Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!sunybcs!sbcs!vallon
From: vallon@sbcs.sunysb.edu (Justin Vallon)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Segmentation (was Re: ARRGH (Strings and things))
Message-ID: <3813@sbcs.sunysb.edu>
Date: 2 Nov 89 20:18:58 GMT
References: <16004@netnews.upenn.edu> <8835@hoptoad.uucp> <16420@dartvax.Dartmouth.EDU> <8852@hoptoad.uucp> <14386@well.UUCP> <8878@hoptoad.uucp> <1989Nov2.053139.23967@agate.berkeley.edu>
Sender: news@sbcs.sunysb.edu
Reply-To: vallon@sblw.UUCP (Justin Vallon)
Organization: State University of New York at Stony Brook
Lines: 38

In article <1989Nov2.053139.23967@agate.berkeley.edu> lippin@math.berkeley.edu writes:
>Recently tim@hoptoad.UUCP (Tim Maroney) wrote:
>>You also ignored the function pointer issue.  I received a letter from
>>a person at Apple that I respect and who is usually right, but not this
>>time.  He says that you should put all function-pointer fetching
>>routines into the main code segment: that way, they will be jump table
>>pointers.  This may work fine for code that uses one or two function
>>pointers and calls them all itself.  However, just imagine passing a
>>function pointer into the jump table to the vertical retrace manager or
>>as a completion routine to the file manager.  Can you say "intermittent
>>crash when A5 is not CurrentA5"?  I knew you could.
>
>Both of you got this one wrong.  Any development system worth its
>salt, and certainly any I've used, will create function pointers as
>pointers into the jump table, precisely so that they're always valid.
>They're truly absolute pointers, so they don't care one whit about
>CurrentA5 (although your code might; that's the well-known gotcha of
>interrupt tasks, and has established solutions.)
>

I seem to sense some confusion here.  I think you two are arguing about
two different things.  Tim said "put all function-pointer FETCHING
routines into the main code segment".  I think this is a little wrong.
I think person at Apple said, or meant to say, that you should put
any function that will be referenced by a function-pointer into the
main segment.  When the main segment is loaded (when you are launched),
all main-segment jump-table entries will be set at the time of _LoadSeg.

Remember that when _LoadSeg is called, ALL jt entries that reference that
segment are updated to a single JMP to the address of the routine in the
locked code segment.  Since you never UnloadSeg(main-seg), these jt entries
will never require a LoadSeg, or A5 to get to your routine in the always-
loaded main segment.

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-Justin
vallon@sbcs.sunysb.edu
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