[efas]

Implicitly, the two signed integers are large enough to fit each other (they are the same size). I would not assume anything about organization in memory though. x=a, y=b; look at arithmetic in pseudo-ops x=a, y=(b-a); there will be an overflow here or after the negation step x=a+(b-a)=b, y=(b-a); x=b, y=(a-b); negate the unitary value could overflow (and set the carry bit) x=b, y=(a-b)+b=a; oops, four operations and not as elegant as XORs can be. Or, the compiler would probably use a third register.