Question

I tried to write a program in Assembly that calculate operand_a + operand_b.

My big problem if that operanda and operandb are representable in 16-byte (4 d-word). So I wrote this code:

mov eax, operanda
mov ebx, operandb
add eax, ebx

But than I understood that my operand is big than 32-bit.

I missed something?

Answer 1

Define another variable, operandc, for the result.

Then do long addition from the least significant dwords to the most significant dwords not forgetting about the intermediate carry’s:

mov eax, dword ptr [operanda]
add eax, dword ptr [operandb]
mov dword ptr [operandc], eax

mov eax, dword ptr [operanda+4]
adc eax, dword ptr [operandb+4]
mov dword ptr [operandc+4], eax

mov eax, dword ptr [operanda+8]
adc eax, dword ptr [operandb+8]
mov dword ptr [operandc+8], eax

mov eax, dword ptr [operanda+12]
adc eax, dword ptr [operandb+12]
mov dword ptr [operandc+12], eax