MC64 — a 64-bit Modula-2 virtual machine

MC64 is a 64-bit generalization of the MCode virtual machine that Borland Turbo Modula-2 for CP/M executed on the Z80. The machine word — the unit of the stack, frame cells and global cells — is widened from 16 to 64 bits, the address space becomes flat and byte-addressable, and the 256-entry opcode map is preserved 1:1: only operand widths and the scalar type mapping differ. The normative reference is docs/mc64-spec.md, §14 of which contains the row-by-row MCode/16 → MC64 migration table.

The machine loads .MC4 image files (a 64-byte "MC64" header + a module descriptor + code + a procedure table) and boots them with the mcint launcher. It is the execution target of the experimental Modula-2 compilers in MyWork/ — V1 (M2c), V2 (M2comp) and V3 — whose code generators emit .MC4 images that this interpreter runs. As of this writing the loader is at the dependency-chain milestone: a guest module can name up to 32 dependencies and call across module boundaries through the 256-slot module table (MTBL).

Quick start

The repository ships prebuilt executables and ready-made sample images, so the first run takes one command:

./mkdemo            # writes boot.MC4 in the current directory
./mcint boot.MC4    # prints "Hello MC64!" and exits 0

The machine model

A stack machine with a single stack shared by values and activation frames, growing down (Push = SP -= 8).

Register	Meaning
IP	instruction pointer — byte address of the next opcode
SP	stack pointer — byte address of the top word
FP	frame pointer — base of the current activation frame
GP	global pointer — base of the current module's data window
OFP	outer frame pointer — static chain / display link
MTBL	module table — MTBL[i] = data base of loaded module i

Data types

Modula-2 type	Width	Stack slots	Notes
BYTE, CHAR, BOOLEAN	8 bits	1	held in the low bits of a slot
CARDINAL	32 bits	1	unsigned, zero-extended
INTEGER	32 bits	1	two's complement, sign-extended
SET	64 bits	1	elements 0..63
LONGINT / LONGCARD	64 bits	1	signed / unsigned 64-bit arithmetic
REAL	64 bits	1	IEEE 754 binary64 (double)
LONGREAL	128 bits	2	IEEE 754 binary128 (quad)
pointer / address	64 bits	1	byte address

Stack slots are always 64 bits; 32-bit values live in the low 32 bits (CARDINAL zero-extended, INTEGER sign-extended), so 32↔64-bit widening is a value no-op. Memory is little-endian everywhere.

Locals live at FP[-1], FP[-2], …; parameter k at FP[k+2]; arguments are pushed in reverse declaration order. ENTER k reserves (255−k)×8 bytes of locals (k=0FFH → none). EXTERN_CALL mod,proc switches GP to MTBL[mod] and jumps through the callee module's procedure table.

Images (.MC4)

A module file is a 64-byte header followed by the image: the module descriptor, its code, a procedure table and (optionally) a dependency-name table.

• File header — magic "MC64" at offset 0; the descriptor itself sits at image offset 0 (i.e. immediately after the 64-byte header) in the generator images.

• Descriptor (image offsets, little-endian):

  offset	field
  0	dependencies — 256 bytes of module-base pointers
  256	link — next module in chain
  264	name — 16 chars, NUL-padded
  280	loadAddr
  288	checksum — u32
  292	flags (bit 2 = TOINIT)
  293	varCount
  294	depCount
  295	pad
  296	procsAddr — location of the procedure table
  304	varSizes — varCount × u64 byte sizes

• Procedure table — N contiguous 64-bit relative byte offsets: procedureAddress(k) = procsAddr + k*8 + table[k].

• Checksum — plain 32-bit sum of the image bytes (file offsets 64 up to end of file), excluding the four checksum bytes themselves (file 352..355 = image 288..291).

• Dependencies — depCount names appended at the image tail (8 NUL-padded bytes each); the loader loads dependency modules into low MTBL slots first (extern module 0 = the first import), then the main module.

The loader places the image at ArenaBase (1 MiB) and allocates the module's globals (varSizes bytes per entry, zero-filled) in an 8-byte-aligned data window right after the image. Historically these files were called .MCD; the 64-bit format was renamed to .MC4 per spec sections 8 and 14.

The toolchain

Tool	Purpose	Sample
mcint	interpreter launcher — boots a single-module image or the main module of a dependency chain	./mcint boot.MC4
mc64	demo wrapper — boots boot.MC4 then prints [vm end]	./mc64
mkdemo	writes boot.MC4 ("Hello MC64!" via the SYSTEM write-string service)	./mkdemo
mkdtest	writes example.MC4, a 24-instruction opcode/composite exercise	./mkdtest && ./mcint example.MC4
mkread	writes readtest.MC4, which reads two stdin lines and echoes them back	printf 'a\nb\n' | ./mcint readtest.MC4
mkdep	writes STAK.MC4 + DEP.MC4, a two-module cross-module dependency-call demo	./mkdep && ./mcint DEP.MC4
trans8to64	translates a classic 16-bit Turbo Modula-2 .MCD binary into a 64-bit .MC4 image	./trans8to64 in.MCD out.MC4

mcint takes the image path as its first program argument; exit status is 0 when the guest reaches end_program (SYSTEM service 0), 1 when the loader or a guest trap aborts. With no argument it prints its usage line.

