Used by the routine at FILL_NEXT_PIXEL.

The textured flood fill of room codes $E6-$FF (DO_ROOM_COMMAND), with the machine stack as its list of places still to fill. A place is x, y in C, B (y up from the bottom), and the list ends at a word whose high byte is $FE, pushed before the first. The fill's map is the 6 KB 32 KB above the screen, laid out like it: the pixels set there bound the fill (room code $E4 $00 clears it and sends the lines there, and $E2 copies the screen there), and the fill sets there every pixel it fills. For each place taken off the list: if it is off the top or the bottom of the screen, or already set in the map, it is dropped; otherwise the fill goes right to the last clear pixel of its run, and from there fills leftwards a pixel at a time (FILL_NEXT_PIXEL), writing the texture's pixels to the screen, setting them in the map, and pushing a place in the row above or below each time a clear run begins there.

This part sets up a run: the map addresses of its pixel and of the pixels above and below it (IY+$7B, IY+$7D, IY+$76), and the texture's two bytes for the row (B' the one for this byte, C' the other). A texture is 16 pixels by 16 rows, in four 8 by 8 cells: bytes 0-7 and 8-15 the left and right cells for the character rows in which y has bit 3 set, bytes 16-23 and 24-31 for the others; each cell's bytes run from the top of its character row. So the pattern lines up with the screen's character cells, and with the colours, whatever the area's shape.