Partial CLS for the ZX81

Did you ever want to clear just the bottom portion of the screen during a BASIC program? Perhaps you have a title across the top line or two and would like for it to remain during the entire program. This little six byte machine code routine will allow just that. That’s right SIX BYTES! Actually the routine call a much longer routine in the ROM and passes on a single byte parameter which is first loaded into the B register.