Used by the routine at TEST_STEP_BOXES.

Called twice a frame from MOVE_PLAYER, through TEST_STEP_BOXES, with the proposed position in E and D and the bit for one axis in A' ($10 for x, $20 for y -- the bits of the player's +$02 that stop APPLY_HEADING moving that way). It walks the player's room's list of door and furniture halves, the same list MAIN_LOOP dispatches, and tests the point against each one's box.

The box is in +$06 (x) and +$07 (y) of the half: the high nibble, signed, times 4 is an offset from the record's own x or y, the low nibble times 4 a size; it runs right from x plus the offset and up from y plus the offset. Inside a box with +$05 bit 3 set -- a shut door -- nothing happens. Inside one with bit 2 set -- a table, the only solid furniture -- the bit is set: the step is refused. Inside any other -- a doorway -- the bit is cleared: the step is allowed although it leaves the room's walk rectangle. So an open door is a channel through the wall, a shut one is wall, and a table is an obstacle.

The lists hold template addresses (INITIAL_STATE onwards) plus ROOM_CONTENTS; taking ROOM_CONTENTS off gives the runtime address, as DISPATCH_FROM_LIST does. An earlier title, "Set up the records that belong to a room", described the walk but not the purpose: nothing is set up.

The check part-way down is the door pairing: if the half named in the list belongs to the other room, eight is added to reach the half in this one.