Routines
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9117: Move the player through a door and redraw everything
Used by the routines at DOOR, DOOR_LOCKED_A and TRAPDOOR_FALL.
Takes the door record in IX, copies its destination into the player's room and position, and then rebuilds the screen: mark the room seen, blank the play area, draw the new room, recolour the panel.
The arrival position is not stored outright. +$02 packs both offsets into one byte, unpacked by rotating it in opposite directions and masking to $1E -- an even number 0 to 30 each way, the vertical one negated. Every door checked holds $34, which comes out as 8 to the right and 6 up, so the player lands just inside the room rather than on top of the doorway they arrived through.
Confirmed by setting IX to four different door records and running from ENTER_ROOM+3, past its first call: doors to rooms $07, $19, $01 and $00 produced exactly the destination and the offset position predicted from their bytes.
Input
IX The door being entered
ENTER_ROOM 9117 CALL DOOR_OTHER_SIDE Swap to the door's other side. The record the player touched describes this room; the one eight bytes away describes where they come out. That reassignment of IX is also why anything testing this routine has to enter below it.
911A LD A,(IX+$01) +$01 is the destination room.
911D LD ($EA91),A
9120 LD A,(IX+$02) Rotate left and mask: the horizontal offset, added to the door's own x.
9123 RLCA
9124 AND $1E
9126 ADD A,(IX+$03)
9129 LD ($EA93),A
912C LD A,(IX+$02) Rotate right three times for the vertical one, which is subtracted rather than added.
912F RRCA
9130 RRCA
9131 RRCA
9132 AND $1E
9134 NEG
9136 ADD A,(IX+$04)
9139 LD ($EA94),A
913C CALL SET_ARRIVAL_HEADING Head the player into the room, away from the wall this door is in; with the nibble set below, STEER keeps them walking that way for fifteen frames.
913F LD A,($EA92) Set the flag that stops PLAYER_AT_DOOR firing again on the way out.
9142 OR $0F
9144 LD ($EA92),A
This entry point is used by the routine at START_GAME.
ARRIVE_IN_ROOM 9147 LD A,($EA91) Record the new room as seen.
914A CALL MARK_ROOM_VISITED
914D CALL CLEAR_PLAY_AREA Blank the play area, draw the new room, colour the panel to match.
9150 CALL DRAW_ROOM
9153 CALL PAINT_PANEL
9156 CALL DRAW_INVENTORY
9159 CALL PLAY_SOUND_NEW_ROOM
915C JP MAIN_LOOP
Each door is one sixteen-byte record whose two eight-byte halves are in the two rooms it joins (+$00 type, +$01 the room this half is in, +$02 the packed arrival offset, +$03 and +$04 the doorway's corner, +$05 orientation and flags, +$06 and +$07 its walk box). The doors tested are the ones in the player's room's list, dispatched by DISPATCH_FROM_LIST.
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