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From: pers@ifi.uio.no (Per Siljubergs}sen)
Newsgroups: comp.sys.mac.programmer
Subject: Access global variables from trap patches?
Summary: No good solutions?
Message-ID: <1990Sep24.231618.29949@ifi.uio.no>
Date: 24 Sep 90 23:16:18 GMT
Organization: Dept. of Informatics, University of Oslo, Norway
Lines: 53
Nntp-Posting-Host: gille.ifi.uio.no
Originator: pers@gille.ifi.uio.no


I have a few questions I can't find any good answers too.
I've looked everywhere, I hope. :-)

I'm writing an extension to the system. It includes INIT
code, VBL task and a couple of trap patches. I need global
data so that the INIT and the patches can communicate.
I would like to use A5 (the SAGlobals module from TN256)
or a relocatable block in the system heap. Anyway, I need
to store A5 or a handle to my globals between the calls
somewhere.

The usual tips regarding stand-alone code don't seem to
help me. Say one of my patches need access to the globals:

   1) The patch can't get it's hands on my A5 world or a
      handle to the globals through the parameters, because
      they (the parameters) must be the same as the trap.

   2) I can't set up A5 before the patch gets called. I have
      no way to predict when the system or the applications
      will call the trap.

Some solutions have come up. They are not very elegant,
I think:

   1) Save the A5 or the handle directly into my stand-alone
      code resource. Will this work with new processors like
      68030 and 68040? These processors have intruction
      cache on the chip. Isn't this solution almost like
      selfmodifying code?

   2) Use a driver for the communication. Overkill?

   3) System 7.0 IPC? Is it possible? Even if it is, I would
      like this to work with System 6.x.x too.

   4) A hack I read in Joel West's book "Programming with
      MPW", is to store the globals as an unpurgeable
      resource in the sys heap at INIT time. The type and ID
      of the resource is known, so my patch rutines can get
      access by a call to GetResource. Will this work at
      interrupt time?

What is the best solution? Other solutions? Please, I need
all the help I can get. I think many others would like some
info on this too.


-Per

Per Siljubergsaasen, Dept. of Informatics, University of Oslo, Norway
EMail: pers@ifi.uio.no