# PIM4 vs GNU Modula-2 vs M2R10 vs TopSpeed V3 vs V3 — feature comparison

**Date:** 2026-10-23 · **V3 state:** `v3-packedset` (suite
275/275, self-hosting fixpoint OK 4,141,580 bytes, corpus 512/612,
0 crashes).

**Scope and sources.**

- **PIM4** — Wirth's *Programming in Modula-2*, 4th edition (the
  language specification, not a compiler).
- **GNU Modula-2 (gm2)** — the GCC front end: PIM and ISO modes plus
  the extensions common in its testsuite.
- **M2R10** — the Kowarsch/Sutcliffe *Modula-2 R10* revision (2012).
  It is a deliberate redesign, **not source-compatible** with classic
  Modula-2.  Cells follow the published R10 grammar and the document
  *“Differences between Modula-2 R10 and classic Modula-2”*.  Some
  features are library-level or outside the base grammar; those are
  marked as such.  A cell saying “not in the R10 grammar” means the
  classic keyword/operator is not in the base grammar, not necessarily
  that no library equivalent exists.
- **TopSpeed V3** — the JPI/TopSpeed (Clarion) V3 dialect as
  reconstructed in `compiler/src/TopSpeed-V3-M2.atg` and
  `docs/OOP.txt`.  This is a historical compiler dialect, not a
  standard; cells describe that grammar/dialect where it is documented.
- **V3** — 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 common/default behaviour; exact flags and modes can
change individual cells.

## Standards & dialect

| Feature | PIM4 | GNU Modula-2 | M2R10 | TopSpeed V3 | V3 |
|---|---|---|---|---|---|
| PIM base language | ✅ (definition) | ✅ (PIM 2/3/4 modes) | 🔄 modern revision, not source-compatible | ✅ PIM-based dialect | ✅ (PIM/ISO subset) |
| ISO 10514-1 base | ❌ (predates ISO) | ✅ | ❌ (independent R10 spec) | ❌ | 🔄 mostly; see missing ISO rows |
| ISO 10514-2 generics | ❌ | ✅ | ❌ (external template engine, not ISO generics) | ❌ (not in the V3 dialect grammar) | ✅ (refinement form) |
| ISO 10514-3 OO | ❌ | ❌ | ❌ | ❌ (Clarion classes, not ISO OO) | ❌ (`CLASS` is Clarion-form) |
| Declaration before use | ✅ strict | 🔄 multi-pass, accepts forward refs | ✅ strict | ✅ strict (single-pass) | ✅ strict for data; 🔄 forward **procedure** calls |
| `FORWARD` declarations | ✅ | ✅ | ❌ (not in the R10 grammar) | ✅ | ✅ |
| `EXPORT QUALIFIED` / `UNQUALIFIED` | ❌ (PIM export list only) | ✅ | ❌ (EXPORT removed; import qualifiers/re-export) | ✅ (`EXPORT [QUALIFIED]`) | ✅ |
| Foreign modules (`DEFINITION MODULE FOR "C"`) | ❌ | ✅ | ⚠️ FFI via pragma, not `FOR "C"` | ❌ (not in the reconstructed grammar) | ✅ |

## Generics (explicit)

| Dialect | Language-level generics |
|---|---|
| PIM4 | ❌ no generic modules |
| GNU Modula-2 | ✅ ISO 10514-2 generic modules (type/const/procedure parameters) |
| M2R10 | ❌ not in the language; R10 uses an **external template engine** (`GENLIB`/`BLUEPRINT`, `##…##` placeholders) expanded before compilation |
| TopSpeed V3 | ❌ no `GENERIC` construct in the reconstructed V3 grammar |
| V3 | ✅ ISO 10514-2 refinement modules: type/value/procedure parameters, constraints, nested generics, generic classes (`docs/analysis_generics.md`) |

## Lexical

| Feature | PIM4 | GNU Modula-2 | M2R10 | TopSpeed V3 | V3 |
|---|---|---|---|---|---|
| Identifiers with `_` | ❌ (classic names) | ✅ | ✅ | ✅ (`_` and `@`) | ✅ |
| Numeric bases | ✅ octal `B`, hex `H` | ✅ `B`/`H` (and more) | ✅ C-style `0b`/`0x`, `U` char-code suffix (octal removed) | ✅ `H`/`C`/`B` suffixes + `0x` | ❌ (`0x`/`0X` only) |
| Nested `(* *)` comments | ✅ | ✅ | ✅ (max 10 levels) | ✅ | ✅ |
| `'…'` and `"…"` strings | ✅ | ✅ | ✅ + C-style escapes | ✅ | ✅ |
| Unicode | ❌ | ❌ | ✅ `UNICHAR` | ❌ | ✅ `UCHAR`/`UString` (extension) |

