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From: nparker@cie-2.uoregon.edu (Neil Parker)
Newsgroups: comp.sys.apple2.programmer
Subject: Re: /RAM
Date: 22 Jun 1996 10:21:57 GMT
Organization: University of Oregon Campus Information Exchange
Lines: 50
Message-ID: <4qghg5$8c3@pith.uoregon.edu>
References: <1996Jun19.234608.1@scvax2.shepherd.wvnet.edu>
NNTP-Posting-Host: cie-2.uoregon.edu

In article <1996Jun19.234608.1@scvax2.shepherd.wvnet.edu>
sh340001@scvax2.shepherd.wvnet.edu (mArK) writes:
>Where is the Aux. portion of the /RAM driver located in the ProDOS file?  If
>ProDOS is loaded at $2000, what's the address of the /RAM driver?  I need a
>copy of it because Micol destroys the Aux. part of the driver and I can't
>reenable /RAM, so I need to have a copy in order to restore the drive before
>enabling it.

An examination of 11 different versions of ProDOS reveals 5 different
locations for the /RAM driver.  If you *really* want to restore the driver
from the PRODOS file, a table is attached below that will tell you where to
look, but be aware that this is Something Mankind Was Not Meant To Know,
and using it will lock you into a specific version of ProDOS.

Before making use of the table, I strongly recommend looking into David
Empson's suggestion about saving and restoring the driver in a file.  You
might also want to consider whether it would be easier to simply reboot.

If you do use the table, remember that you need the version number of the
ProDOS file you're planning to load the driver from.  This may not
necessarily be the same as the ProDOS version that you booted, because the
user can switch disks without telling your program.  For this reason, you
should restore both the main and aux bank portions of the driver, to guard
against the possibility that your original main-bank-half may not be
compatible with your restored aux-bank-half.

As far as I can tell, the aux bank half of the driver is always 512 bytes.
The ProDOS installation code installs the main bank half by copying 153
bytes, but examining the code reveals that copying 130 bytes is sufficient
for all existing versions of ProDOS.

                        file offset of     file offset of
     ProDOS version    main bank driver    aux bank driver
     --------------    ----------------    ---------------
     1.0.1 thru 1.1.1        $C00               $A00
 (*) 1.2 exp                 $D00               $B00
     1.3 thru 1.7            $E00               $C00
     1.8, 1.9                $F00               $D00
     2.0.1, 2.0.3            $D00              $3100

(*) "1.2 exp" is "1.2 Experimental", which I found on the IIGS Sales Demo
disk (the one with the saxaphone music at the beginning).  I don't seem to
have a non-"experimental" version 1.2 anywhere in my disk collection.

               - Neil Parker
-- 
Neil Parker                       | No cute quote, no cute ASCII art, no cute
nparker@cie-2.uoregon.edu         | disclaimer, no deposit, no return....
nparker@cie.uoregon.edu           | (This space intentionally left blank:
http://cie-2.uoregon.edu/~nparker |                                           )