Re: Disassembly of the Z80 ROM of the C128

From: Marko Mäkelä <>
Date: Sat, 25 Feb 2017 21:09:25 +0200
Message-ID: <20170225190925.laidalehiwf75jpw@hp>
On Sat, Feb 25, 2017 at 10:00:43AM -0500, David Wood wrote:
>Zimmers does have the 4k z80 bios as a separate image in the firmware 

That file existed already on the FUNET archive that was maintained by me 
and later moved to Zimmers.

>I thought the z80 source was in the wild, but I guess I'm mistaken.

I have not seen any source code, but I have not really looked for it.

I vaguely remember that my modification was simply to prevent the C128 
from switching to C64 mode when -GAME or -EXROM is asserted on the 
cartridge port. This would allow me to leave a C64 cartridge connected 
all the time, and I could decide myself whether to switch to C64 mode, 
by holding the C= key. For that simple modification, I did not have to 
disassemble much of the Z80 BIOS, because the Z80 does not do much more 
than check the signals from the MMU GPIO pins, and then switch to the 
8502 processor.

If this was the case, the modification probably consisted of zeroing 
some bits in the ROM, in the immediate operand of the bitwise-and 
operation with the MMU register, so that the condition would never hold.


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Received on 2017-02-25 20:00:02

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