Re: Strange address line on VIC-II

From: Gerrit Heitsch <>
Date: Fri, 17 Feb 2012 17:48:28 +0100
Message-ID: <>
On 02/17/2012 08:18 AM, Bil Herd wrote:
> 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".

Well, the thing is, this address line doesn't need to be tristate, that 
part is handled by the 74LS258. Also, the rest of the C64 handles A7 
(the line that is not multiplexed) the same as A6 (the line that is 
multiplexed with '1'). The moment the '1' appears on the pin, the 
74LS258 will have switched to the address bits generated by the CIA.

In other words, the A6 line could have been made to behave like A7 but 
wasn't and at least the circuit of the C64 doesn't give any hint about 
the 'why'.


       Message was sent through the cbm-hackers mailing list
Received on 2012-02-17 17:00:13

Archive generated by hypermail 2.2.0.