Strange address line on VIC-II

From: Gerrit Heitsch <>
Date: Thu, 16 Feb 2012 18:15:25 +0100
Message-ID: <>

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 2012-02-16 18:00:03

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