Re: Additional SIDs

From: Jim Brain <>
Date: Thu, 01 Dec 2011 21:46:06 -0600
Message-ID: <>
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
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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 Brain

       Message was sent through the cbm-hackers mailing list
Received on 2011-12-02 04:00:03

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