一个C语言中unsigned和signed的汇编不同的插曲

一个C语言中unsigned和signed的汇编不同的插曲

void main()
{
unsigned int  a = 0xffffff;
unsigned int len = sizeof(int);

for (int i = 0; i < len; i++)
{
unsigned char temp = ((char *)(&a))[i];
printf("the value is %x\n", temp);

}
}

对应的汇编语言如下：

11: void main()
12: {
00DB4C80  push        ebp
00DB4C81  mov         ebp,esp
00DB4C83  sub         esp,0F0h
00DB4C89  push        ebx
00DB4C8A  push        esi
00DB4C8B  push        edi
00DB4C8C  lea         edi,[ebp-0F0h]
00DB4C92  mov         ecx,3Ch
00DB4C97  mov         eax,0CCCCCCCCh
00DB4C9C  rep stos    dword ptr es:[edi]
13:     unsigned int  a = 0xffffff;
00DB4C9E  mov         dword ptr [a],0FFFFFFh
14:     unsigned int len = sizeof(int);
00DB4CA5  mov         dword ptr [len],4
15:
16:     for (int i = 0; i < len; i++)
00DB4CAC  mov         dword ptr [ebp-20h],0
00DB4CB3  jmp         main+3Eh (0DB4CBEh)
00DB4CB5  mov         eax,dword ptr [ebp-20h]
00DB4CB8  add         eax,1
00DB4CBB  mov         dword ptr [ebp-20h],eax
00DB4CBE  mov         eax,dword ptr [ebp-20h]
00DB4CC1  cmp         eax,dword ptr [len]
00DB4CC4  jae         main+64h (0DB4CE4h)
17:     {
18:         unsigned char temp = ((char *)(&a))[i];
00DB4CC6  mov         eax,dword ptr [ebp-20h]
00DB4CC9  mov         cl,byte ptr a[eax]
00DB4CCD  mov         byte ptr [ebp-29h],cl
19:         printf("the value is %x\n", temp);
00DB4CD0  movzx       eax,byte ptr [ebp-29h]     (注意这个地方，是movzx，程序在这里是零扩展)
00DB4CD4  push        eax
00DB4CD5  push        offset string "the value is %x\n" (0DB6CD0h)
00DB4CDA  call        _printf (0DB1320h)
00DB4CDF  add         esp,8
20:
21:     }
00DB4CE2  jmp         main+35h (0DB4CB5h)
22: }
程序运行结果是
the value is ff
the value is ff
the value is ff
the value is 0
请按任意键继续. . .

void main()
{
int  a = 0xffffff;
int len = sizeof(int);

for (int i = 0; i < len; i++)
{
char temp = ((char *)(&a))[i];
printf("the value is %x\n", temp);

}
}
汇编结果是：
11: void main()
12: {
01204C80  push        ebp
01204C81  mov         ebp,esp
01204C83  sub         esp,0F0h
01204C89  push        ebx
01204C8A  push        esi
01204C8B  push        edi
01204C8C  lea         edi,[ebp-0F0h]
01204C92  mov         ecx,3Ch
01204C97  mov         eax,0CCCCCCCCh
01204C9C  rep stos    dword ptr es:[edi]
13:      int  a = 0xffffff;
01204C9E  mov         dword ptr [a],0FFFFFFh
14:     unsigned int len = sizeof(int);
01204CA5  mov         dword ptr [len],4
15:
16:     for (int i = 0; i < len; i++)
01204CAC  mov         dword ptr [ebp-20h],0
01204CB3  jmp         main+3Eh (01204CBEh)
4000
01204CB5  mov         eax,dword ptr [ebp-20h]
01204CB8  add         eax,1
01204CBB  mov         dword ptr [ebp-20h],eax
01204CBE  mov         eax,dword ptr [ebp-20h]
01204CC1  cmp         eax,dword ptr [len]
01204CC4  jae         main+64h (01204CE4h)
17:     {
18:          char temp = ((char *)(&a))[i];
01204CC6  mov         eax,dword ptr [ebp-20h]
01204CC9  mov         cl,byte ptr a[eax]
01204CCD  mov         byte ptr [ebp-29h],cl
19:         printf("the value is %x\n", temp);
01204CD0  movsx       eax,byte ptr [ebp-29h]    （注意这里是movsx，是带符号的扩展）
01204CD4  push        eax
01204CD5  push        offset string "the value is %x\n" (01206CD0h)
01204CDA  call        _printf (01201320h)
01204CDF  add         esp,8
20:
21:     }
01204CE2  jmp         main+35h (01204CB5h)
22: }
01204CE4  xor         eax,eax
01204CE6  push        edx
01204CE7  mov         ecx,ebp
01204CE9  push        eax
01204CEA  lea         edx,ds:[1204D0Ch]
22: }
程序运行结果是
the value is ffffffff
the value is ffffffff
the value is ffffffff
the value is 0
请按任意键继续. . .