Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!linus!philabs!cmcl2!harvard!topaz!uwvax!puff!lishka
From: lishka@puff.UUCP (Christopher Lishka)
Newsgroups: net.micro.cbm
Subject: Re: Turning BASIC ROM into RAM...
Message-ID: <904@puff.UUCP>
Date: Tue, 6-May-86 13:23:32 EDT
Article-I.D.: puff.904
Posted: Tue May  6 13:23:32 1986
Date-Received: Thu, 8-May-86 08:03:18 EDT
References: <2167@caip.RUTGERS.EDU>
Organization: U of Wisconsin CS Dept
Lines: 62
Summary: yes, it is, but you must copy the rom to ram

In article <2167@caip.RUTGERS.EDU>, KOCYAN@BLUE.RUTGERS.EDU writes:
> From: KOCYAN@BLUE.RUTGERS.EDU
> 
>    On the Commodore 64, is it possible to turn the BASIC ROM between addresses
> $A000 and $BFFF into RAM, and yet keep the BASIC routines there so that they
> can be modified?
>    I've tried changing the data-direction-register (location $0000), but that
> didn't seem to help at all.
>    Any advise would be greatly appreciated.  Thanks in advance.

	Yes, it is possible to do this.  However, the Ram that lies beneath the
BASIC Rom is empty...i.e. if you flip out BASIC ROM you have nothing below it
to tell the machine what to do!

	The solution is pretty simple...before you flip out the BASIC Rom, make
sure you copy the Rom to Ram.  This will work by just peeking and poking
because of a funny quirk (which was intentional) in the c-64: when there is Rom
on top of Ram, any peek to a location in tha area yeilds what is in Rom;
however, if you poke to the same location the value will be stored in the Ram
underneath the Rom, even though you can't 'see' it while the Rom is still
there.

	The practical value of this nifty trick is that you can transfer any
Rom to the Ram beneath it (and there is Ram beneath ALL Rom on the c-64)
by just:

	a) making sure that Rom is flipped in

	b) performing the following operation in BASIC (although this will
	   work in PASCAL and C and ML):

	   poke x, peek(x): rem x is memory location

After you have copied the range you want (with a loop), just flip out the Rom
and...VOILA!  You now have what was in Rom encoded in Ram, just waiting to be
changed! (Isn't this fun?)  

	Anyways, this can be done with the KERNAL and character sets too.  To
flip in/out the Rom you need to poke to location 1 in memory...consult your
handy reference manual for more details (I don't have mine here with me).  If
you have any other problems, just email to me (using reply or the addresses 
below...hopefully they'll work).


		lishka@rsch.wisc.edu

			or

		talcott!uwvax!puff.wisc.edu!lishka@rsch.wisc.edu

			or
	
		uwvax!puff.wisc.edu!lishka@rsch.wisc.edu


p.s. my apologies if those pathes don't work...I'm still trying to desperately
     make sense out of this mailing sytem (and I don't have any documentation)
-- 
				Chri Lishka
				U.W. Madison

	-my views are not my own; rather, they are those of my pets.