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Discussion of Z80 CPM chip layout by Ruud Baltiss.
Discussion of C64 Z80 Cartridge on Genie
C64 CPM Files at retro-archive or local copy.
A lot of information on the technical details including pictures, bios, bootstrap and utility source codes can be found here.
It also appears that other 3rd party manufacturer's jumped into trying to get CP/M to run on the C64. Here is a referance to a Z-80 Video Pak by DATA 20 Corporation that appears to offer a Z80 processor along with selectable 40 or 80 columns.
Zimmers C64 CPM Boot Disks
Note that the C64 CPM was never known for it's compatibility - if you know of a collection of files that work with C64 CPM please let me know so I can provide a reference.