x86 Assembly - segmentation fault?
 

Howdy folks:

Just began exploring x86 assembly the other day and I'm having a few problems with a practice program that I am trying to write that displays a string and then displays it again backwards.

First, the code below assembles and links fine. But I am getting a segmentation fault at the lds and les instructions in the revstr procedure. Can anyone see what is causing this?

Second, I thought that placing the variable name within brackets retrieved the data itself not the address. movsb needs the addresses of the two strings that it will operate on, but when I try to copy the addresses into DS:SI and ES:DI I get an assembler error that says:

reversestring.asm:42: error: invalid combination of opcode and operands

The documentation that I have on how to use movsb shows it with brackets around the variables. What does this mean, because I know that it isn't attempting to move the variables values into the registers, because the values have no use here.

lds si, [Message]
les di, [MessageReversed]

It seems that it would make more sense to have it

lds si, Message
les di, MessageReversed

Or am I misunderstanding the lds and les instructions? Perhaps someone could attempt to give me a better explanation of these instructions than the one that I have. I tried reading the Intel Instruction Set Reference entry on lds/les and as anyone who has been there knows, the manual is not exactly targeted towards a novice assembly programmer.


Anyways, below is the source. Any assistance would be greatly appreciated. Even better than an answer would be a pointer in the right direction (as far as reading topics that would assist me in finding the answer).

Thanks,
JT

*****************************************************************************

;ReverseString
;Displays a text string both normally and in reverse


[SECTION .text]

extern puts
global main

main:
push ebp
mov ebp, esp
push ebx
push esi
push edi

push dword Message
call puts
add esp, 4

call revstr

push dword MessageReversed
call puts
add esp, 4

pop edi
pop esi
pop ebx
mov esp, ebp
pop ebp
ret

revstr: ;reverse 'message' and store in 'messagereversed'
lds si, [Message]
les di, [MessageReversed]
std
mov cx, 40
rep movsb
ret


[SECTION .data]

Message DB "This is a sentence.", 10, 0

[SECTION .bss]

MessageReversed resb 40

Your problem is here:
STD tells movsb to decrement memory addresses when moving bytes, and since si is set to the beginning of Message, it reads before Message and segfaults. BTW, [Message] loads the value of Message, not the address.