Used by the routines at CLEAR_THING_BOX and REDRAW_OBJECT.

Krumlinde's RTN_Composite_To_Screen. The pixels the game draws on the screen once a room is drawn go through here (the colours are ATTRI's): a moving object's new picture (from REDRAW_OBJECT), the box on the panel for the thing in use (CLEAR_THING_BOX, SHOW_THING_IN_USE), and every character printed (PRINT, by way of the second entry, three instructions in).

The rectangle's top left corner is x, y at IY+$64 and IY+$65 (y counted up from the bottom of the screen, as everywhere in the game), its width in bytes at IY+$73 and its height in rows at IY+$67. A row is written as its width plus one screen bytes, since the image is shifted to x's pixel within its byte, and each byte is made from four buffer pages ($D8 to $DB: the 1 KB after the clean copy, from offset 364 of SOURCE_AFTER_TEMPLATES), indexed by one count that goes on through the rows: where page $D8 has a bit set, the screen keeps what it has; elsewhere, where page $D9 has it clear, the pixel comes from the image (IX); where $D9 has it set, it comes from page $DB if page $DA has it set too (when bit 1 of IY+$71 says pages $DA and $DB are in use), and otherwise from the clean copy of the room, the byte 32 KB above the screen byte (the copy RESTOR and room code $E2 make).

The callers fill the pages. For a moving object, REDRAW_OBJECT and the routines it calls (CULL_OBJECT, SORT_AND_DRAW_BEHIND, DRAW_SPRITE) fill them from the objects that overlap the rectangle: the object's own mask, what stands in front of it, and what moves behind it (see there). The printer clears pages $D8 and $D9, so a character is drawn as it is; CLEAR_THING_BOX sets every bit of page $D9, which puts the clean copy back over the box. The pixels to the left of x in each row's first byte are marked in page $D8 by this routine itself, so that they stay as they are.

The shift is made by jumping into a run of seven RRCAs: the start rewrites the operand of the JR before them. It also rewrites the JRs round the steps for pages $DA and $DB (to skip them unless they are in use) and the one before a row's last byte (to skip it when x is on a byte boundary, as nothing spills into it). Rows above the top of the screen (y above 191) are skipped, and it stops at the bottom of the screen.

At this entry the image's address is the word at IY+$68, and the routine starts three rows before it, as REDRAW_OBJECT's rectangle does; at the second, COMPOSITE_FROM_IX, three instructions in, HL is the address to start from.