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From: ranger@ecsvax.UUCP (Rick N. Fincher)
Newsgroups: net.micro.apple
Subject: Re: Apple 2E/enhanced monitor rom
Message-ID: <1786@ecsvax.UUCP>
Date: Mon, 7-Jul-86 09:21:15 EDT
Article-I.D.: ecsvax.1786
Posted: Mon Jul  7 09:21:15 1986
Date-Received: Tue, 8-Jul-86 07:26:48 EDT
References: <222@neoucom.UUCP>
Organization: UNC Educational Computing Service
Lines: 41

> 	Everything was fine when we used regular Apple 2s and 2Es.
> Now our application bombs, as the monitor ROM (under DOS 3.3) seems
> to do something that regularly shuts off interrupts for 250-300 uS.
> 
> support.  What I was wondering, does anybody know if I can
> de-enhance my computer by putting old ROMs in it?  Anybody happen
> to know if the EF and CD ROMs can be replaced with 2764s?  It would
> save us a lot of time to burn our own.
> 

   This is an odd problem since the manual for the new ROMs claims that
the new monitor does not disable interrupts for an extended period. 
The old ROMs would disable interrupts for extended periods while 80 col
screen updating was going on because the old ROMs couldn't tolerate
interrupts except in main memory.  The new Roms support interrupts in
Aux memory by saving the memory state in effect when the interrupt
occurred and setting the machine to a known state before going to the
interrupt handling routine.  This shouldn't take anywhere near 250-300
usecs though.  With the new ROMs the accumulator is no longer saved in
location $45, it is put on the stack instead.  This location is used by 
DOS 3.3 and the monitor so storing the accumulator here trashed things.
Your application probably uses a modified version of DOS to avoid this proble  
problem.  If your application is looking for the contents of the
accumulator at $45 it will not work with the new ROMs.

You should have gotten the manual  "About your enhanced Apple IIe:
Programmers Guide" when you upgraded, if not see you dealer for a 
copy.  It will help you modify your code to work with the new 
interrupt handler.  If you are using commercial software call the
manufacturer and see if they have a new version.

   If you really want the old ROMs back, see Don Lancaster's column
"Ask the Guru" in Computer Shopper.  From about January to May of
this year he had a series of articles on installing both sets of
ROMs in your IIe and switching between them.  He had detailed in-
structions on burning ROMs for the Apple. (Sorry I can't be more 
precise on the dates of those articles, but you can call Don for
help, his number is in his column).

Rick Fincher