[bodhiandpysics1]

When trying to understand C, it's always useful to look at what the pdp-11 did. the pdp-11 didn't have a mod instruction; but its div instruction can be used instead. Div can only take an even numbered register as its dest. That register holds the quotient. The odd numbered register numbered one greater then dest holds the remainder, which for a negative number will naturally be negative. so div easilly be used to calculate a mod-just ignore the quotient, but you'll get a negative value.