Access is denied ?

[gelatine1]

Hi I have the code below but something weird happens. It compiles without any problem but when I run it I get the message 'Access is denied.' Why am I getting this message?
This is my code:

Code Select Expand


.386
.model flat, stdcall
option casemap :none

include \masm32\include\windows.inc
include \masm32\include\kernel32.inc
include \masm32\include\masm32.inc

includelib \masm32\lib\kernel32.lib
includelib \masm32\lib\masm32.lib

.data
HelloWorld db "Hello World!", 0

.code
tst:
pop eax ;get address of HelloWorld from stack
inc eax ;increase that address
xor edx,edx ;edx is set to 0
mov [eax],edx ; the second character of HelloWorld is set to 0
ret

start:

lea eax,HelloWorld
push eax ;invoke tst, addr HelloWorld
call tst ;invoke tst, addr HelloWorld

push eax
call StdOut ;invoke StdOut, addr HelloWorld

push 0
call ExitProcess ;invoke exitprocess,0

end start



Thanks in advance

[GoneFishing]

Learn to use a debugger
First, when I assembled your code the real entry point was label tst . So I moved it below the start label.
Next, in your tst code you get the return address of the caller in eax and not an address of the string.

[Vortex]

To get the address of the string :

Code Select Expand


mov   eax,OFFSET HelloWorld


[GoneFishing]

...

[jj2007]

With a few changes, you will see the "H" as expected:
tst:
      pop edx  ; yep, you forgot the return address!!
      pop eax ;get address of HelloWorld from stack
      push edx
      inc eax ;increase that address
      xor edx,edx ;edx is set to 0
      mov [eax],edx ; the second character of HelloWorld is set to 0
      ret
     
start:
      lea eax, HelloWorld
      push eax
      push eax ;invoke tst, addr HelloWorld
      call tst ;invoke tst, addr HelloWorld
     
      ; push eax?? The inc'ed one?? No...
      call StdOut ;invoke StdOut, addr HelloWorld

It might be a good idea to give yourself  and us a time to look at output:

Sleep(3000) is somewhat suboptimal. Either use an intelligent IDE (like RichMasm) that keeps the console open if it detects a console app with no inkey, or
   include \masm32\macros\macros.asm
   includelib \masm32\lib\msvcrt.lib
   inkey chr$(13,10, 10, "bye")

[gelatine1]

So in fact eax remains a global variable ? I assumed that i had a local storage of eax in my tst 'function' and that it got removed after the ret but clearly it doesnt work like that.
Thanks jj2007 and others too

[jj2007]

So in fact eax remains a global variable ?

No, eax is a register. This is not C/C++...

Read about register preservation in ttt