Used by the routine at DRAW_CELLS.

The second half: the things LIST_THINGS_IN_CELL listed are drawn one at a time, each chosen by a pass along the list. The first entry not yet drawn is the candidate (IX). Every later entry not yet drawn (IY) is compared with it, and the outcome goes through DEPTH_TABLE: either the candidate stays, or IY is further back and becomes the candidate, and the pass carries on from IY. At the end of the list the candidate is drawn (DRAW_SPRITE) and its entry marked drawn by clearing bit 7 of its high byte -- every record's address has it set -- and the next pass starts; DRAW_LIST_AT keeps the place just past the candidate's entry, DRAW_LIST_NEXT just past IY's. The list is empty of undrawn things when a pass finds none, and the routine returns.

The comparison is two-dimensional: a record is a box in U and V (its position at +1 and +3, its half-sizes at +8 and +9) and nothing in the town has a height. On each axis each side of the box is worked out as (the low byte of the position plus or minus the half-size) / 2 + 64 -- only the low bytes, since the things listed share a cell, and RRA shifting the carry of the 9-bit sum back in, so a side from 128 below the cell to 128 past it becomes a byte from 0 to 255. Then 0 if the candidate is wholly beyond IY on that axis (its low side at or above IY's high side), 2 if IY is wholly beyond the candidate, 1 if they overlap: U counts once, V three times, and the town turned round adds 9, which reverses what "further back" means. Further back is smaller U and larger V, as in Knight Lore and Alien 8: higher up the screen (PROJECT_CELL).

A new candidate is not compared with the entries the pass has already gone by, so an order among three things can come out wrong; but a pass never goes back to the top, so it cannot go round in a circle, and Filmation's chain of candidates is not needed.