It's not clear to me how [simple unconditional] mov could possibly do the job alone. I believe it could only work if it incorporates "magic" memory locations - e.g., storing at location x executes math combining location x and location y in some way and alters location z. This simply begs the question by moving logic behind the curtain.

I think the single instruction which can do the entire job without any magic assist is subneg x, y, z:

Subtract location x from location y; store the result in location y; and branch to location z if result is less than 0; else proceed to next.

Or various trivial variations of the same idea.

Any complication beyond this is no more than syntactical sugar and performance optimization.