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From: 850747c@aucs.uucp (Vitamin)
Newsgroups: comp.sys.atari.8bit
Subject: Re: XL/XE RAM
Summary: Using the RAM under the ROM
Message-ID: <1989Oct25.011529.13507@aucs.uucp>
Date: 25 Oct 89 01:15:29 GMT
References: <47017@bbn.COM> <1367@clyde.Concordia.CA>
Reply-To: 850747c@aucs.UUCP (Vitamin)
Organization: School of Computer Science, Acadia Univ., Nova Scotia
Lines: 21

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
>
The address bus is wired so that if you try to access a register from
$D000-D7FF, you access a device register instead, regardless of the way
the memory 'windows' are configured.  Therefore, only 62K is available
at any time.  (If you have >64K, the extra memory is accessed through
a memory window at $8000.)  You might want to switch off all interrupts,
and copy the OS ROM in segments to the RAM underneath, then reenable the
interrupts, so you can still leave the core of the O.S. running, and 
overwrite parts of the O.S. that you won't be using (like the floating 
point routines, or the printer or cassette drivers, for example).  This
seems to me to be a bit more practical, since unless your program is 
purely computational, you will probably want some sort of interface
between the user and the computer, and if the O.S. is gone, displaying
things becomes a bit complicated.
                                                         -Chris