM
  constructs (generics, coroutines, foreign modules, ISO strings/SYSTEM/
  procedure types, empty string as `CHAR`), and documents its chosen gm2
  compatibility (`__FILE__`/`__LINE__`, scalar→`ARRAY OF BYTE`, same-size
  `VAR`, bare proc values, variant extensions, set `CASE` labels, libc
  prototypes).  Its hard gaps are the large ISO features (**exceptions,
  complex arithmetic, `PACKEDSET`, finalization/protection,
  multi-dimensional open arrays, ISO OO**) and its **32-bit word model**
  (`BYTE` granularity, `SHORTREAL`, `REAL` width), plus a **65536-bit
  set span limit**.

## V3-specific deviations and limits

- **Strict data declaration order** (PIM-conforming); forward
  **procedure** calls are allowed where strict PIM would require
  `FORWARD`.
- **32-bit word model:** `INTEGER`/`CARDINAL`/`WORD`/`BYTE` are 32-bit;
  `LONGINT`/`LONGCARD` are 64-bit; every `REAL`/`SHORTREAL` is 8 bytes;
  `COMPLEX*` is a placeholder.  `ARRAY OF BYTE` is a compatibility
  descriptor, not an 8-bit buffer.
- **Sets:** bases limited to bool/char/enum/small subranges; total span
  ≤ 65536; no `SET OF CARDINAL`.
- **No runtime range checks** on subrange assignment (V3 is already
  lenient for the integer family).
- **Multi-dimensional open arrays** are not supported: V3 represents an
  array level as its own descriptor, whereas gm2 passes a flat block —
  accepting the corpus files would compile but miscompile.
- **Single-invocation compilation:** definition, implementation and
  program units are compiled together into one image; there is no
  incremental object/archive format.
- **OO** is the Clarion-form `CLASS` dialect (single inheritance,
  virtual dispatch), not ISO 10514-3.

## Sources

- V3: `docs/STATUS.md`, `docs/features.md`, `docs/gaps/README.md`,
  `docs/analysis_gaps.md`, `docs/summary_session-2026-10-22.md`; the
  gates in this document were run at `v3-proc-types`.
- gm2: the GCC Modula-2 testsuite and `gm2-libs`/`gm2-libs-iso`
  definition modules in the sibling `gcc-git` tree.
- PIM4: N. Wirth, *Programming in Modula-2*, 4th edition.