SCUMM v5 — Per-Opcode Encoding Reference

Authoritative per-opcode encoding + semantics table for SCUMM v5, transcribed from the ScummVM wiki. This is the companion to opcodes.md (which covers the dispatch infrastructure — byte encoding, param-mode bits, var-ref scope, the expression mini-VM, branch semantics). When implementing or decoding an opcode, check the encoding here rather than guessing.

Sources

• ScummVM Technical Reference — OpCodes, at https://wiki.scummvm.org/index.php?title=SCUMM/V5_opcodes
• Cross-check decoded bytecode against real MI1 when a layout is ambiguous.

Encoding notation

Opcodes are non-orthogonal — many exceptions to the param-bit rule.

Opcode index (hex → mnemonic)

High-bit variants of an opcode are param-mode variants of the same base instruction — dispatch is per full byte.

Encodings (selected — the ones we decode/implement)

Every encoding begins with the opcode byte itself; the Operands column lists what follows it in the byte stream.

Flow / variables

Scripts

Objects

drawObject evicts same-box objects. When a draw displaces another object covering the same box (the eviction described in objects.md), the displaced object's state also reverts to 0 — MI1 room 31's rat-hole loop (#207) re-picks among state-0 frames and would spin forever otherwise.

getVerbEntryPoint falls back to the default verb. It returns the object's script entry for verb, or its 0xFF default-verb entry when the exact verb has none (SCUMM matches entry || 0xFF). This fallback is the opener for edge exits: a plain walk-to (verb 11) on an exit whose only verb is 0xFF reads truthy here → sentence #2 runs it → loadRoom (the room-78 "can't exit").

getDist clamps and saturates. The metric is Chebyshev — max(|dx|,|dy|) — and an unresolvable id yields 0xFF. When an operand is an object, its point is first clamped into actor-standable walkboxes (adjustXYToBeInBox) before measuring; MI1 room 36's guard dogs depend on that clamp.

Actors

Actor id 0 is shorthand for Ego in every actor opcode, resolved through VAR_EGO (global #1) — MI1's boot positions Guybrush with putActor 0 ….

Room placement and the camera. putActorInRoom does not load the room — a subsequent actorFollowCamera on an actor placed in another room is what triggers the room switch (how MI1's boot enters room 38).

actorOps $13 sub-opcodes (actor[p8], then subops until $FF):

$08 default/init resets ignore-boxes, scale, walkbox (to unassigned), forceClip (to 0), and the chore frames, but does not reset facing — only game-start init does. Room 60's teaching machine and room 51's cannon rely on the facing surviving the reset.

Camera / room

loadRoomWithEgo sets VAR_WALKTO_OBJ to the entry object, and the value must survive the room change — room 58's maze ENCD branches on it after a breakHere. Afterwards the camera re-snaps to Ego and follow re-engages.

roomOps $33 sub-opcodes (the wiki lists further ones):

Intensity values above 255 brighten; intensity is always computed against the room's load-time base palette (room 29's reveal, room 63's map fade).

Verbs / cursor / text

verbOps $7A — verbID[p8] sub-opcode... $FF:

A verb's charset is fixed at new-time; a later $02 setName must not re-capture the then-current charset (MI1's sentence-line verb #100 depends on this).

cursorCommand $2C — sub-opcode:

Cursor and userput are nesting counters, not booleans: the hard on/off forms set them to 1/0, the soft forms increment/decrement. After every subop the values are mirrored into VAR_CURSORSTATE and VAR_USERPUT.

print $14 — actor[p8] sub-opcode (and printEgo $D8 — the same, actor = Ego implicitly):

String substitution codes — printed strings embed $FF-introduced control codes:

The $04–$07 assignments are bytecode-verified against MI1's sentence line #100 (INPUT §6); some transcriptions instead give $06 = var and $08 = object/verb name, which that bytecode disproves. The wiki lists further codes ($09 sound, $0A actor name, $0E colour) not yet pinned against bytecode here.

Sound / system