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Room-map object interactions

src/game/rooms/mechanics.asm owns $9C60-$9DB0. It connects runtime RoomMap cells, object-record initialization, and Dana's block magic. The adjacent coordinate/object overlap predicate at $9DB1-$9DCF is separated because it has a stable general-purpose calling contract.

Applying a map tile to an object

ApplyMapTileInteractionToObject accepts:

  • A: the current map tile;
  • Y and MapInteractionY on entry: the packed map-cell index;
  • MapInteractionObjectPointer: destination object record;
  • MapInteractionDirection: direction bits used for negative tile classes.

Tiles below $F8 have their upper two bits cleared in RoomMap. The packed cell is converted to pixel coordinates, optionally offset according to the tile sign and direction, and stored in object bytes 7 and 10. The common header initializer writes state $C6, variant $04, sentinel $FF in byte 2, and a tile-dependent action byte. Nonnegative source tiles are then queued for redraw through BuildAndPublishRoomMapCellUpdate.

The entry has six external callers. HandleDanaHeadCollision and RemoveBlockAtTarget supply the magic-spark record. The four enemy paths are UpdateNuelGhostMapInteraction, AllocateSingleLinkedEnemy, UpdateType0CTo0FMapInteraction, and ApplyForwardEnemyMapInteraction. Every call uses the semantic symbol and the shared zero-page aliases.

Creating a block

TryCreateBlockAtMapCell receives the target cell in MapInteractionY and a destination effect object through MapInteractionObjectPointer. For ordinary empty/blockable tile classes it checks whether the target coordinate overlaps the active fireball and then each of the seventeen enemy records.

The enemy scan advances by $14 bytes. Its compact pointer update adds $13 with the carry returned by the no-overlap predicate, producing the full object stride without a separate increment.

Depending on occupancy, the routine either:

  • marks an overlapping enemy record;
  • positions and initializes the effect record from the fireball or enemy;
  • writes a new negative block value to RoomMap;
  • advances the low two-bit variant of an existing $08-$0F tile.

Shared object header initializer

InitializeObjectStateHeader at $9D99 initializes object bytes 0 through 3 through MapInteractionObjectPointer:

ObjectStateOffset      = MapInteractionY
ObjectTypeOffset       = MapInteractionX
ObjectCachedTypeOffset = $FF
ObjectActionOffset     = A, unless A is negative

Eleven static calls use this symbol: eight external call sites and three paths inside this module. The scratch names reflect this calling convention; the gameplay meaning of every possible byte value remains open until the individual object behaviors are reconstructed.

Coordinate/object overlap

CheckCoordinateOverlapWithObject starts with carry set. A nonnegative object state returns immediately as no overlap. Active records are tested against MapInteractionX/MapInteractionY using the integer X/Y fields at offsets 10 and 7. Carry clear denotes overlap inside the routine's asymmetric $15 by $1D comparison window; carry set denotes no overlap.