This is how the checksum is calculated in Acorn's Superform program P%=buffer%+1000 [ OPT2 CLC LDY#&FF TYA .L1% ADC buffer%+255,Y DEY BNE L1% TAX DEY CLC TYA .L2% ADC buffer%-1,Y DEY BNE L2% STX buffer%+511 STA buffer%+255 RTS ] CALL buffer%+1000