Adding 64 bit numbers using 32 bit arithmetic

Question

How do we add two 64 bit numbers using 32 bit arithmetic??

Answer 1

The C code previously posted is unnecessarily verbose:

unsigned a1, b1, a2, b2, c1, c2;

c1 = a1 + b1;
c2 = a2 + b2;

if (c1 < a1)
    c2++;

The 'a1' in the 'if' can be replaced with 'b1' as well. On overflow, c1 will be less than both a1 and b1.