Adding 64 bit numbers using 32 bit arithmetic

Question

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

Answer 1

If the 64-bit numbers are (a₂,a₁) and (b₂,b₁), where x₂ is the high 32 bits taken as unsigned, and x₁ is the low 32 bits taken as unsigned, then the sum of the two numbers is given below.

c1 = a1 + b1

c2 = a2 + b2

if (c1 < a1 || c1 < b1)
   c2 += 1