了解汇编语言中的cmpb和循环 [英] understand cmpb and loops in assembly language

问题描述

我有一个具有以下汇编代码的函数string_length

I have a function string_length that has the following assembly code

0x08048e90 <+0>:     push   %ebp
0x08048e91 <+1>:     mov    %esp,%ebp
0x08048e93 <+3>:     mov    0x8(%ebp),%edx     // assign whatever I declared into edx
0x08048e96 <+6>:     mov    $0x0,%eax          // assign eax = 0
0x08048e9b <+11>:    cmpb   $0x0,(%edx)        // compare edx to byte of 0 (null..?)
0x08048e9e <+14>:    je     0x8048ea9 <string_length+25>   // if equal, jump to +25
0x08048ea0 <+16>:    add    $0x1,%eax          // else, add 1 to eax
0x08048ea3 <+19>:    cmpb   $0x0,(%edx,%eax,1) // compare byte 1*eax+edx with 0, 
0x08048ea7 <+23>:    jne    0x8048ea0 <string_length+16>   // if not equal, back to +16
0x08048ea9 <+25>:    pop    %ebp               // pop ebp
0x08048eaa <+26>:    ret

由于函数名是string_length，所以我假设它将返回字符串中有多少个字符.

Since the function name is string_length, I am assuming it will return how many characters is in the string.

我感到困惑的是

cmpb   $0x0,(%edx)

这是否将指向edx的内容与0字节进行比较，而ASCII中的0为空.?

is this comparing whatever is pointed to edx to the byte of 0, and 0 in ASCII is null..?

和

cmpb   $0x0,(%edx,%eax,1)

以字节为单位比较1 * eax + edx.如果edx是字符串，这是否意味着edx将首先转换其ascii值，然后执行计算?

is comparing, in bytes, 1*eax+edx. If edx is a string, does that mean edx will first be converted its ascii value and then perform the calculation?

推荐答案

此:

cmpb   $0x0,(%edx)

采用EDX指向的字节(即包含地址)并将其与零比较.这个:

takes a byte that EDX points to (i. e. contains the address of) and compares it to zero. This:

cmpb   $0x0,(%edx,%eax,1)

接受EDX + EAX指向的字节并将其与零进行比较. EDX充当字符串基本指针，EAX充当索引.小数位数为1，因为我们正在处理字节.这样想整个循环:for(eax=0; edx[eax] != 0; eax++).

takes a byte that EDX+EAX points to and compares it to zero. EDX serves as the string base pointer, EAX is the index. Scale is 1 because we're working with bytes. Think of the whole loop this way: for(eax=0; edx[eax] != 0; eax++).

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