Problems with FreeBSD assembly

David Jackson djackson452 at
Thu Nov 12 15:38:33 UTC 2009

Charlie Kester wrote:
> On Wed 11 Nov 2009 at 17:32:41 PST Charlie Kester wrote:
> One more thing:
>> Notice that the system call number (or any other dword) should also be
>> pushed onto the stack before the int 80h.
> The reason for this is given at the top of the page:
>    although the kernel is accessed using int 80h, it is assumed the
>    program will call a function that issues int 80h, rather than issuing
>    int 80h directly.
> So the extra dword pushed onto the stack takes the place of the return
> address from the function the kernel expects to have been called.  
> And since you're not actually using as a return address, it doesn't
> matter what value it actually has.  The kernel doesn't use it for
> anything; it just expects it to be there in a properly arranged stack
> frame.
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The push eax is what made it work. So that was the problem. Stdin and 
stdout do not need to opened before they are used, as in C. Thank you 
everyone for your help on this, that solved it.

Here is the code that works:
        section .data
        hello   db      'Hello, World!', 0xa
        hbytes  equ     $ - hello

        section .text
        global  _start
        push    dword hbytes
        push    dword hello
        push    dword 1
        mov eax,0x4
        push eax
        int 0x80
        add esp,16

        push    dword 0
        mov eax,0x1
        push eax
        int 0x80

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