## Types

| Feature | PIM4 | GNU Modula-2 | M2R10 | TopSpeed V3 | V3 |
|---|---|---|---|---|---|
| Basic integer/real/bool/char types | ✅ | ✅ | ✅ (+ `OCTET`, `LONGCARD`) | ✅ PIM types | ✅ |
| Subranges | ✅ | ✅ | ✅ (`[lo..hi] OF Base`) | ✅ | ✅ (typed CHAR/enum subranges) |
| Enumerations | ✅ | ✅ | ✅ extensible | ✅ | ✅ |
| Sets (`SET OF`) | ✅ any finite ordinal (impl. limit) | ✅ | 🔄 `SET OF <enumType>` only | ✅ range/base/enum bases | 🔄 bool/char/byte/16-bit/enum/small-int; span ≤ 65536; no `SET OF CARDINAL` |
| `PACKEDSET` | ❌ (ISO) | ✅ | ❌ | ❌ | ✅ (1/2/4 bytes, span ≤ 32) |
| Fixed / multi-dimensional arrays | ✅ `[a], [b] OF` | ✅ | ✅ `ARRAY count, count OF` (zero-based) | ✅ | ✅ both `[a], [b]` and `[a, b]` |
| Open arrays (1-D) | ✅ | ✅ | ✅ (`BARE ARRAY OF`) | ✅ | ✅ |
| Multi-dimensional open arrays | ❌ (ISO addition) | ✅ | 🔄 one-dimensional `BARE` arrays | ❌ | ❌ (descriptor-per-level representation) |
| Records | ✅ | ✅ | ✅ extensible | ✅ | ✅ |
| Variant records | ✅ | ✅ | ❌ removed (extensible records replace them) | ✅ | ✅ (plus empty arms/`ELSE`) |
| Pointers, `NIL`, opaque types | ✅ | ✅ | ✅ (`OPAQUE` keyword) | ✅ | ✅ |
| Procedure types | ✅ (`PROCEDURE`) | ✅ (`PROCEDURE`, `PROC`) | ✅ (comma-separated formals) | ✅ | ✅ (`PROCEDURE`, `PROC`; comma/unnamed forms) |
| Complex types (`COMPLEX`, `LONGCOMPLEX`) | ❌ | ✅ (ISO) | ❌ (not in the base R10 grammar) | ❌ | ❌ (`COMPLEX64` is a `REAL` placeholder) |
| SYSTEM sized types (`WORD32`, `CARDINAL8`, `BITSET32`, …) | ❌ | ✅ | 🔄 `OCTET` (8-bit), `LONGCARD`, `LONGBITSET` | 🔄 `SHORTINT`/`LONGINT`/`SHORTCARD`/`LONGCARD` family | ✅ 8/16/32 names; `WORD`/`WORD32` 32-bit |
| `BYTE` granularity | ✅ 8-bit | ✅ 8-bit | ✅ `OCTET` 8-bit | ✅ (PIM `BYTE`) | ✅ `BYTE`/`SBYTE`/`WORD8`/`CARDINAL8`/`INTEGER8` are 8-bit; `ARRAY OF BYTE` is a real buffer |
| `SHORTREAL` (32-bit) | ❌ | ✅ | ❌ (REAL/LONGREAL) | ⚠️ dialect-dependent | ❌ (all reals are 8 bytes) |
| `ADDRESS` arithmetic (`ADDADR`/`SUBADR`/`DIFADR`) | ❌ | ✅ | ❌ | ❌ | ❌ |
| Absolute variables `VAR [addr]: T` | ❌ | ✅ (extension) | ✅ (pragma `ADDR`) | ❌ | ❌ |
| Set ordering (`<`, `<=`, `>`, `>=`) | ❌ (`=`/`#`/`IN` only) | ✅ (extension) | ❌ | ❌ | ❌ |

