段故障移动(%eax)， %ebx在这个循环的第二次迭代?

Question

我是汇编新手，我试图一次打印字符串的一个字符，目前为止有这个。

.equ  STDOUT,1
.equ  WRITE,4
.equ  EXIT,1
char_string:
.asciz "hello, world"

.text
.globl _start
_start:
movl $char_string, %eax
call print_str
movl $EXIT, %eax
int $0X80

print_str:
mov (%eax), %ebx
movl $WRITE, %eax
movl $STDOUT, %ebx
movl $char_string, %ecx
movl $1, %edx
int $0x80
inc %eax
cmpl $0, %ebx
jne print_str
je out_of_loop
out_of_loop:
ret

然而，当我尝试编译时，我在行中得到分段错误move (%eax), %ebx这有什么问题?我该如何修复它?我试图将字符串的指向字符移动到%ebx用于打印，然后稍后我增加eax以移动到字符串中的下一个字符。

Answer 1

崩溃的直接原因是使用eax作为系统调用的返回值。但是，您的代码在其他方面也是错误的。我已经注释了你的代码:

print_str:
mov (%eax), %ebx           # loads 4 bytes not 1
movl $WRITE, %eax
movl $STDOUT, %ebx         # overwrites ebx you loaded
movl $char_string, %ecx    # uses the starting address instead of iterating
movl $1, %edx
int $0x80
inc %eax                   # eax is return value of system call by now
cmpl $0, %ebx              # ebx is $STDOUT, see above
jne print_str
je out_of_loop             # makes no sense to jump to next instruction
out_of_loop:
ret

一个可能的解决方案是:

print_str:
mov %eax, %ecx             # address of char to print
movl $STDOUT, %ebx
movl $1, %edx
print_str_loop:
cmpb $0, (%ecx)            # test for terminating zero byte
je out_of_loop
movl $WRITE, %eax          # reload eax as it is return value from a previous iteration
int $0x80
inc %ecx                   # point to next character
jmp print_str_loop
out_of_loop:
ret