Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!keith
From: keith@Apple.COM (Keith Rollin)
Newsgroups: comp.sys.mac.programmer
Subject: Re: StripAddress...
Message-ID: <38276@apple.Apple.COM>
Date: 2 Feb 90 03:33:51 GMT
References: <1990Feb2.010425.28126@oracle.com>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 52

In article <1990Feb2.010425.28126@oracle.com> gstein@oracle.uucp (Gregory Stein) writes:
>
>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.
>
>I've been programming the Mac for almost six years now, but this whole
>issue has me kind of stymied.  Am I missing something here?  For a
>concrete example, let's define a structure like this:
>
>    struct foo {
>	int a, b, c;
>    } **bar;
>    bar = (struct foo **) NewHandle(sizeof(struct foo));
>
>Now, when I say "(*bar)->a" is this 32-bit clean?  For this example
>and the current system software it is because the flag bits in the
>handle represented by bar are zero, so there is no problem (the object
>is not locked, purgable, or a resource).  What happens if I do
>something like:
>
>	HLock((Handle)bar);
>	(*bar)->a = f();		/* f() and g() move memory */
>	(*bar)->b = g();
>	HUnlock((Handle)bar);
>
>Now, is this fragment still 32-bit clean?  I would assume it isn't
>because when the dereference (*bar) occurs, I pick up the flag bit in
>the master pointer.  

Of course it's 32-bit clean. In 24-bit mode, the hardware effectively ignores
the high bits, so we used them for tag bits. In 32-bit mode, we naturally can't 
use the high byte for flags, or you would still be limited to a 24-bit address 
space. So when you are in 32-bit mode, the Memory Manager puts the bits 
elsewhere, freeing up the upper byte for address lines (there's actually a 
parallel 512 megabyte space in machines with the 32-bit Memory Manager, so 
that we can store the tag bits there  :-)

You only have to use StripAddress if you need to do master pointer comparisons
or pointer arithmetic (e.g. subtract two pointers to find the size of data
between them).

-- 
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"Argue for your Apple, and sure enough, it's yours" - Keith Rollin, Contusions