From: Gideon Zweijtzer (
Date: 2003-02-03 22:36:51

Dear all,
I have seen this fenomenon also in the C-64 ROM, when I was debugging my
6510 CPU with the logic analyzer. I had some triggers on erroneous
conditions, and one of them was "writing to a ROM area", which happened
exactly in the same code fragment as you mention below! A very bad
programming practice, IMHO!
-----Original Message-----
[] On Behalf Of Baltissen, R (Ruud)
Sent: Monday, February 03, 2003 08:04
To: ''
Subject: RE: VIC20 KERNAL Bug
Hallo Nicolas, 
> Now I think I found the reason, the following code in the 
> .C:fd62   B1 C3      LDA ($C3),Y 
> .C:fd64   91 C3      STA ($C3),Y 
> .C:fd66   99 14 03   STA $0314,Y 
> This copies some vectors from ROM to RAM, but why does it 
> contain an STA command back to ROM? 
Because this code is also used to move data from the $0314 RAM-area to
another RAM area defined by the user in $C3. This routine starts at
$FD57. The VIC-20 uses this routine only once and enters this routine
with CLC some bytes above. This performs the C3 -> 0314 copy. A SEC does
it ythe other way around.
But the trick/problem is that a JUMP-vector at $FF8B points to $FD57.
God only knows which external software uses this vector. And does this
software enter the routine with a set or reset Carry?
> I think I'm going to patch this out with two NOPs, 
Defenitly NO. 
   / __|__ 
  / /  |_/     Groetjes, Ruud 
  \ \__|_\ 


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