If you want to build CP/M Plus, the second and third links in the "CP/M 3" section at www.cpm.z80.de contain the CP/M 3 source tree and build scripts, for cross-compiling under DRDOS (second link) or UNIX (third link).

In the case of the UNIX one in particular, it should just be a matter of installing ZXCC and Thames, unpacking the source tree and typing 'make'.

If you want to build CP/M Plus, the second and third links in the "CP/M 3" section at www.cpm.z80.de contain the CP/M 3 source tree and build scripts, for cross-compiling under DRDOS (second link) or UNIX (third link).

In the case of the UNIX one in particular, it should just be a matter of installing ZXCC and Thames, unpacking the source tree and typing 'make'.

Update 2024: Since CP/M has now been fully open-sourced, I have put the CP/M 3 source tree and scripts on my website, and added similar archives for CP/M 2.0, 2.1 and 2.2.

The assembly source for the CCP and BDOS is in 8080 mnemonics. To convert to Z80, I would use XIZ.COM to translate to Z80 mnemonics. It should then be possible to build relocatable files with pasmo --prl (you may also need to replace $ characters in labels with _, as pasmo may try to parse them as variable names).