Used by the routine at NEW_GAME.

Run at every new game (NEW_GAME). Fills the six pages from MIRROR_TABLES, the six from SHIFT_TABLES, and REVERSE_TABLE with the tables the drawing of sprites and tiles reads to put a byte at any even pixel.

The six pages from SHIFT_TABLES up are every byte value shifted right two, four and six places, in pairs: page $FA holds a byte shifted right two, the part that stays in its own byte, and page $FB the two bits that fall out into the next byte; $FC and $FD the same for four places, $FE and $FF for six. The loop builds them the other way about: it shifts each value left across two bytes, two places at a time, and stores the pair after each shift from the top page down, so the high byte of a shift left by eight less s is the shift right by s. The six from MIRROR_TABLES are the same for every value with its bits reversed first, for tiles drawn mirrored; and REVERSE_TABLE is every value reversed, for mirroring sprites (TURN_SPRITE) and tiles on a byte boundary.

Only even shifts are built because nothing is drawn at an odd pixel: the drawing ignores bit 0 of x. Knight Lore, Alien 8 and Pentagram build seven shifts, complemented; these are three, and plain. Checked in the simulator: after this routine every byte of the thirteen pages is as described, and the page between them, UNUSED_F800, is not touched.