Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!bloom-beacon!mintaka!think!samsung!aplcen!haven!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga
Subject: Re: Amiga Exception Vectors
Message-ID: <19459@grebyn.com>
Date: 13 Mar 90 21:36:16 GMT
References: <2780@castle.ed.ac.uk>
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 22

In article <2780@castle.ed.ac.uk> aiajms@castle.ed.ac.uk (-=Andy=-) writes:
>
>I am currently designing a mini operating system round the 68000 here at
>Uni. I thought I might have a look at the Amiga Exception vectors and
>found to my suprise that the system stack pointer seemed to be directed
>to location 0, ie on top of itself. The PC pointed to an area of memory
>in my fast ram which seemed fair enough but several of the other vectors
>seemed to be pointing to the area between the Amiga ROM and the top of
>the 68000 memory map, which according to the fabby (ahem) Amiga Manual
>does not exist! It does however mention that the exception table has
>some sort of overlay and I notice from the Gary chip that it has some
>sort of Overlay input, presumably for some weird sort of decoding.

	The memory map of the Amiga looks different between normal
runtime and just after a reset.  That "overlay" signal you see tells the
Gary chip to place the ROM beginning at address 0, which is the reset
condition.  After powerup, the ROM startup routines negate the 'overlay'
signal, ROM moves to
$FC0000, and chip RAM appears at 0.  So therefore, when you peek
location 0 (Reset SSP) and 4 (Reset PC), what you see is not really used
at reset time.  To see the real SSP and PC, you should peek the first
two long words in ROM.