Building from source

Everything is ISO Modula-2, compiled with GNU Modula-2:

# in a build directory containing the sources (cp src/*.mod src/*.def .)
for m in Memory Local Global Stack Instruction Extended Interpreter Loader2 \
         Console FileIO; do
  gm2 -fiso -c "$m.mod"
done

gm2 -fiso -o mcint mcint.mod \
    Loader2.o Interpreter.o Extended.o Instruction.o Stack.o Global.o Local.o \
    Memory.o Console.o FileIO.o
gm2 -fiso -o mc64 MC64.mod \
    Stack.o Memory.o Console.o Global.o Loader2.o Interpreter.o Extended.o \
    Instruction.o Local.o FileIO.o
gm2 -fiso -o mkdemo mkdemo.mod FileIO.o Console.o
gm2 -fiso -o mkdtest mkdtest.mod FileIO.o Console.o
gm2 -fiso -o mkread mkread.mod FileIO.o Console.o
gm2 -fiso -o mkdep mkdep.mod FileIO.o Console.o
gm2 -fiso -o trans8to64 trans8to64.mod FileIO.o Console.o

One build rule matters (see spec §16.1): only the program module is passed to the link step; library modules are given as their precompiled .o. Passing several .mod files at once makes gm2 emit one main per module and the link fails with duplicate-main errors — and linking all *.o indiscriminately runs every module's init body at startup.

Project layout

src/       VM library modules (Memory, Local, Global, Stack, Instruction,
           Extended, Interpreter, Loader2, Console, FileIO) and the seven
           tool programs (mcint, MC64, mkdemo, mkdtest, mkread, mkdep,
           trans8to64) — .mod/.def sources
MC4/       generated sample images (boot.MC4, example.MC4, readtest.MC4,
           STAK.MC4, DEP.MC4)
docs/      mc64-spec.md (normative spec), session-summary.md, SESSION.md,
           m-code-summary-64.md, m-code-summary.md, porting-plan-m2sp.md
root       built executables, *.o, reference files (skills.*,
           gnu-m2-grammar.txt, M2c-git instructions.txt, SESSION.MD5.trans8to64)

Documentation

• docs/mc64-spec.md — the normative specification: memory model, registers, types, frames, addressing, module linkage, the on-disk format (§8), the host interface (§9.3), the extended and quad sub-dispatches (§10), the 256-opcode instruction-set reference (§11), the MCode/16 → MC64 migration table (§14) and the GNU Modula-2 build notes (§16).
• docs/m-code-summary-64.md — condensed summary of the 64-bit machine.
• docs/session-summary.md — day-by-day log of loader/interpreter milestones: build recipes, verified behaviors, hard-won tools-and-gotchas.
• docs/porting-plan-m2sp.md — plan for porting Niklaus Wirth's M2SP single-pass compiler to ISO gm2 with an MC4 code generator.
• docs/SESSION.md — working notes.

Verification status

All of the following are green with the freshly built binaries:

• ./mkdemo → boot.MC4; ./mcint boot.MC4 → Hello MC64! (0); ./mc64 → Hello MC64! + [vm end] (0); ./mcint → usage (0); ./mcint nope.MC4 → loader fatal (1).
• ./mkdtest → example.MC4; ./mcint example.MC4 — 24/24 checks: g after 10*2+1 = 2047, add/sub/mul/div/mod, bit_and/or/xor, power2, the extended 0x40 sub-dispatch (uc_add/uc_mul/uc_div/uc_mod, long_negate, build_field_mask), real_add 3.5+2.25 = 5, dup/swap, global/local dword loads, copy_block → abc (0).
• ./mkread → readtest.MC4; printf 'a\nb\n' | ./mcint readtest.MC4 echoes both lines through SYSTEM service 2 (0).
• ./mkdep → STAK.MC4 + DEP.MC4; ./mcint DEP.MC4 executes an extern call into STAK through MTBL[0] and prints cross-module ok (0).

Related projects

Siblings in MyWork/:

• m2compiler-V1 — M2c, the single-pass compiler (Coco/R M2c.atg + SymTab/MGen), step 11, 158/158 tests; the original .MC4 consumer.
• m2compiler-V2 — M2comp, the Blaise front/back (AST) compiler, step 7, 65/65 tests; its run_tests.sh defaults to MCINT=../m-code-64/mcint.
• m2compiler-V3 — the larger successor generation.
• m-code — the original 16-bit Turbo Modula-2 reference material and MCode sources (used only as Modula-2 sources, never binary analysis).
• CocoGm2 — the GNU-Modula-2 port of Coco/R (CR) used to build the compiler front ends.

Working conventions

• The interpreter is written against docs/mc64-spec.md only — no disassembly or analysis of classic .MCD binaries.
• Deterministic build order (§16.1), pure ISO dialect (gm2 -fiso).
• Every instruction in the demos is hand-encoded: mkd*.mod generators are the known-good .MC4 encoders used as ground truth for the compilers.
• SESSION.MD5.trans8to64 stores a baseline MD5 of trans8to64.mod: any step that rewrites that file without updating the lock note is a session bug.