Adding 64 bit numbers using 32 bit arithmetic

Question

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

Answer 1

Add the least significant bytes first, keep the carry. Add the most significant bytes considering the carry from LSBs:

; x86 assembly, Intel syntax
; adds ecx:ebx to edx:eax
add eax, ebx
adc edx, ecx