C，大会：认识开关状态，EDX ECX EAX [英] C, Assembly : understanding the switch condition, edx eax ecx

问题描述

C，大会：认识开关状态，EDX ECX EAX

C, Assembly : understanding the switch condition, edx eax ecx

我在C开关条件code和汇编code。
不过好像很随意对我有什么设置EDX或EAX或ECX？
我如何告诉EDX，EPX，ECX，EBP之间的差异？即使是教科书不给我足够的解释。

I have a switch-condition code in C and assembly code. But it seems very arbitrary to me what to set edx or eax or ecx? How do I tell the difference between edx, epx, ecx, ebp? Even the textbook does not give me the sufficient explanation.

#include <stdio.h>

// Enumerated type creates set of constants
// numbered 0 and upward
typedef enum {MODE_A, MODE_B, MODE_C, MODE_D, MODE_E} mode_t;

int switchmode(int *p1, int *p2, mode_t action)
{
  int result = 0;
  switch(action) {
    case MODE_A:
      result = *p1;
      *p1 = *p2;
      break;
    case MODE_B:
      *p2 += *p1;
      result = *p2;
      break;
    case MODE_C:
      *p2 = 15;
      result = *p1;
      break;
    case MODE_D:
      *p2 = *p1;
      /* Fall Through */
    case MODE_E:
      result = 17;
      break;
    default:
      result = -1;
  }
  return result;
}

int main(int argc, const char * argv[])
{
  int num1 = 10;
  int num2 = 20;

  printf("MODE_A: %d \n", switchmode(&num1, &num2, MODE_A)); // 10
  printf("MODE_B: %d \n", switchmode(&num1, &num2, MODE_B)); // 40
  printf("MODE_C: %d \n", switchmode(&num1, &num2, MODE_C)); // 20
  printf("MODE_D: %d \n", switchmode(&num1, &num2, MODE_D)); // 17
  printf("MODE_E: %d \n", switchmode(&num1, &num2, MODE_E)); // 17
  printf("NULL: %d \n", switchmode(&num1, &num2, (mode_t) NULL)); // 20
    return 0;
}

和组装code是

参数：在P1的％ebp + 8
            在P2的％ebp + 12
            在EBP％+ 16的动作
注册：导致EDX％（初始化为-1）

Arguments: p1 at %ebp+8 p2 at %ebp+12 action at %ebp+16 Register: result in %edx (initialized to -1)

.L17: // MODE_E
  movl $17, %edx
  jmp .L19
.L13: // MODE_A
  movl 8(%ebp), %eax
  movl (%eax), %edx
  movl 12(%ebp), %ecx
  movl (%ecx), %eax
  movl 8(%ebp), %ecx
  movl %eax, (%ecx)
  jmp .L19
.L14: // MODE_B
  movl 12(%ebp), %edx
  movl (%edx), %eax
  movl %eax, %edx
  movl 8(%ebp), %ecx
  addl (%ecx), %edx
  movl 12(%ebp), %eax
  movl %edx, (%eax)
  jmp .L19
.L15: // MODE_C
  movl 12(%ebp), %edx
  movl $15, (%edx)
  movl 8(%ebp), %ecx
  movl (%ecx), %edx
  jmp .L19
.L16: // MODE_D
  movl 8(%ebp), %edx
  movl (%edx), %eax
  movl 12(%ebp), %ecx
  movl %eax, (%ecx)
  movl $17, %edx
.L19: // default
  movl %edx, %eax // set return value

我的问题是
我怎么知道什么是EAX？它是其中一个参数？
如果是这样，二，我知道这是EAX和ECX和如何EDX？

My question is how do I know what is eax? Is it one of the arguments? If so, how di I know which is the eax and ecx and which is edx?

在此先感谢！

推荐答案

在大多数组装code，无论是在％eax中是从函数的返回值。这就是为什么在.L19块无论是EDX％被移动到％eax中。回首过去，无论你设置结果来的东西中，code举动，重视到％edx中，这样，当code跳到.L19的值将是默认情况下，放在％eax中。

这看起来像一个32位系统，因此它具有8个通用寄存器：

In most assembly code, whatever is in %eax is the value returned from the function. That is why in the .L19 block whatever was in %edx is moved into %eax. Looking back, wherever you set result to something, the code moves that value into %edx so that when the code jumps to .L19 the value will be placed in %eax by default.
This looks like a 32-bit system, so it has 8 general-purpose registers:

• ％EBX，％ecx中，％EDX，％ESI，和％EDI是寄存器，大多存储的任何数据是必要的他们在任何给定的时间保持。


• ％eax中拥有一个函数，如返回值调用它的功能将被期待在％eax中该函数的返回值它更改％eax中之前。


• ％EBP和ESP％的，基本上管理和栈各种函数的调用上分配空间的特殊功能寄存器。

要看到你的论点被放置，看看那里的数据是在code被引用。例如，在.L13的程序放置的％ebp + 8（其为p1）插入％eax中，然后在该地址（* p1）为宿值到％EDX。所以，EDX％现在拥有的价值由P1指向。


寄存器不要拿着函数的自变量。不要以为％edx中包含的参数之一开关模式（）默认情况下。参数始终放置在堆栈中。

To see where your arguments are being placed, look at where that data is being referenced in the code. For example, at .L13, the program places %ebp+8 (which is p1) into %eax, then places the value at that address (*p1) into %edx. So, %edx now holds the value pointed to by p1.

The registers DO NOT hold the arguments to the function. Don't think that %edx contains one of the arguments to switchmode() by default. Arguments are always placed on the stack.

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