Path: utzoo!censor!becker!douglee
From: douglee@becker.UUCP (Doug Lee)
Newsgroups: comp.sys.amiga.tech
Subject: Re: OVL line in Amiga 2000
Message-ID: <813@becker.UUCP>
Date: 24 Aug 89 05:45:45 GMT
References: <8908181206.AA07159@jade.berkeley.edu>
Reply-To: douglee@becker.UUCP (Doug Lee)
Organization: G. T. S., Toronto, Ontario
Lines: 35

In article <8908181206.AA07159@jade.berkeley.edu> U211344@HNYKUN11.BITNET (Olaf 'Rhialto' Seibert) writes:
>I bought a new Amiha 2000 this week, and looking through the schematics
>in the back of the (German) user's manual, I noticed the familiar
>(from my A1000) memory overlay line (OVL). It went from a CIA (as
>is to be expected) to GARY.
>
>This makes me wonder. The 2000 doesn't need a boot rom, does it? The
>start of the kickstart ROM has the same structure as the boot rom
>in the 1000. (I know; I disassembled it). Does the OVL line maybe
>map a PART of the kickstart rom to location zero? Or is there a
>boot rom anyway?
>
The reason for this is that the boot rom code must start at location zero.
When you reset the 68000, this causes an exception to be generated, which
looks for a jump vector at location 0. The first two words point to the
supervisor stack pointer and the last two words to the program counter.
I realize this is ambiguous as the last two words are not necessarily
the second set of two, but this is paraphrased from the Motorola manual 
itself.

When the machine is turned on, the OVR line causes the boot rom to be mapped 
into location zero. as soon as things get going, this bit is toggled by a write 
to high ram and you once again have ram at location zero.
This is all necessary because the exception vectors go frm location zero
to $255. On the Amiga, you want to be able to program these, so ram should start
at 0 once it's booted. All this good stuff about the 68000 is described in
the MC68000 Programmer's reference manual Pg 35-40 5th edition.
	    
>        Olaf.Seibert;


-- 
Doug Lee
douglee@becker
416-461-5357