Used by the routines at 27008, 27026, 28807, 29394 and 29735. Each byte of the SRB corresponds to a segment of 8 character squares on the screen, the leftmost of which will be at x=0, x=8, x=16 or x=24. Each bit set in an SRB byte corresponds to a character square in the segment that needs refreshing. Bit 0 corresponds to the leftmost character square in the segment, bit 7 to the rightmost character square.