Re: VIC-registers

From: Marko Mäkelä (
Date: 1998-02-25 22:06:15

On Wed, 25 Feb 1998 wrote:

> In my case writing to $D031 puts the 6510 in DMA-mode. The question is
> how many and what kind of PRGs use the RAM under the I/O area

The answer is that most crunchers move the crunched data to top of memory
(so that it ends at $ffff) and decrunch it from there backwards.  You
cannot assume that the RAM at $d000-$dfff won't be modified.  Even if most
programs themselves wouldn't access this area, the decruncher will.  And
to my knowledge, GEOS does use this area as well.

> So if anybody else has another idea of creating an extra I/O area not
> using IO1 and IO2 and wants to share it with me, (s)he is welcome. 

The $d700-$d7ff area is quite safe, it's free on the C128 as well.  But no
matter which area you choose, it is a good idea to make the expansion
software-disablable, to make it possible to disable it e.g. with one POKE. 
My memory expansion works this way (I wired the 6821's IRQ line to one of
its active-high chip selection lines), and I can select the C128 and C64
mode KERNALs with the MMU lines -GAME and -40/80, respectively.  (I
removed the misfeature of switching to C64 mode when -GAME or -EXROM are
active during boot.)


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