Used by the routines at screen_north and screen_south.

Gives the object its new room number and starts the room-entry walk: 3 in the top nibble of +$0C, three frames of walking straight in with the controls and the room's walls ignored, so the player clears the archway.

For the player -- type 16 to 79, both forms and both halves -- the rest of this frame is abandoned. The two return addresses on the stack, to move_player_apply and player_move_and_draw, are dropped, and both player records are copied to plyr_spr_1_scratchpad and plyr_spr_2_scratchpad: the player as he entered this room, which lose_life copies back when a life is lost, so he reappears at the door he came in by. The copies' types are replaced by 120, the first frame of the materialising sparkle, with the real types kept in +$10 for upd_127 to restore. Then the new room is built from game_loop.

Only the player reaches this code in this build, so the return for other types is never taken.