为什么在将数据从寄存器移动到内存时需要使用 [ ](方括号)，而在其他情况下不需要? [英] Why do I need to use [ ] (square brackets) when moving data from registery to memory, but not when other way around?

问题描述

这是我拥有的代码，它运行良好:

This is the code I have and it works fine:

section .bss
    bufflen equ 1024
    buff: resb bufflen

    whatread: resb 4

section .data

section .text

global main

main:
    nop
    read:
        mov eax,3           ; Specify sys_read
        mov ebx,0           ; Specify standard input
        mov ecx,buff        ; Where to read to...
        mov edx,bufflen     ; How long to read
        int 80h             ; Tell linux to do its magic

                            ; Eax currently has the return value from linux system call..
        add eax, 30h        ; Convert number to ASCII digit
        mov [whatread],eax  ; Store how many bytes has been read to memory at loc **whatread**

        mov eax,4           ; Specify sys_write
        mov ebx,1           ; Specify standart output
        mov ecx,whatread    ; Get the address of whatread to ecx
        mov edx,4           ; number of bytes to be written
        int 80h             ; Tell linux to do its work

        mov eax, 1; 
        mov ebx, 0; 
        int 80h

这是一个简单的运行和输出:

Here is a simple run and output:

koray@koray-VirtualBox:~/asm/buffasm$ nasm -f elf -g -F dwarf buff.asm
koray@koray-VirtualBox:~/asm/buffasm$ gcc -o buff buff.o
koray@koray-VirtualBox:~/asm/buffasm$ ./buff
p
2koray@koray-VirtualBox:~/asm/buffasm$ ./buff
ppp
4koray@koray-VirtualBox:~/asm/buffasm$ 

我的问题是:这两条指令是什么:

My question is: What is with these 2 instructions:

        mov [whatread],eax  ; Store how many byte reads info to memory at loc whatread
        mov ecx,whatread    ; Get the address of whatread in ecx

为什么第一个使用 [] 而另一个没有?

Why the first one works with [] but the other one without?

当我尝试将上面的第二行替换为:

When I try replacing the second line above with:

        mov ecx,[whatread]  ; Get the address of whatread in ecx

可执行文件将无法正常运行，它不会在控制台中显示任何内容.

the executable will not run properly, it will not shown anything in the console.

推荐答案

使用方括号和不使用方括号基本上是两种不同的事情:

Using brackets and not using brackets are basically two different things:

括号表示给定地址的内存中的值.

A bracket means that the value in the memory at the given address is meant.

不带括号的表达式意味着地址(或值)本身.

An expression without a bracket means that the address (or value) itself is meant.

示例:

mov ecx, 1234

意思是:将值1234写入寄存器ecx

Means: Write the value 1234 to the register ecx

mov ecx, [1234]

意思是:将内存中1234地址的值写入寄存器ecx

Means: Write the value that is stored in memory at address 1234 to the register ecx

mov [1234], ecx

意思是:将ecx中存储的值写入内存1234地址

Means: Write the value stored in ecx to the memory at address 1234

mov 1234, ecx

... 没有意义(在此语法中)，因为 1234 是一个无法更改的常数.

... makes no sense (in this syntax) because 1234 is a constant number which cannot be changed.

Linux写入"系统调用(INT 80h，EAX=4)需要写入值的地址，而不是值本身！

Linux "write" syscall (INT 80h, EAX=4) requires the address of the value to be written, not the value itself!

这就是为什么你不在这个位置使用括号！

This is why you do not use brackets at this position!

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