A clever homebrew modulus implementation

Question

I\'m programming a PLC with some legacy software (RSLogix 500, don\'t ask) and it does not natively support a modulus operation, but I need one. I do not have access to: modulus

Answer 1

How many bits are you dealing with? You could do something like:

if dividend > 32 * divisor  dividend -= 32 * divisor
if dividend > 16 * divisor  dividend -= 16 * divisor
if dividend > 8 * divisor  dividend -= 8 * divisor
if dividend > 4 * divisor  dividend -= 4 * divisor
if dividend > 2 * divisor  dividend -= 2 * divisor
if dividend > 1 * divisor  dividend -= 1 * divisor
quotient = dividend

Just unroll as many times as there are bits in dividend. Make sure to be careful about those multiplies overflowing. This is just like your #2 except it takes log(n) instead of n iterations, so it is feasible to unroll completely.