## Statements & procedures

| Feature | PIM4 | GNU Modula-2 | M2R10 | TopSpeed V3 | V3 |
|---|---|---|---|---|---|
| `IF`/`CASE`/`WHILE`/`REPEAT`/`LOOP`/`EXIT`/`WITH`, `RETURN`, `HALT` | ✅ | ✅ | 🔄 no `WITH`/`GOTO`; `LOOP`/`EXIT`, `FOR … IN` | ✅ + `IN`/`IS`, `<<`/`>>` | ✅ |
| `FOR` over any ordinal (CHAR/enum) | ✅ | ✅ | 🔄 `FOR … IN` over ranges/iterables (loop variable declared in the header) | ✅ classic `FOR … TO` | ✅ (since 2026-10-22) |
| `GOTO` / labels | ✅ | ✅ | ❌ (not in the R10 grammar) | ✅ | ✅ |
| `INC`/`DEC` on enumerations | ❌ (integer) | ✅ | ❌ replaced by `SUCC`/`PRED` | 🔄 dialect-dependent | ✅ (any ordinal) |
| Empty string `""`/`''` as `CHAR` | ❌/unclear (ISO rule) | ✅ | 🔄 (char literals compatible with `ARRAY OF CHAR`; empty-to-`CHAR` not explicit) | ✅ | ✅ (ISO rule) |
| `LENGTH` | ❌ (ISO adds it) | ✅ | 🔄 library/pervasive, not in the base grammar | ❌ | ✅ |
| `HIGH` | ✅ | ✅ | ❌ (not in the R10 grammar) | ✅ | ✅ (including string `CONST`) |
| `TSIZE`/`SIZE`/`ADR` | ✅ | ✅ | 🔄 SYSTEM/library forms | ✅ | ✅ |
| Runtime subrange range checks | ⚠️ error per language; checks implementation-defined | ✅ | ✅ (R10 safety) | 🔄 | ✅ assignment/value actual/`RETURN`/`INC`/`DEC`; `FOR` final increment exempt |
| `SHIFT`/`ROTATE` | ⚠️ implementation SYSTEM extension | ✅ | ❌ | ✅ (`<<`/`>>` operators) | ✅ |
| `TBITSIZE` | ❌ | ✅ | ❌ | ❌ | ❌ |
| ISO exceptions (`EXCEPT`, `RETRY`, `EXCEPTIONS`) | ❌ | ✅ | ❌ (not in the base R10 grammar) | ❌ | 🔄 `EXCEPT`/`RETRY`/`SYSTEM.THROW`, `EXCEPTIONS`/`M2EXCEPTION`, index/range/CASE traps; `functionException`/`FINALLY` pending |
| Module finalization (`FINALLY`) | ❌ | ✅ | ❌ | ❌ | ❌ |
| `__FILE__` / `__LINE__` / `__FUNCTION__` | ❌ | ✅ | 🔄 COMPILER pseudo-module introspection | ❌ | ✅ / ✅ / ✅ |
| `__BUILTIN__` / `__ATTRIBUTE__` pragmas | ❌ | ✅ | ✅ language pragmas `<* … *>` | ⚠️ compiler directives | ✅ (parsed, ignored) |
| Inline `ASM` | ❌ | ✅ | ⚠️ optional `ASSEMBLER` pseudo-module | ❌ (not in the reconstructed grammar) | ❌ |

