Used by the routine at DO_ROOM_COMMAND.

Where the command interpreter (DO_ROOM_COMMAND) sends every code below $E6 that it has not dealt with itself -- it takes the points ($00-$BF), $C0, $CF, $D0 and $D6, and in object mode the objects. A cascade of comparisons; a code that none of them matches ($D1, $D3, $D4, $D7-$DF and $E3) does nothing and has no operand. The lengths of every code are laid out command by command in the rooms (ROOM1) and parts (PART1).

$D2 draws a line from the second point to the first, both ends included (DRAW_LINE; DO_ROOM_COMMAND also calls it after each point while mode bit 5 is on). It is Bresenham's: a step along the longer distance every pixel, and one along the shorter whenever the running error passes the longer. The first pixel goes through PLOT_LINE_START; the rest are plotted here with the same pen, stepping the screen address a line or a pixel at a time rather than working it out afresh.

$C1-$CE set and clear the mode bits (MODE), which the interpreter reads before every command. Bit 0 ($CB on, $CC off): lines clear pixels. Bit 1 ($CD on, $CE off): lines flip pixels. Bit 2 ($C3, $C4): after each point the second point is moved to it, so that with bit 5 the points make a joined line. Bit 3 ($C5, $C6): the second point moves along with the first. Bit 4 ($C1, $C2): points are relative -- their bytes are added to the point before, and those of $C0 and $E4 $05 to the second point and the origin. Bit 5 ($C7, $C8): a line from the second point to each new point. Bit 6 ($C9, $CA): the column bytes are negated or complemented, which mirrors the drawing left to right. Bit 7 is object mode, which $E4 $04 turns on. How the points use bits 2 to 6 is DO_ROOM_COMMAND's.

$D5 n repeats what follows, up to the $D6 that DO_ROOM_COMMAND handles, n times: the count goes in REPEATS and the place to go back to in REPEAT_FROM, so there is one repeat at a time. $E0 n and $E1 n draw part n (PART1) in the middle of the room's commands and come back to them; $E1 keeps the drawing's state as it was -- both points, the origin, the mode and the repeat, which it saves and restores -- and $E0 lets the part change it. Either way object mode ends with the part. $E2 copies the screen into the clean copy: from then on, what has been drawn so far bounds the fills.

$E4 has a second byte. $E4 $00 clears the clean copy and sends the lines into it, where they bound a fill without being seen; $E4 $01 sends them back to the screen. Room 2 uses the pair to lay a second texture over part of a wall that is already textured, inside an outline drawn only in the clean copy (measured: without them the fill finds its first point already set and does nothing); part 16 does the same. $E4 $04 turns object mode on, and $E4 $05 x y z sets the origin (ORIGIN, ORIGIN_Y, ORIGIN_Z) that is added to the place of each object a room's commands put down -- rooms 13 and 17 draw one part's objects several times over, moving the origin each time. Both first place the object table's things in the room (PLACE_ROOM_OBJECTS), once in each drawing of a room, whichever comes first -- so the table's things are always placed with the origin at 0, and their places are the room's own. Any other second byte does nothing.

Of these codes only $CE occurs in no room or part. Room 26 is the only one to turn flipping on, for its last line, which crosses a wall that is already textured; its drawing ends soon after, so nothing needs to turn it off (measured: with the line drawn in set pixels instead, 36 pixels of the room come out different).