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From: siegel@endor.harvard.edu (Rich Siegel)
Newsgroups: comp.sys.mac.programmer
Subject: Re: StripAddress...
Message-ID: <1461@husc6.harvard.edu>
Date: 2 Feb 90 08:07:00 GMT
References: <1990Feb2.010425.28126@oracle.com>
Sender: news@husc6.harvard.edu
Reply-To: siegel@endor.UUCP (Rich Siegel)
Organization: Symantec Language Products Group
Lines: 40

In article <1990Feb2.010425.28126@oracle.com> gstein@oracle.uucp (Gregory Stein) writes:
>[ I posted this once before and received no response, so I'm trying
>  again... ]
>
>Geez, it seems like StripAddress needs to be called *everywhere* if
>you want to be 32-bit clean.  If you maintain any kind of information
>within a handle, it appears that you are required to call StripAddress
>to access that information.  Now, sure, I can dereference, strip it,
>and store it so that I don't have to call StripAddress again (say, in
>a sequence of stores), but as soon as I call a routine that moves
>memory, I'll have to do it again.

	Not quite. You should call StripAddress if you want to be sure
that the address you get by dereferencing a handle is clear of any
extra information such as flag bits. This is useful if you're planning to
do any kind of address arithmetic. However, in the normal case, you
can double-indirect a locked (or purgeable, or resource) handle and 
still get the right thing, because the system only uses the low-order
24 bits of whatever addresses you use.

	When the system runs in 32-bit mode, the flag bits will be kept
in some place other than the high byte of the handle, since the system
will use all 32 bits of the address. This means that you can still
single-and double-indirect handles safely, but you should NOT use
$00FFFFFF as a mask to get rid of the flag bits, since you'll smash
the upper 8 bits of a 32-bit address and end up with a pointer to hell. :-)


R.
~~~~~~~~~~~~~~~
 Rich Siegel
 Staff Software Developer
 Symantec Corporation, Language Products Group
 Internet: siegel@endor.harvard.edu
 UUCP: ..harvard!endor!siegel

"When someone who makes four hundred and fifty dollars an hour wants to
tell you something for free, it's a good idea to listen."

~~~~~~~~~~~~~~~