MOVSB汇编语言代码如何工作？

Question

I cannot for the life of me understand why the below IA-32 assembly code works. I will explain the part I do not understand by walking through the code, line by line, below.

section .text
   global main
main:
   mov ecx, len
   mov esi, s1
   mov edi, s2
   cld
   rep movsb
   mov edx,20 ;message length
   mov ecx,s2 ;message to write
   mov ebx,1 ;file descriptor (stdout)
   mov eax,4 ;system call number (sys_write)
   int 0x80 ;call kernel
   mov eax,1 ;system call number (sys_exit)
   int 0x80 ;call kernel
section .data
   s1 db 'Hello, world!', 0
   len equ $-s1
section .bss
   s2 resb 20

First, ecx is set to the length of the string so that the rep will repeat movsb for all the characters in the string. Next, esi is set to equal the string and edi is set to be 20 bytes. Next, we clear the direction flag so that movsb copies in the right direction. Finally, we get to the movsb mnemonic which will move a character from the memory address specified in esi to the memory address specified in edi. That is repeated for each character in the string.

The problem I have is that if you look at esi and edi, they are NOT set to be the address of s1 and s2. They are set to be the value of s1 and s2. If we wanted to set the registers to be the address, i thought we would have to use "mov esi, [s1]" and "mov edi, [s2]". That however, is not what the code says. The code is from a tutorial to assembly programming, so should be right.

Thanks a lot for helping me get to the bottom of this.

Magnus

Answer 1

The problem I have is that if you look at esi and edi, they are NOT set to be the address of s1 and s2. They are set to be the value of s1 and s2. If we wanted to set the registers to be the address, i thought we would have to use "mov esi, [s1]" and "mov edi, [s2]".

You've got it backwards.
mov esi,foo is what you'd use to put the address of foo into esi , and mov esi,[foo] is what you'd use to put the value located at the address of foo into esi .

Note that the syntax differs between different assemblers. What I said above is true for NASM (which is what you've indicated that you're using), but when using MASM/TASM you'd use mov esi,OFFSET foo to get the address, while mov esi,foo and mov esi,[foo] both would get the value.