RE: Strange address line on VIC-II

From: Bil Herd <>
Date: Fri, 17 Feb 2012 02:18:00 -0500
Message-ID: <>
Without getting out the schematic I would say that the VIC addresses on
the bus are also a function of the BA line and so has a tristate state in
addition to the driven state where the processor is active.  During
Tristate the A6 line comes from the processor and so still needs to go
through the MUX "normally".  If A6 wasn't needed by the VIC then it would
make "clean" sense to drive it as floating would be disastrous and a weak
pullup really doesn't work at speeds as it would have to go from driven
low to floating high in time for /RAS, meanwhile when it floats through
the linear region of the MUX's they might "sing".

Actually you don't have to think of it as being driven high, it's driven
to the correct address for VIC operation which turns out to always be a
"1". :)

-----Original Message-----
[] On Behalf Of Gerrit Heitsch
Sent: Thursday, February 16, 2012 12:15 PM
Subject: Strange address line on VIC-II


the address lines on VIC-II as used in the C64 are a mix of multiplexed
(A0-A5/A8-A13) and static (A7-A11). That's easy enough to explain from the
way the memory is set up.

But then there is A6 which is, according to the datasheet, multiplexed
with a static '1'. From the schematics that doesn't make any sense since
it's routed through the 74LS258 just like A7 and as soon as A6 switches to
a static '1' the 74LS258 switches too and the 2 address bits supplied by
the CIA are valid.

Is there a known reason why it was designed that way? Was it maybe used in
the CBM-II series in some way?


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Received on 2012-02-17 08:00:16

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