You could change the stack pointer to the top of the area of memory you wanted to fill and then use PUSH to fill at I think 11 clock cycles per two bytes. It was faster than unrolled LDI or LD (HL),A followed by INC HL. It would be filling memory in the wrong direction for a Rainbow processor but you could use it for repeating patterns. I think I did a checkerboard pattern that would shift every frame and it was pretty smooth.