Re: basic system call question

["Joseph A. Knapka" <jknapka@earthlink.net>]

Erik Mouw wrote:
> 
> On Mon, May 14, 2001 at 08:51:07AM +0100, Ralph Corderoy wrote:
> > > Why is the syscall "int 0x80"?  Is this where the system is actually
> > > executed with the arguments previously listed?  It is not the return
> > > value, and the system call number is already 42.
> >
> > The `int 0x80' generates the interrupt that switches to the kernel
> > which in turn seems to examine the %eax register to see what system
> > call is required.  I'd guess all system calls generate the same
> > interrupt, 0x80, and the only thing to distuinguish between them is the
> > value of %eax.
> 
> Your guess is correct (for i386, that is ;).

Which reminds me of a question I had some time ago, but wasn't
able to investigate at the time. The kernel_thread() function,
called to spawn kernel threads, seems to invoke int0x80 with
the contents of EAX undefined. Am I insane, or what? I suppose
something about the calling convention used in the kernel
must gaurantee the EAX value is something useful, but is that
documented anywhere? I have not been able to figure out why
this code works.

<http://lxr.linux.no/source/arch/i386/kernel/process.c#L439>

Thanks,

-- Joe
 

-- Joseph A Knapka
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