From: Gideon Zweijtzer (gideonz_at_dds.nl)
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!
Gideon
-----Original Message-----
From: owner-cbm-hackers@cling.gu.se
[mailto:owner-cbm-hackers@cling.gu.se] On Behalf Of Baltissen, R (Ruud)
Sent: Monday, February 03, 2003 08:04
To: 'cbm-hackers@cling.gu.se'
Subject: RE: VIC20 KERNAL Bug
Hallo Nicolas,
> Now I think I found the reason, the following code in the
> VIC20 (PAL) KERNAL ROM:
> .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
\ \__|_\
\___| http://Ruud.C64.org
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