Between the two sets of tables, and neither read nor written in stage 1's measured run: the snapshot's zeros. It is where the tables' addressing -- page $F0 or $F8 plus bits 1 and 2 of x -- would put a shift of 0, which is never looked up: a sprite or tile on a byte boundary takes a path of its own (DRAW_SPRITE, PUT_TILE). The mirrored set's page for 0 would be $F0, inside the attribute buffer.