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Future software may allow desk accessories to have their own globals by
changing register A5 when the accessory is entered and exited. This can cause problems
for applications that patch traps without following certain rules.
[Jun 01 1986]
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Introduction
If your
application patches any traps, it's important that the patches not depend on
register A5. This is because you may have intercepted a trap used
by a desk accessory.
If you need access to your globals within your patch, you can save A5
(on the stack, perhaps), load A5 from the low-memory global CurrentA5
(this is guaranteed to be correct for your application), do whatever you have to do
within your patch, then restore A5 on the way out. Note that if you make
any traps within your patch (or call the "real" version of the routine
you patched), you should restore the caller's A5 before
doing so.
There are several ways of depending on A5 within a patch that you
should watch out for:
- Are you making any references to your global variables, or those of any units
that you're using, such as
thePort from QuickDraw? These are accessed
with A5-relative references with negative offsets.
- Are you making any inter-segment subroutine calls? These are accessed with
A5-relative references with positive offsets.
- Are you using any system calls (either traps or "glue" routines) which will
depend on
A5 during their execution? In this case, you need to be sure that you restore
the caller's A5 before executing the call.
To be safest, patched traps should follow the same rules as interrupt handlers.
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Note:
In general, applications should not have to patch any traps, and
risk compatibility problems if they do! If you'd like help in removing your dependence on patching,
please contact Macintosh Developer Technical Support.
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References
The Operating System Utilities
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Downloadables
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Acrobat version of this Note (44K).
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