在 GAS.intel-syntax 程序集中在 memory 标签和寄存器上使用“偏移”和“[]”有什么区别？

Question

Using GCC, compiling for 32-bit Intel architecture, with .intel_syntax noprefix . Let's suppose I have the following .data section:

A: .int 0x1, 0x2

What is the difference between the following, when used as an operand: A , [A] , offset A ? Also, what is the difference when [..] is used on a register and on a memory label?

The difference between writing the register simply and putting it into [..] was answered here well.

Answer 1

Let me give you a brief explanation.
[] brackets mean something like "do not work with the content, but with the address". When used labelled DATA, you can omit them (it depends on your assembler's syntax, but in MASM it definitely works like that). Why? There is no way of working with data in memory directly; instead, you just work with a data somewhere in memory (on some address) . So no disambiguity can happen, you always work with data on address. When you use them with registers, it's a quite different story:

MOV EAX, 10

simply loads 10 (0x0000000A) in EAX register. You work directly with the register. But:

MOV EAX, 666
MOV BYTE PTR [EAX], 77

loads 77 into memory adress 666. The BYTE PTR directive is necessary, because assembler doesn't know if it should use 1, 2, 4 etc. bytes. The [EAX] says "do not work with EAX, instead, work with ADDRESS (memory location) contained in EAX.

If you want to find out a difference between [VAR] , VAR and OFFSET VAR , try to step-by-step this code:

.DATA
VAR DWORD 77

.CODE
    MOV EAX, VAR
    MOV EBX, OFFSET VAR
    MOV ECX, [VAR]
    MOV EDX, OFFSET [VAR]

You will clearly see the difference.