Re: Additional SIDs

From: Ingo Korb <>
Date: Thu, 01 Dec 2011 22:47:25 +0100
Message-ID: <>
"Richard Atkinson" <> 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

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

[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

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Received on 2011-12-01 22:00:33

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