🚀 go-pugleaf

"Martin Cote" <vlrcfmnw@videotron.ca> wrote in message
news:3BFDF24B.9E4D2F05@videotron.ca...
> Hi,
>
> I'm trying to write some C modules in assembly.  I'm using 'gcc' and
> 'as' under linux.  I'm trying some test to 'get the feel' of this hybrid
> programming.  I'm trying a simple 'write_char' function that display the
> char passed in parameter on the screen.  Here's my source code :
> extern write_char(char);
> ----------------------------------
> ..globl write_char
>
> write_char:
>  pushl %ebp
>  movl  %esp,%ebp
>  movl  $4,%eax
>  xorl  %ebx,%ebx
>  incl  %ebx
>  movl  4(%esp),%ecx
>  movl  %ecx,char
>  leal  char,%ecx
>  xorl  %edx,%edx
>  incl  %edx
>  int   $0x80
>  leave
>  ret
>
> char: .byte 0,0,0,0
> ----------------------------------
>
> And the C main :
>
> ----------------------------------
> extern write_char(char);
>
> int main() {
>   write_char(97);
>   return 0;
> }
> ----------------------------------
>
> The problem is that it get a 'core dump' ;)
> There is something that I did not undersand in the way to pass
> parameters to a C function (with the %esp register).
>
> If anyone can help, I would be really grantful...
>

1. You should not attempt to by-pass the libc and call the OS directly using
the software interrupt.

2. You cannot write into the code segment - it is a no-no! Move the
temporary data elsewhere. The standard way is to allocate the temp location
in the stack. Do not think of writing self-modifying code - it is a
hardware-protected impossibility.

My recommendation is to start the assembly language training without messing
first with the operating system.

An usable template for the assembler routine can be obtained by compiling
the corresponding routine skeleton with GCC using the -S switch.

Tauno Voipio
tauno voipio @ iki fi