loop: lodsb Get number in odd position sub al, dl Subtract ASCII zero add bl, al Add to total dec cx One fewer character jz. done Empty string = stop dec si We don't need the zero itself std Seek backwards. push ds 'REP DS:SCASB' is a bad idea, because the 286 has a pop es bug where it 'forgets' the 2nd prefix if interrupted! mov di, si DI = pointer xor ax, ax Zero to test against xor bl, bl BL = S1+S2 mov dx, 0 A30h DH = 10 (divisor), DL = '0' (ASCII zero) xor cx, cx Set counter to 65535 dec cx cld Seek forwards repnz scasb Find zero dec di SCASB goes one byte too far xchg si, di SI = pointer, DI = end (or rather, beginning) mov cx, si CX = counter sub cx, di jcxz. luhn: push es Keep original ES, and set ES=DS so SCASB can be used. Returns with carry clear if the string passes, carry set if the string fails. text jmp demo Check whether the 0-terminated string at DS:SI passes the Luhn test. Run the routine on the argument given on the CP/M command lineĭemo: lxi h,80h Zero-terminate the command line argumentīits org 100 h section. Rc If carry, it is not divisible, return (carry set) Rz If zero, it is divisible, return (carry clear) Lchk: sub b Trial subtraction, subtract 10 Ldone: mov a,d See if total is divisible by 10 Ldiv: inr c Find two digits using trial subtraction

Mvi c,9 10-1, compensate for extra subtraction loop Mov d,b D = S1+S2 (we don't need to keep them separate) Returns with carry clear if the string passes, carry set Ĝheck if the 0-terminated string at HL passes the Luhn test. OI CL16+15,X'F0' zoned to char (zap sign) Use it to validate the following numbers: Write a function/method/procedure/subroutine that will validate a number with the Luhn test, and S1 + s2 = 70 which ends in zero which means that 49927398716 passes the Luhn test If s1 + s2 ends in zero then the original number is in the form of a valid credit card number as verified by the Luhn test.įor example, if the trial number is 49927398716:.Sum the partial sums of the even digits to form s2.Multiply each digit by two and sum the digits if the answer is greater than nine to form partial sums for the even digits.and every other even digit in the reversed digits: and every other odd digit in the reversed digits and sum them to form the partial sum s1 Reverse the order of the digits in the number.Those companies using credit card numbers that can be validated by the Luhn test have numbers that pass the following test: The Luhn test is used by some credit card companies to distinguish valid credit card numbers from what could be a random selection of digits.

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Random empty debit card numbers working