writeonce/runtime/src/CODE-LOGIC.md

runtime/src — how the VM is put together

Written 2026-08-14, when the runtime grew the systems stdlib and json. Read this before changing a file here; the normative contracts are docs/plan/oop-vm/00-wob-format.md (the .wob format, opcodes, builtin ids) and 08-builtin-surface.md (what each builtin means in source terms). wob.h is the machine-readable twin of the first: constants there and prose there must never disagree.

The files, in dependency order

file	what it owns
wob.h	every format constant: header offsets, field kinds, opcodes, builtin ids, trap codes, the 16-byte object header, the class descriptor
obj.h/.c	the per-shard arena, object allocation, wo_str (header + length + inline bytes, no NUL)
cont.h/.c	multi and map as native classes: struct heads in the arena, backing arrays malloc'd, map lookup a linear scan over parallel key/value arrays
gc.h/.c	the kind-directed dispatcher (wo_drop_kind/wo_drop_obj), refcounting for @gc, and the budgeted Bacon–Rajan cycle collector
borrow.h/.c	the borrow word: shared counts and the exclusive sentinel
loader.h/.c	parse and validate an image; the validation contract in its header comment is exactly what the interpreter may then assume
vm.h/.c	the register interpreter: window-overlap calls, dual-flavor dispatch, traps, unwinding, catch frames
builtin.h/.c	the pure builtins: print, containers, text
sysio.c	the OS half: fs, time, env, net, proc
json.c	json.encode / json.decode, driven by class metadata
main.c	the CLI: find an image (argument or embedded trailer), build argv, call the entry, map its result to an exit code

builtin.c's wo_builtin is the single entry point the interpreter calls; it forwards ids at or above WO_B_SYS_FIRST to sysio.c and the json pair to json.c. Splitting by translation unit keeps the kernel-touching code and the format-walking code out of the hot builtin switch.

Two invariants worth stating plainly

The loader is the only validator. Everything the interpreter skips checking — opcode ranges, register operands, jump targets, builtin arities, window sizes, table ordering — is checked once at load. The exceptions are deliberate and documented: GETF/SETF field indexes and receiver shapes stay runtime checks, because registers are untyped (the spec's residual-check doctrine). If you add an opcode or a builtin, its validation goes in loader.c's switch and its arity in b_arity, or the interpreter is running unvalidated bytes.

Traps never leak. A trap unwinds frames innermost-outward, and in each one the drop-table entry governing that frame's current instruction says which registers hold owned or counted values. The governing instruction is the trapping pc for the innermost frame and the CALL (saved pc − 1) for every outer one. Registers are nulled as they are released, so window overlap cannot double-free.

try/catch (the catch stack)

TRY A sBx pushes {depth, handler pc, error register}; ENDTRY pops it. On a trap with a catch frame live, vm_trap:

1. fills vm->caught (the same structured error the uncaught surface prints),
2. unwinds every frame above the catching one, exactly as an uncaught trap would,
3. releases what the try region owned in the catching frame — the difference between the drop entry at the trapping instruction and the entry at the handler pc, which is why the compiler must record an entry at the handler,
4. points that frame at the handler and returns 0, so TRAPF reloads and keeps interpreting.

A frame that returns pops the catch frames it registered (DROP_CATCHES), so a return out of a try region cannot leave a handler aimed at a dead window. With ncatch == 0 every trap behaves byte-for-byte as it did before the feature existed — that is the property to preserve when touching this code.

Records the VM fills but does not know

Three builtins return a record: fs.stat, time.local, proc.run, plus err_fill for a catch arm. The VM cannot name a source type, so the compiler passes the record's class id as the call's last argument and the builtin fills fields by index. The field order is therefore a contract, written beside each case in sysio.c and mirrored in compiler/src/types.ml's predeclared records. Change one side and the other silently writes to the wrong slot.

Class metadata and json (.wob v2)

The class table carries, per field, its name constant, the class it refers to (or a json-raw marker) and a container field's element kinds. That is what lets json.c be one implementation for every shape instead of per-type generated code:

• encode takes the top-level value's static kind from the compiler, because a register alone cannot say whether it holds an i64 or a pointer. Everything nested comes from object headers (which carry class_id) and the class table.
• decode parses and binds straight into the target class: keys matched against field names, a nested object built as that field's class, an array as a multi of that field's element kind, unknown keys skipped, absent keys left as the zero word (nil). Malformed input yields nil rather than trapping — that is what makes json.decode(t) as T a checked decode.

Two limits are inherent to the kind byte and are documented, not bugs to discover: a Bool field encodes as 0/1, and a fractional JSON number decodes by truncation.

Program mode

main.c accepts an entry taking no arguments or exactly one multi Text. The list holds the program's own arguments — not the program name, and not the image path a wovm image.wob args... invocation carries — so args[0] is the first real argument. The entry's return value is the process exit code (low byte); a trap is exit 1 with the fixed trap N in METHOD at line L: MESSAGE line on stderr, which the conformance harness parses.

Where to look when something breaks

• A wild pointer inside a builtin usually means the compiler put the wrong thing in a register: check the method's disassembly (woc --dump-bc) before suspecting the C.
• make -C runtime wovm-asan builds the sanitized binary; the unit suites (just wovm-test) run every test/test_*.c under ASan+UBSan in both dispatch flavors, so a fallback-only bug cannot hide.
• runtime/test/wob_build.c is an independent image assembler. A builder/loader disagreement shows up as a unit-test failure, which is the point of having two encoders.