大会EAX寄存器复位没有道理

问题描述

我有以下组装code：

I have the following assembly code:

; File: strrev.asm
; A subroutine called from C programs.
; Parameters: string A
; Result: String is reversed and returned.


    SECTION .text
    global strrev
_strrev: nop
strrev:
    push    ebp
    mov ebp, esp

    ; registers ebx,esi, and edi must be saved if used
    push ebx
    push edi

    xor esi, esi
    xor eax, eax
    mov ecx, [ebp+8]    ; load the start of the array into ecx
    jecxz   end     ; jump if [ecx] is zero
    mov edi, ecx

reverseLoop:
    cmp byte[edi], 0
    je  reverseLoop_1
    inc     edi
    inc eax
    jmp reverseLoop



reverseLoop_1:

    mov esi, edi    ;move end of array into esi
    mov edi, ecx    ;reset start of array to edi

reverseLoop_2:
    mov al, [esi]
    mov bl, [edi]
    mov     [esi], bl
    mov [edi], al
    inc edi
    dec esi
    dec eax
    jnz reverseLoop_2

end:
    pop edi     ; restore registers
    pop ebx
    mov esp, ebp    ; take down stack frame
    pop ebp
    ret

，直到你开始通过reverseLoop_2循环工作正常。使用GDB，EAX被列为是11，它应该是（这是我通过一个单独的C程序传入的字符串的长度）。这是展现在调试器：

Which works fine until you start looping through reverseLoop_2. Using gdb, eax is listed as being 11, which it should be (this is the length of the string I passed in through a separate c program). This is show in the debugger as:

Breakpoint 2, reverseLoop_2 () at strrev.asm:40
40      mov al, [esi]
(gdb) display $eax
1: $eax = 11

不过，如果我通过程序到下一行一步，它重置为0。

However, if I step through the program to the next line, it resets to 0.

(gdb) next
41      mov bl, [edi]
1: $eax = 0

我需要EAX是preserved自多少次一个保持跟踪reverseLoop_2需要循环。为什么重置为0，来电后MOV？

I need eax to be preserved since its the one keeping track of how many times reverseLoop_2 needs to loop. Why is it resetting to 0 after the call to mov?

推荐答案

如果你使用 EAX 作为一个循环计数器，你不应该写它里面的循环：

If you're using eax as a loop counter, you shouldn't write to it inside the loop :

reverseLoop_2:
  mov al, [esi]

记住人是最显著字节 EAX ：

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