Re: Need help fixing a C128 DCR

From: Marko Mäkelä <>
Date: Thu, 8 Jul 2010 09:52:06 +0300
Message-ID: <20100708065206.GC2614@x60s>
On Wed, Jul 07, 2010 at 10:46:40PM -0700, Brenda wrote:
>> Some updates about the C128 (in case someone's interested).
>maybe its too obvious but have you tried a different monitor and 
>monitor cable?

How would that fix garbage characters on the screen?

My bet is on the low RAM. The stack lives in $0100-$01ff, and if an RTS 
instruction reads a garbage return address from the stack, it would 
freeze the system.

Does the computer enter C64 mode when you hold down the Commodore key or 
tie the EXROM line to ground while powering it up? I seem to remember 
that the C128 KERNAL initializes the screen colors ($d020/$d021) earlier 
than the C64 KERNAL. If you don't get the blue screen in C64 mode but do 
get the green/gray in C128 mode, then that could narrow down the search.

In any case, the Z80 processor should be running fine, because it is the 
Z80 that switches the control to the 8502. There are two checks for 
entering the C64 mode, both in the Z80 BIOS and in the C128 KERNAL. I 
know, because I patched the Z80 BIOS in my system in some way, IIRC 
something that was related to controlling my 1-megabyte internal memory 
expansion <>.


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Received on 2010-07-08 07:00:11

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