Entered by the JR in SPRITE_ROW, as many 8-byte units from the end as the sprite is wide. Each unit takes the next mask and image off the stack and does one buffer byte: cleared under the mask (CPL, OR E, CPL), the image ORed in, stored. The last unit has no INC BC, so a row leaves BC on its last byte, which the step patched in at SPRITE_NEXT_ROW allows for.

This first unit is for a fifth byte, and no sprite in the game is wider than four (graphics 2, 76 to 79 and 152 are four; read from the graphic table): it can never run, which is why the code map took it for data. Knight Lore, Alien 8 and Pentagram unroll the same five units.