"Richard Atkinson" <rga24@cantab.net> writes: > No idea who > decided $D420 was a good idea, that takes way more in the way of address > decoding. Obviously easier in this day of programmable logic devices, > but what about a good old 74LS139 for decoding $D400 further? If you don't mind mirrors of both SIDs every 0x40 bytes and you're adding the second SID internally[1]: SID1_CS = SID_CS | A5 SID2_CS = SID_CS | !A5 Let's say you have just one 7400 (quad NAND) available. First invert both incoming signals: !A5 = A5 nand A5 !SID_CS = SID_CS nand SID_CS then you can generate the CS signals for the two SIDs with the remaining two NANDs: SID1_CS = !SID_CS nand !A5 SID2_CS = !SIG_CS nand A5 -ik [1] I don't see any simple way to determine if an access to $D4xx is meant for the SID, the character ROM or RAM Message was sent through the cbm-hackers mailing listReceived on 2011-12-01 22:00:33
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