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From: jean@maxwell.Concordia.CA ( JEAN GOULET )
Newsgroups: comp.sys.atari.8bit
Subject: Re: XL/XE RAM
Summary: some suggestions
Message-ID: <1367@clyde.Concordia.CA>
Date: 17 Oct 89 17:44:21 GMT
References: <47017@bbn.COM>
Sender: sccs@Clyde.Concordia.CA
Reply-To: jean@maxwell.Concordia.Ca ( JEAN GOULET )
Organization: Concordia University, Montreal Quebec
Lines: 22

In article <47017@bbn.COM> slackey@BBN.COM (Stan Lackey) writes:
>I am a neophyte in terms of using the RAM under XE ROM.  Can someone
>tell me, when ROM is shut off, are the device registers shut off too?
>Do you get a linear space of 64K of just RAM?  If not, are the device
>registers accessed in the same way as when ROM is on?
>Thanks.  -Stan

Stan, the address space between $D000 and $D7FF is always reserved for the
device registers, and thus the RAM in that address space is unaccessable.
Thus, you can only use 62K RAM in a '64K' 800XL, and 126K in a '128K' 130XE.

I might add that if you plan to use the 'hidden' RAM, don't forget that
when you switch out the OS, the character set disappears with it, as do
the interrupt routines.  One way to handle this is to copy the ROM character
set to the RAM beneath, and disable all interrupts using SEI, and POKE NMIEN
($D40E) with the necessary value (and the same for IRQEN - $D20E).

                                             Jean Goulet
                                             Electrical Engineering
                                             Class of '89
                                             Concordia University
                                             Montreal, Canada