## Modules, compilation, libraries

| Feature | PIM4 | GNU Modula-2 | M2R10 | TopSpeed V3 | V3 |
|---|---|---|---|---|---|
| Definition / implementation / program modules | ✅ | ✅ | ✅ (+ blueprints) | ✅ | ✅ |
| Local modules | ✅ | ✅ | ❌ removed | ✅ | ✅ |
| Separate compilation | ✅ | ✅ (object files/archives) | ✅ compilation units | ✅ | 🔄 one invocation compiles all units into one image |
| Generics | ❌ | ✅ ISO 10514-2 | ❌ external templates (`GENLIB`/`BLUEPRINT`) | ❌ | ✅ ISO 10514-2 refinement |
| Object orientation | ❌ | ❌ | ❌ (extensible records only) | ✅ Clarion-form `CLASS`, `VIRTUAL`, inheritance | 🔄 Clarion-form `CLASS` (single inheritance, virtual dispatch), not ISO 10514-3 |
| PIM coroutines (`PROCESS`, `NEWPROCESS`, `TRANSFER`, `IOTRANSFER`) | ✅ | ✅ | 🔄 `COROUTINE` type/`YIELD` (R10 phase II) | ✅ PIM-style | ✅ (`IOTRANSFER` degrades to a transfer) |
| ISO `COROUTINES` module | ❌ | ✅ | ❌ | ❌ | ❌ (PIM interface only) |
| Standard library | ❌ (none defined) | ✅ PIM + ISO libraries | ✅ redesigned R10 library | ✅ TopSpeed libraries (historical) | 🔄 subset: ISO/PIM I/O, strings, storage, SysClock, EXCEPTIONS; no termination/coroutine libs |
| Self-hosting | n/a | ✅ (part of GCC) | n/a (reference compiler under development) | n/a (historical commercial compiler) | ✅ (byte-identical 3-stage fixpoint) |
| Backend / target | n/a | GCC (many targets) | reference compiler | x86/DOS/Windows (historical) | 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.
- **GNU Modula-2** 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).
- **M2R10** is a **redesign, not an extension**: no variant records, no
  local modules, no `WITH`, no `EXPORT`, no `GOTO`, no `FORWARD`; it
  adds `OCTET`, `UNICHAR`, `LONGBITSET`, `ALIAS OF`, `OPAQUE`,
  extensible records/enumerations, `FOR … IN`, `SUCC`/`PRED`,
  `CAST`, structured literals, pragmas and a redesigned library.  Its
  “generics” are an external template engine, not ISO generic modules.
- **TopSpeed V3** is a PIM-based **dialect with OO**: Clarion-form
  `CLASS`/`VIRTUAL` and inheritance are the main additions; it has its
  own lexical forms (`_`/`@`, `H`/`C`/`B` literals, `<<`/`>>`,
  `WITH`/`IN`/`IS`).  It has no language-level generics or ISO
  exceptions.
- **V3** is a PIM/ISO compiler with the opposite bias to gm2: it rejects
  gm2 leniencies (declaration order), implements the common ISO/PIM
  constructs (ISO 10514-2 generics, coroutines, foreign modules, ISO
  strings/SYSTEM/procedure types, empty string as `CHAR`), and documents
  its chosen gm2 compatibility (`__FILE__`/`__LINE__`/`__FUNCTION__`,
  scalar→`ARRAY OF BYTE`, same-size `VAR`, bare proc values, variant
  extensions, set `CASE` labels, libc prototypes).  ISO exceptions are
  **mostly complete** (`EXCEPT`/`RETRY`/`SYSTEM.THROW`, `EXCEPTIONS`/
  `M2EXCEPTION`, index/range/CASE traps) with `functionException` and
  `FINALLY` pending.  Its hard gaps are **complex arithmetic,
  finalization/protection, multi-dimensional open arrays, ISO OO** and
  parts of the **word model** (`SHORTREAL`, `REAL` width), plus a
  **65536-bit `SET` span limit** and a **32-bit `PACKEDSET` limit**.

## V3-specific deviations and limits

- **Strict data declaration order** (PIM-conforming); forward
  **procedure** calls are allowed where strict PIM would require
  `FORWARD`.
- **Word model:** `BYTE`/`SBYTE`/`WORD8`/`CARDINAL8`/`INTEGER8` are
  true 8-bit types (`SIZE` 1), `WORD16`/`CARDINAL16`/`INTEGER16` are
  true 16-bit types (`SIZE` 2); `INTEGER`/`CARDINAL`/`WORD`/`WORD32`
  are 32-bit; `LONGINT`/`LONGCARD` are 64-bit; every `REAL`/`SHORTREAL`
  is 8 bytes; `COMPLEX*` is a placeholder.
- **Sets:** bases limited to bool/char/byte/16-bit/enum/small subranges;
  `SET` total span ≤ 65536; no `SET OF CARDINAL`.  `PACKEDSET` uses
  1/2/4-byte storage with a 32-bit span limit (wider spans are a loud
  `230`).
- **Runtime range checks** are emitted for subrange assignment, value
  actuals, `RETURN` and `INC`/`DEC` (`QbeGen.CheckRange`, abort trap).
  The post-increment of a `FOR` control variable is not checked: the
  final successor may legitimately exceed a full subrange.
- **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-2026-10-23.md`; the
  gates in this document were run at `v3-packedset`.
- gm2: the GCC Modula-2 testsuite and `gm2-libs`/`gm2-libs-iso`
  definition modules in the sibling `gcc-git` tree.
- M2R10: the published R10 grammar (m2r10-grammar.gll) and
  *“Differences between Modula-2 R10 and classic Modula-2”*
  (Kowarsch/Sutcliffe, 2012).
- TopSpeed V3: `compiler/src/TopSpeed-V3-M2.atg` (reconstructed V3
  grammar) and `docs/OOP.txt` (Clarion class model).
- PIM4: N. Wirth, *Programming in Modula-2*, 4th edition.