0和dword 0有什么区别？

Question

As the question states, What's the difference between (for example) mov eax, 0 and mov eax, dword 0 ?

I've been using cmp statements, and I can't catch the difference. Is one an address and the other a numerical value?

Answer 1

As stated, no difference for the MOV instruction. For CMP, you would have difference between

CMP memory address of a qword,a 32 bit immediate

is different from

CMP memory address of a qword,an 8 bit immediate

as the comparison will be done after a sign extension, and hence, with a smaller dimension immediate number, expecially when its a negative number, some caution is advised.

Have fun programming...

Answer 2

It does not make a difference with a register since the name of the register already tells the assembler "how big" the data item (in this case, 0 ) is:

mov   eax, dword 0    ; move a 4-byte 0 into eax
mov   eax, 0          ; move a 0 into eax (eax is 4 bytes, so move 4 bytes)

However, there are cases where you might want to be able to specify how large the value is since there's a choice. In x86 32-bit assembly, the following would push a 4-byte 0 value on the stack:

push   0             ; push 4-byte 0 onto the stack

If you wanted to push a 2-byte 0 value, you would use:

push   word 0

If you wanted to be explicit so it's clear for the 4-byte immediate push, you could use the dword :

push   dword 0

If you want to move an immediate value to memory, the size specifier becomes necessary because the assembler doesn't know how large the data item is otherwise. For example, consider the following code in nasm assembly:

        section  .bss
num     resb     4

        ...
        section  .text
        ...
        mov    [num], 0

This will generate an error:

 error: operation size not specified

So you need to specify the size, say, 4-bytes:

        mov    [num], dword 0

Answer 3

They are exactly the same thing, it's just assembler syntax.

As a side note, xor eax, eax is generally preferred since it generates a two byte instruction which is much smaller.