Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!darrenr
From: darrenr@munnari.oz.au (Darren James REED)
Newsgroups: comp.sys.amiga
Subject: Re: Amiga Exception Vectors
Keywords: Reset
Message-ID: <3338@munnari.oz.au>
Date: 13 Mar 90 23:43:50 GMT
References: <2780@castle.ed.ac.uk>
Sender: news@cs.mu.oz.au
Reply-To: darrenr@munmurra.UUCP (Darren James REED)
Organization: Comp Sci, University of Melbourne
Lines: 19


 
   Yes, you are right, there are a couple of overlay vectors that are provided by one of the chips, be it gary, agnus or whatever.  However it is only present (I assume) when those chips receive a RESET signal so at all other times, the first 8 bytes are then RAM.
   To check them out, the only possible way I know would be to (perhaps) use the following code:

   move.l  #Chk_Reset,$64.s  (trap intercept vector 0)
   trap    #0
   ....
Chk_Reset:
   reset              put external chips in RESET state
   move.w  #$2700,SR
   move.l  $0,a0      a0=reset address
   move.l  $4,a1      a1=stack pointer
   rte

   It is likely that the above routine will do nasty things to the amiga and may guru it.   As long as the other chips are in the reset state long enough for the above instructions to execute, it should work.
   It is a rather dirty way of doing it but given the nature of the 680x0 series, the only way.

Darren.