Determining when NASM can infer the size of the mov operation

Question

Something has got me confused in x86 assembly for a while, it\'s how/when can NASM infer the size of the operation, here\'s an example:

mov ebx, [eax]
         


        

Answer 1

In your first case it can determine it without problems, since EBX is a 32bit register. But in the second one you're using EAX as an address, not as a destination register so nasm developers took the safe route and make the developer choose the size.

If you did mov [eax], 1, what could nasm determine from that? Do you want to set the byte, 16bit or 32bit block of memory to 1? It is totally unknown. This is why it's better to force the developer to state the size.

It would be entirely different if you said mov eax, 123456 since then the destination is a register.