On Wed, 25 Feb 1998 email@example.com 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.) Marko
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