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

From: Andrew Vardy (
Date: 1998-04-22 17:39:29

On Wed, 22 Apr 1998, Andre Fachat wrote:

> 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

Also, the same thing occurs with the C128 in 2Mhz aka Fast mode.  At
least, if I unplug my Swiftlink.

> 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)?

$D700-D7FF is another free space.

Without unplugging anything, I typed:

LDA $D720,Y


The result: Acc. is left with $FD.

(And the color in color RAM was all $D.)

Got the same result when I typed it again.

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