Re: MAX Machine PLA equations

From: Segher Boessenkool <>
Date: Sun, 7 Aug 2016 12:40:34 -0500
Message-ID: <>
On Sun, Aug 07, 2016 at 12:51:40PM +0100, smf wrote:
> On 06/08/2016 02:20, Segher Boessenkool wrote:
> >which seems sane, except for the VIC accesses to VIC and CIA.  What
> >drives A15,A14 during such accesses, anyway?

> If 
> you try then you'll most likely get some open bus artifacts on the bus, 
> with the possibility of occasional bytes read from CIA (which may be bad 
> news for the TOD and interrupt handling).

Yes, that.  Programming the VIC to read from VIC or CIA addresses is bad
news all around (and not useful in any case).

> The main difference 
> here is that VIC itself knows it's doing the bus access, so it could 
> ignore it.

But it doesn't.

> Quite what will happen if it doesn't ignore it, as it will be 
> trying to drive the bus while reading from it, which it most likely was 
> never designed to do.

That works fine.  Address and data bus are completely separate.  VIC
knows at all times whether to drive the address resp. data bus.  It does
not care who is driving its input signals.


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Received on 2016-08-07 18:00:03

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