Used by the routine at DRAW_EDGE.

Draws an edge picture -- a height byte, then two bytes a row, the bottom row first -- at a pixel x and a line y, ORed into the screen buffer (BUFFER). The buffer's lines run up the screen, 24 bytes apiece: line $48 is the bottom of the play area and $B7 its top, and BUFFER_ADDRESS turns x and y into an address. Rows above the top are cut; rows below the bottom are skipped by CLIP_EDGE.

SP reads the picture, a row a POP, the real stack pointer kept in SAVED_SP; the game never enables interrupts. Four cases: x a multiple of 8 or not (bits 1 and 2 of x; bit 0 is ignored, so things move in steps of two pixels), and the first face or the second (DRAW_PASS), which is drawn mirrored. A shifted row covers three buffer bytes, through a pair of pages built by MAKE_TABLES: the part of each picture byte that stays in its buffer byte and the part that falls into the next. The pages at SHIFT_TABLES shift; the ones at MIRROR_TABLES reverse a byte and shift it, for the mirrored face, which also takes the row's two bytes the other way round. An aligned mirrored row reverses each byte through REVERSE_TABLE.