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Re: x86 D flag: what's its status thru the program?
On Tue, 6 Jul 2004, Denis Zaitsev wrote:
> On Mon, Jul 05, 2004 at 07:19:06PM +0200, Lukasz Michal Rak wrote:
> > > > The convention is that the D flag in IA32 is cleared. So if you call a
> > > > subroutine you have to clear D flag before making call.
> > >
> > > Do you mean that 'I have to clear the D flag if I had set it somewhere
> > > before?' Else what is the sense of the convention?
> > Yes. Other code may assume that D is cleared and for example after calling
> > your code will not check state of D flag, so if you will left D flag set
> > this may cause errors.
> >
> > > > The subroutine can change the state of D flag, but before returning
> > > > the routine has to clear the D flag.
> > >
> > > If the subroutine can change the flag, that it will violate the
> > > convention, won't it? And what is the strength of the convention,
> > > again?
> > >
> > Subroutine can change the D flag, but before exiting must clear it.
> >
> > The convention should make you free from checking the state of flag
> > and clearing at the beginning of your code.
> >
> > > > Hope I helped.
> > >
> > > Thanks, but I still don't understand something. Again: if every
> > > procedure keeps the flag clear at exit, then why the same every
> > > procedure clears it at entry? In a suspicion, that caller has changed
> > > the flag against the rules?
> > Just to be sure that the D flag is cleared. The convetion is one thing and
> > reality other.
>
> Ok. Thanks! It's the thing that I didn't understand. BTW, where is
> this convention documented, if either?
>
I don't know where it is documented. Maybe in some documents about C
calling convention. I let you know if I will find something.
Lukasz