了解ATT大会（立即） [英] Understanding ATT Assembly (immediate)

问题描述

可以说，我有以下装配线

lets say i have the following assembly lines

movl   $-1, %edi
movl   $1, %edx

我究竟在存入％EDI /％EDX寄存器。

What exactly am I storing into %edi/%edx registers.

基本上，如果我是这个code转换成C程序，我会被initalizing一些变量为-1和1，因为这就是我看到它，这就是我觉得我越来越糊涂了。

Basically if I were to convert this code into a C program, would I be initalizing some variables to -1 and 1 because that's how I see it and that's where I think I'm getting confused.

我明白，眼前=某个常数，但是这是什么意思？

I understand that immediate = "some constant" but what does that mean?

推荐答案

有四种方式来加载的东西到寄存器：

There are four ways to load something into a register:

1. 立即值 - 在AT＆amp; T公司的汇编，这是使用 $号，并加载特定的值（数字）入册。需要注意的是数量不必是一个数值，它可能是，例如， MOVL $的printf，％eax中 - 这将printf的加载功能的地址到寄存器 EAX 。

1. Immediate value - in AT&T assembler, that's using a $number, and it loads that particular value (number) into the register. Note that number doesn't have to be a numeric value, it could be, for example, movl $printf, %eax - this would load the address of the function printf into register eax.

从另一个寄存器， MOVL％eax中，％EDX - 我们现在有 EAX 值复制到 EDX 。

From another register, movl %eax, %edx - we now have eax value copied into edx.

从一个固定的内存位置， MOVL MYVAR，％EAX - MYVAR 的内容是 EAX 。

From a fixed memory location, movl myvar, %eax - the contents of myvar is in eax.

从另一个寄存器内存位置， MOVL（％eax中），％EDX - 现在， EDX 有任何32位值是在 EAX 的地址。当然，假设它实际上是一个好存储位置 - 如果不是，我们有一个段错误。

From a memory location in another register, movl (%eax), %edx - now, edx has whatever 32-bit value is at the address in eax. Of course, assuming it's actually a "good" memory location - if not, we have a segfault.

如果这是C code时，code可loook有点像这样：

If this was C code, the code may loook a bit like this:

1）

int x = 42; 

int (*printfunc)(const char *fmt, ...) = printf;

2）

int x = 1;  
int y = 2; 
..., 
x = y;     // movl  %eax, %edx

3）

int x = myvar;

4）

int x = *myptr;

编辑：
几乎一切，这是一个源移动指令也可以是算术运算，如加$ 3％EAX A源将在C相当于<$的C $ C> X + = 3; 。

Almost everything that is a "source" for a move instruction can also be a source for arithmetic operations, such as add $3, %eax will be the equivalent in C of x += 3;.

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