On 12/1/2011 4:40 PM, Segher Boessenkool wrote: >> Strikes me $D7xx is the best place for an internal SID modification, >> being as that is spare in the C128 and actually decoded. No idea who >> decided $D420 was a good idea, that takes way more in the way of >> address decoding. > > On a C64, there is no spare chip select you can use (you cannot use #IO1 > or #IO2, de00 resp. df00, since cartridges already use those). So you > split #SID into two using one of the address pins. A8 is traditional > (putting the second SID at d500), A5 (d420) of course also works fine. > A8 makes the second SID available at d700 as well, nice for C128 > compatibility. > > > Segher > > > Message was sent through the cbm-hackers mailing list If a PAL/GAL or CPLD option was a consideration, I'd vote for a new register at $d41f that sets the IO space for both (or more) SIDs. The upper 4 bits would be the SID#2 high nybble, while SID#1 would be low nybble. You could create a register "knocking" setup to prevent unwanted movements. Jim -- Jim Brain brain@jbrain.com www.jbrain.com Message was sent through the cbm-hackers mailing listReceived on 2011-12-02 04:00:03
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