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From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: Programming questions
Message-ID: <2303@cbmvax.UUCP>
Date: Fri, 4-Sep-87 21:55:19 EDT
Article-I.D.: cbmvax.2303
Posted: Fri Sep  4 21:55:19 1987
Date-Received: Sun, 6-Sep-87 02:00:18 EDT
References: <4119@udenva.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 19

In article <4119@udenva.UUCP> pbrody@udenva.UUCP (Paul Brody ) writes:
> 
> What is the purpose of the memory overlay bit on the 8520 chip ? When he
> tries to use it he gets a Guru error of FF17, which is undocumented.

The OVL or overlay bit is used to map ROM over the video RAM starting at
address 0 to allow the 68000 to find it's way into the normal ROM location.
It is set high by the hardware reset and then the software sets it low
right after it jumps to the normal ROM address range.

It doens't make too much sense for a program to mess with this bit.  Either
you have to already be executing out of ROM, or you need to be executing
code in RAM copatible with that in the ROM at the corresponding addresses
when the floor drops out from under you.

-- 
George Robbins - now working for,	uucp: {ihnp4|seismo|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)