Re: 74ls612, clocking 6502and others...

From: Andre Fachat (
Date: 1998-04-22 01:02:53

Marko Mäkelä wrote:
> > Then explain me the $DE00/$DFFF trick of Marko :-)
> Explain it to me too. :-)  By the way, a similar effect occurs also with
> the 1541 and with the VIC-20.  Haven't you ever wondered why reading from
> a non-existing address returns the page number, e.g. reading from $20xx
> returns $20?  Could it by any means have anything to do with the fact that
> the high-order byte of the address was on the data bus right before the
> read operation? :-) But why does it work?  The VIC-20 and the 1541 have

Does that work with indexed access as well? I mean, trying to read
something from a non-existant page with an overflow when adding the 
index register. Does this read the high-byte of the address or the value 
of the address one page below the actual read (it is read before the 
overflow carry is added to the high byte of the address)?

If it is the second one, I assume it's something to do with capacitance,
i.e. the internal (input) pullups cannot get the lines high in time 
without active (output) NMOS pullups. The signal lines have a capacitance
and the together with the resitors you get an exponential rise with 
tau ~ (R * C).This would also explain why more
than a few components on the bus then suddenly change the value read
to $ff (I read the address high-byte on the bus with only a few cards in my
CS/A65 computer, but with more cards I read $ff instead - when R gets 
small enough - more components have more pullups in parallel - the time
till high is read gets smaller and and a high is read.)


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Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG

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