[ColorForth] Question regarding if

[Alex Thiel <alex.lists@xxxxxxxxxx>]

On Friday 12 July 2002 16:55, Bob Shafer wrote:
> (As I found out last time I tried to be helpful on this list... :-)
>
> The '?' word is a macro that expects to be preceded by a literal value
> during compilation, and which compiles the x86 TEST instruction of that
> literal with whatever's on the top of the stack at runtime. The TEST
> instruction AND's the bits of its operands, and sets the processor flags
> accordingly, but doesn't actually change either of it's operands.
>
> So, something like "4 ? if foo then" will skip foo unless AND'ing 4 with
> the top of stack would have been non-zero.
>
> I hope that makes sense, and that I got it right this time. :-)

Yes, thanks. 
Not being a native speaker, I was always wondering what a "literal value" is.  

:)
	Alex

>
> On Fri, 12 Jul 2002, Kristopher Johnson wrote:
> > The ? (question-mark) word will test the top-of-stack and set CPU flags.
> >
> > So you may be able to write your word like
> >
> > : tst ? if 1 ; then 2 ;
> >
> > (I can't test this myself.)
> >
> > -- Kris
> >
> > --- Alex Thiel <alex.lists@xxxxxxxxxx> wrote:
> > > Hello,
> > >
> > > I want to write a word that test the TOS for a non-zero value. I have
> > > tried
> > >
> > > : tst if 1 ; then 2 ;
> > > : foo 0 tst 1 tst ;
> > >
> > > Execution of foo leaves 0 2 1 2 on the stack, i.e. pushing a constant
> > > does not
> > > seem to affect the flags. Is this expected behavior?
> > >
> > > As a workaround I do
> > >
> > > : tst 0 or if 1 ; then 2 ;
> > >
> > > Does anyone have a better idea?
> > >
> > > Cheers,
> > >
> > > 	Alex
> >
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