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From: james@uw-atm.UUCP (James M Synge)
Newsgroups: comp.sys.amiga
Subject: Re: Register state in an Exception Handler
Message-ID: <48@uw-atm.UUCP>
Date: Sat, 8-Nov-86 23:19:29 EST
Article-I.D.: uw-atm.48
Posted: Sat Nov  8 23:19:29 1986
Date-Received: Mon, 10-Nov-86 00:55:17 EST
References: <2210@mtuxo.UUCP> <46@uw-atm.UUCP> <971@cbmvax.cbmvax.commodore.COM>
Distribution: net
Organization: Dept. of Atmospheric Sciences, U. of Washington
Lines: 32

In article <971@cbmvax.cbmvax.commodore.COM>, higgin@cbmvax.commodore.COM (Paul Higginbottom) writes:
> In article <46@uw-atm.UUCP> james@uw-atm.UUCP (James M Synge) writes:
>
> >I've tried writing an exception handler and had some problems.  One thing
> >I notice you're not dealing with in your assembly section is the fact that
> >you don't know what state your registers are in, except D0 and A0, which
> >contain the signal mask and address of your ExceptData.  SO, when your
> >code is adding 1 to hadIntr, you don't know where that 1 is being
> >added.  It is some offset from (A4), but, since A4 is in an unknown state,
> >we don't know where that is.
> >
>
> This is not really true.  The "rules" of the OS dictate that routines
> leave registers untouched except d0/d1/a0/a1 which are used for passing
> args and results and cannot be guaranteed to be munged during some call.
> Thus, a4 should always be in a healthy state for a program's assembler
> sections to access any variables in the C or assembler parts.

  Lets get this clear: The rules do indicate what the state of your registers
will be after a call you make returns.  The rules DO NOT define the state
of any registers other than D0 and A1 on entry to an exception handler.

(when I said D0 and A0 in my previous posting I didn't have my manuals with me).

All the registers which defined the state of your task previously are pushed
onto the stack before your exception handler is called.
-- 
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James M Synge, Department of Atmospheric Sciences, University of Washington
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