How can I turn an odd number even, or vice versa, in Motorola 68000 Assembly?

Question

Basically if I had a number in D1, and wanted it to ALWAYS be even, how would I make sure it is never odd?

I know it has something to do with the AND instruction. But when I tried that, it would always subtract 1. So it would turn odd numbers even and even numbers odd.

How can I basically do if n is odd, sub 1

Answer 1

and your number with -2.

In 2's complement representation -2 is a number with all bits set to one except the lowest one (11111...110), so, used as a mask, it always kills just the low bit of your number. This forces it to be even (it works correctly even for negative numbers).

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As for the "viceversa" in the title: to do the opposite (=force every even number to the next odd), just or with 1. This sets the low bit to be 1, which obtains the required effect.

Answer 2

Odd numbers end with 1 in binary, and even numbers end with 0 in binary. What you really want is to make the last binary digit be 0, regardless of what it started as. (That'll subtract 1 from odd numbers, and leave even numbers unchanged.)

The way to do this is to AND with 1111...1110, where all binary digits are 1 except the last digit, which is 0. You can construct this by doing a bitwise negation on 0000...0001, which of course is just 1.

So if your number is n, you want to compute n & (~1).