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

From: Marko Mäkelä (
Date: 1998-04-22 23:24:05

On Wed, 22 Apr 1998, Andrew Vardy wrote:

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

Unless you know the horizontal raster position, it will only work with
probability 125/130 in your case (121/126 on PAL machines).  Remember the
five refresh cycles per line during which the CPU is slowed down, and the 
video chip will read from $3FFF-$3F00.

> Without unplugging anything, I typed:
> A1300
> LDY #$FF
> LDA $D720,Y
> GF1300
> The result: Acc. is left with $FD.
> (And the color in color RAM was all $D.)

Of course, if you are reading the color RAM ($d81f), you will get data
from there. BTW, the 4 high bits from color RAM are not random.  I've
written a program that runs in color RAM.  Most of the color RAM is filled
with zeros, and the video bank is initialized so that it will supply
opcodes like $60 most of the time.  The stack is filled with $DA I think,
so the program will be fetched from $dadb most of the time.  It took me
two weeks to write the program.  It changes the screen color when you
press space.  It is almost trivial to write a program to $de00-$dfff once
you know all of the video timing, but writing one to color memory is
extremely difficult, since in that case also the value of the program
counter matters.


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