Subj : Re: Commodore history - The Commodore 128 by The 8-Bit Guy
To   : All
From : Martijn van Buul
Date : Tue Oct 23 2018 09:40 am

* Etienne von Wettingfeld:

> IIRC it has to do with the ability to auto-boot CP/M.

Well... Close :)

The C128 was designed to be compatible with all official C64 addons -
which included the C64 CP/M cartridge, which had compatibility issues
on later revision C64s already. Designing in the Z80 (so the CP/M cartridge
would be redundant, and no longer an issue..) was cheaper than trying to work
around the issues.

There were other problematic cartridges: Cartridges that (ab)used Ultimax
mode to hijack the reset-, NMI- and interrupt vectors. This included some
of Commodore's own (including the Magic Voice). These would cause problems,
since the system hadn't been configured for C64 mode yet.

This is the real reason the c128 initially boots using the Z80: It probes
whether the system needs to boot in CP/M, 128 or 64 mode, while the 8502
was still halted.

Get it from the horse's mouth (Not that I'd call Bil a horse, mind):

https://www.reddit.com/r/c128/comments/682da4/c128_ama_from_bil_herd/

--
Martijn van Buul - [email protected]
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