Used by the routine at ENTER_ROOM.

Clears every object record but the player's two, finds the player's room in ROOMS, and sets the room's colour (ROOM_ATTR) and its half-sizes and floor (ROOM_EXTENT, from ROOM_SIZES). Then fills the records from the top, ROOM_FIRST, downwards: first the scenery, then the objects. Nothing checks that the 48 records are enough.

Scenery: each entry is a template number and one more byte, up to an $FF. A scenery template (SCENERY_TABLE) is a list of 8-byte pieces -- graphic, U, V, Z, the three half-sizes, flags -- ended by a zero, each piece its own record. Every piece of the entry takes the entry's second byte as its +$08: for a doorway that is the room it leads to (FIRST_PILLAR).

Objects: groups, each a header byte (bits 0-2 how many less one, bits 3-7 the template) and a position byte for each. An object template (OBJECT_TABLE) is a list of 5-byte pieces -- graphic, the three half-sizes, flags -- ended by a zero, each piece its own record, all at the same place. A position byte gives U = 72 + 16 times bits 0-2, V = 72 + 16 times bits 3-5, and Z = the floor + 12 times bits 6-7; the object's +$08 is this room.

Template 31 in a header is not a template: it moves TEMPLATES_AT on 64 bytes, to a second page of templates, and the byte after it is skipped. No room uses it (it never ran). PLACE_NUDGE would add 8 to U (bit 0), 8 to V (bit 1) and the rest to Z; it is cleared here and nothing sets it but the unreached code at SET_PLACE_NUDGE.

The count at +1 of the room's record, of the bytes from there to the end, is what stops the build, not the end of the data: the scenery can use it up, or a group be cut short.