Event Runtime

Event Runtime

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Single reference for how the Amiga executable loads, triggers, and interprets event.dat scripts. File layout is in 06-event-dat-format.md; opcode semantics in 07-event-script-opcodes.md.

Core routines

Address	Label	Role
0x92F2	event_dat_loader	Copy current location record into A4-$47C8 (max 2220 bytes)
0x1754A	event_system_init	Scan triplet table; compute string anchor + script start
0x175E2	event_tile_scanner	Match party tile + cond flags; dispatch matching event id
0x17262	event_handler_pool_seek	Skip N 0xFF-delimited records in script pool
0x172BC	(era gate)	Compare era_low vs cached attrib+0x0F before VM runs
0x172CA	event_script_interpreter	Opcode dispatch via table @ 0x17494
0x1750C	event_script_fetch_next	Read next byte; loop or cleanup on 0xFF
0x155BE	(read helper)	work_buf[parse_pos++] — script byte fetch
0x157FC	event_token_skip_helper	Skip N tokens using length table @ A4-$6CC8
0x15884	event_text_resolve_u8	Map u8 string index → string table pointer
0x171AC	event_script_cleanup	End script (OP_0F, 0xFF, abort)
0x1280	main scheduler	If tile runtime < $80 and pending latch → scanner thunk

Thunk JSR -$7D3A(A4) from the main loop reaches 0x175E2 (vector @ $25C6E).

Loader (0x92F2)

1. screen_mode_id = A4-$79F2 (byte).
2. Index header: entry = header[screen_mode_id] (6 bytes/entry, BE offset + BE length).
3. Clamp length to 0x8AC; memcpy into A4-$47C8 (event_work_buf).
4. Blob pointer from A4-$1108 (event_blob).

Locations 0–59 align with map.dat / attrib.dat screens. Locations 60–70 use the same index but hold global quest banks, castle blobs, and meta reference text (see inventory in EXTRACTED/event_location_inventory.json).

Init (0x1754A)

Walk work_buf from offset 0:

1. Read 3-byte triplets (pos, event, cond) until 00 00 00.
2. Store terminator index in A4-$7954 (event_script_anchor).
3. Read LE u16 at terminator → add to anchor → final string-table base in $7954.
4. Store terminator + 5 in A4-$5C44 (event_script_start).

If no 00 00 00 group exists (locations 63, 65, 68), init never completes normally — those records use the queued dispatch path instead.

Tile scanner (0x175E2)

Each pending step:

context = byte_table_6BE6[ facing_index_55D7 ]
party_tile = (coord_a << 4) | coord_b     // Y in high nibble, X in low
if event_script_anchor == $FFFF → call 0x1754A
parse_pos = event_script_start
for each triplet (pos, event_id, cond):
    if pos != party_tile → continue
    if (cond & context) == 0 → continue
    pool_seek(event_id) → optional era gate → interpreter
    clear triplet bytes (one-shot)
if queued_event_id != $FF:
    rebuild anchor from work_buf[0..1] as BE u16; seek queued id; re-init
if left trigger tile → clear visited bit7 + tile_rt bit7
else → pending_event_latch = 1

Condition flags (triplet byte 2, AND-masked with context_mask_tbl[facing_index]):

Value	Meaning
0x10	DIR_W? (west-facing context)
0x20	DIR_S? (south-facing context)
0x40	DIR_E? (east-facing context)
0x80	DIR_N? (north-facing context; also used for ENTER tiles)
0xC0	DIR_N? + DIR_E?
0xF0	ANY_DIR (all four facing bits set)

context_mask_tbl @ A4-$6BE6 (ghidra mm2_data_00.bin off 0x1418): [0x10, 0, 0x20, 0, 0x40, 0, 0x80, 0] indexed by facing index 0/2/4/6 (W/S/E/N). There is no “match all facings” cond byte — 0x10 door labels require west-facing only.

Interpreter loop

172CA: op = read_script_byte()
       dispatch jump_table[0x17494][op]  // op in 0..0x32
1750C: if op == 0xFF → cleanup
       if script_abort → exit
       goto 172CA
1748C: script_abort = 1

Invalid opcode (>= 0x33) branches to 0x1748C.

Era gating (two layers)

1. Screen gate @ 0x172BC: era_low (A4-$79B5) must equal cached attrib.dat byte 0x0F for the current screen (A4-$A9F5).
2. Script ops OP_22 / OP_23: range-check era or day-of-year into cond_flag (A4-$7951).

A4 memory map (event-related)

Anchor A4 = $7FFE. Canonical signed offsets (see mm2_gamestate.h).

Session / interpreter

Offset	Field	Notes
-$79F2	screen_mode_id	Indexes event.dat header
-$79F1	coord_b	Party X (column)
-$79F0	coord_a	Party Y (row)
-$7956	event_parse_pos	Script PC in work buf
-$7954	event_script_anchor	String base; $FFFF = need init
-$7952	pending_event_latch	Set on movement; cleared @ 0x1280
-$7951	cond_flag	Predicate result; gates OP_10/11
-$7950	exit_flags	ESC / modal bits
-$79EA	script_abort	Set by OP_29 / invalid op
-$79B5	era_low	Current era index
-$79B1	last_move_key	'N'/'S'/'E'/'W'

Queued / secondary dispatch

Offset	Field	Notes
-$5C44	event_script_start	Restored to parse_pos each scan
-$5D46	queued_event_id	$FF = none; path @ 0x176B6
-$5D42	saved_cond_flag	OP_26/27 party select
-$5D44	string_walk_index	String resolver @ 0x15884

Tile / map runtime

Offset	Field	Notes
-$55D7	facing_index	0/2/4/6 → context table
-$55D6	tile_runtime_flags	Bit7 active event; <$80 allows scanner
-$55C8	event_busy_sentinel	$FF during script
-$55BA	tile_table_a	OP_21 writes
-$54BA	tile_visited_flags	OP_14 clears bit7
-$6BE6	context_mask_tbl	Facing → cond context byte

Buffers / pointers

Offset	Field	Size
-$47C8	event_work_buf	2220
-$6CC8	opcode_len_tbl	256×16-bit deltas
-$1108	event_blob	ptr → event.dat
-$110C	map_blob	ptr → map.dat
-$A9F5	attrib_era_gate_cache	byte

Token skip table (A4-$6CC8)

Helper @ 0x157FC for OP_10, OP_11, OP_2B:

1. Read count byte N.
2. Repeat N times: read token byte B at parse_pos; parse_pos += word_table[B].

Table is indexed as word[B×2] (68000 move.w (a0,d1.l), d0 after asl.l #1). Dump / rebuild: python tools/dump_event_token_table.py.

Tools

Tool	Purpose
tools/decode_event.py	Decode, decompile, predicates, consumed items
tools/event_location_inventory.py	JSON inventory of 71 locations
tools/validate_event_map.py	Triplet vs map collision 0x80 check
tools/mm2_event_sim/	Interactive TUI simulator
EXTRACTED/decomp/mm2_event_codec.{h,c}	C load/decode/encode

See also

• 14-game-state-struct.md — full A4 workspace
• 21-map-dat-format.md — collision event bit
• 12-attrib-dat-format.md — era gate byte 0x0F