[fpc-pascal]runtime 201 with swapword function

[James Mills]

On Fri, Jul 25, 2003 at 04:30:35PM +0200, Florian Klaempfl wrote:
> James Mills wrote:
> >On Fri, Jul 25, 2003 at 04:21:36PM +0200, Jonas Maebe wrote:
> >
> >>On vrijdag, jul 25, 2003, at 16:22 Europe/Brussels, James Mills wrote:
> >>
> >>
> >>>Why does the following result in a runtime error ?
> >>>
> >>>program example;
> >>>
> >>>const
> >>>	THE_PORT = 9999;
> >>>
> >>>function swapWord(w: Word): Word;
> >>>begin
> >>>	swapWord := (w SHL 8) OR (w SHR 8);
> >>>end;
> >>
> >>In Pascal, all calculations are performed using the base type (which is 
> >>longint in FPC), so if w > 255, then w shl 8 > high(word) and will 
> >>result in a range check error when assigning to swapWord. Even if we 
> >>calculated everything using words, you'd still get an overflow error, 
> >>this time when doing the shl.
> >
> >
> >How can I correct this ?
> >This bit of code was taken directly from server.pp in one of the old
> >2002 mailing threads...
> 
> Turn range checking off for this piece of code or do
> 
> function swapWord(w: Word): Word;
>   begin
>      swapWord := ((w SHL 8) OR (w SHR 8)) and $ffff;
>   end;

Least now I know the reason :)

cheers
James

> 
> 
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