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From: unierik@uts.uni-c.dk (Erik Bertelsen)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Unsafe use of an object field as a var or > 4 byte parameter
Message-ID: <UNIERIK.91Mar20213111@uts.uni-c.dk>
Date: 20 Mar 91 20:31:11 GMT
References: <4749@syma.sussex.ac.uk>
Sender: unierik@uts.uni-c.dk (Erik Bertelsen)
Followup-To: comp.sys.mac.programmer
Organization: UNI-C
Lines: 12
In-Reply-To: paulr@syma.sussex.ac.uk's message of 19 Mar 91 14:09:17 GMT

When calling a procedure with a non-VAR parameter larger than 4 bytes,
the address of the value is pushed on the stack and the entry code within the
procedure makes a copy of the parameter when the procedure is called.
This is the problem if the procedure call causes a segment load which
relocates the object whose field is passed as a parameter.

{$push}{$h-}  doit {$pop} is safe when the called procedure is in the same
segment as the caller or in a resident segment.

Regards
Erik Bertelsen
UNI-C, The Danish Computing Centre for Research and Education.