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From: norlin@uokmax.ecn.uoknor.edu (Norman Lin)
Newsgroups: comp.sys.atari.8bit
Subject: XL/XE RAM
Message-ID: <1989Oct18.155242.31230@uokmax.ecn.uoknor.edu>
Date: 18 Oct 89 15:52:42 GMT
Reply-To: norlin@uokmax.UUCP (Norman Lin)
Organization: University of Oklahoma, Engineering Computer Network, Norman, OK
Lines: 34


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

If by "device registers" you are referring to what I call "hardware registers"
located in memory beginning at $D000, then no, the device registers are NOT
shut off.  These memory locations have no RAM; thus, it is technically
inaccurate, for instance, to say that the 800XL has 64K RAM; it actually has
only 62K RAM.  That means that when you shut off the OS to get to the RAM
underneath, your extra 14K (not 16K) will have a nice big gap right in the
middle of it.

As to your second question, yes, the device registers are accessed in the same
way as when the ROM is on.  Be aware, of course, that when the ROM is off,
no system interrupts are processed, and stuff like changing the screen colors
and such will be done directly through the hardware registers.

Now, I'm talking about the 800XL.  I have little experience with the 130XE,
but I believe its architecture is identical (except that the extra bits
in the $D301 memory switch are used to control extra 16K banks, but the
hardware register locations still have no RAM underneath, so I think the
130XE has only 126K RAM), so the information should be equally applicable
to both the 800XL and the 130XE.

Hope this answers your question.  Switching out the OS to get to the RAM
underneath, though, is not usually worth the trouble.


                                    --Norman Lin (A.K.A. NARC)