Entered by the JR in SPRITE_ROW, as many 18-byte units from the end as the sprite is wide. H is the first of the pair of pages MAKE_TABLES built for the shift: page H gives a byte shifted right, the part that stays in its own buffer byte, and page H + 1 the bits that fall out into the next. Each unit finishes the buffer byte in A -- cleared under the shifted mask (CPL, OR, CPL) and the shifted image ORed in -- stores it, and begins the next byte with the parts of the mask and image that fall into it, left in A for the next unit. The byte the last unit begins is stored after it.

Pentagram's tables are complemented and its unit ANDs and XORs; Nightshade's are plain and the unit complements around the OR, as the byte-aligned run does. This first unit is for a fifth byte and never runs (see SPRITE_ALIGNED_RUN); the code map took it for data.