Re: Prodos Woes

[Jeff Blakeney <CUTjblakeney@sympatico.ca>]

On Sun, 14 Dec 2003 07:24:35 GMT, "Bender MX" <no@morespam.com> wrote:

>  I was trying to 'Bsave' and 'Bload' lo-res / text screens
>under Prodos (emulator) tonight ...
>
>It's a No-Go - what gives?  Does Prodos not allow
>Bload filename,a1024  ???

Like others have already mentioned, you've run into ProDOS' memory
protection system.

I wrote an article that was published in the spring 1996 issue of II
Alive magazine about this and how to circumvent it.  The reason I even
researched this was because I was trying to convert some older, single
file DOS 3.3 games onto a 3.5" ProDOS floppy and some of them tried to
load themselves over the text screen.  I found out what caused the "NO
BUFFERS AVAILABLE" error and wrote a program launcher in Applesoft
that gave me the option to deprotect the text screen before launching
a program to avoid it.  The program launcher I wrote is called
Selector v1.0 and is on the ground FTP site and maybe one or two
others as well.

The quick and easy way to disable just the text screen under ProDOS is
to do a POKE 48984, 192.  This leaves all the other areas protected
that ProDOS protects by default (those being page $00, $01 and $BF) by
modifying ProDOS' memory protect bitmap which is at $BF58 to $BF6F.

However, to do it properly you should only modify the bits that you
are really concerned about.  In this case it is bits 0 to 3 of byte 0
of the bitmap.  This can be done easily in assembly using this code:

     LDA #$BF58
     AND #$F0
     STA #$BF58
     RTS

To do it in Applesoft is a little more involved and I won't bother
typing it in here unless someone is really interested in seeing the
code I hacked together years ago.  :-)

-- 
 Jeff Blakeney - Dean of the Apple II University in the
                 Apple II Community on Syndicomm.com
 CUT the obvious from my address if you want to e-mail me