Re: What am I missing?
The issue with e.g. 128-bit integers in C is that there’s no access to the carry flag, so what would be a sequence of 4 add-with-carry instructions (on a 32-bit CPU) ends up much more complicated. (If you’re lucky the compiler will spot what you’re doing and use add-with-carry, but do you know what you have to write in C for it reliably do that?)
c = a + b;
bool carry = c<a;
f = d + e + (carry ? 1 : 0);
Is that correct? Does that produce the optimal two-instruction sequence?
.... now write a version for signed arithmetic!