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From: commodorejohn on 15 Apr 2010 20:36
Okay, this has been bugging me for a while. Is it possible to access
the RAM that's underneath the character ROM and register area at $D000-
DFFF? _Mapping The Commodore 64_ hints that it is possible, but
doesn't say exactly how to do it, and the only control I can see for
this area is the I/O-ROM select in register $01. It's not *vitally*
important, but it would be nice if this area were available for use.
Anybody know the answer?
From: Joe Forster/STA on 15 Apr 2010 21:13
> Is it possible to access
> the RAM that's underneath the character ROM and register area at $D000-
> DFFF? _Mapping The Commodore 64_ hints that it is possible, but
> doesn't say exactly how to do it, and the only control I can see for
> this area is the I/O-ROM select in register $01.

See the top of my C64 memory map at http://sta.c64.org/cbm64mem.html .
Clear the low three bits of memory address $0001 and you'll see the
RAM at $D000-$DFFF.
From: commodorejohn on 15 Apr 2010 21:26
Wow, interesting. Having to have _everything_ switched out puts some
limits on what you can do with it, but fortunately it'll work nicely
for my purposes. Thanks for your help!
From: Robert Roland on 16 Apr 2010 04:27
On Thu, 15 Apr 2010 18:26:01 -0700 (PDT), commodorejohn
<commodorejohn(a)gmail.com> wrote:

>Having to have _everything_ switched out puts some
>limits on what you can do with it, but fortunately it'll work nicely
>for my purposes.

You can copy ROM to RAM before you bank the ROM out. That way,
everything will work normally until you start messing with it.

Writing to a ROM address will write to the RAM underneath, even if the
ROM is banked in.
--
RoRo
From: Martijn van Buul on 16 Apr 2010 07:16
* Robert Roland:
> You can copy ROM to RAM before you bank the ROM out. That way,
> everything will work normally until you start messing with it.

ISTR that the standard IRQ routine assumes I/O is mapped in, though; if
RAM is mapped in instead you'll be facing problems first time an interrupt
hits.

--
Martijn van Buul - pino(a)dohd.org
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