I'm trying to renew my old project from 2017 to be able to run CP/M on MEGA65 with i8080 (for now, later: Z80) emulated in software equipped with a custom CBIOS for the task. The 65816 CPU also seems to be a possible target to port future Z80 emulation (I wouldn't do for the current i8080 since I will rewrite the emulator anyway).
For now, i8080 CPU (which is the original CPU for CP/M, not the Z80) is emulated. A custom CP/M v2.2 is provided dispatching the work to the native 65xx implementation via HALT opcodes. For now, original DR's CPM (BDOS and CCP) v2.2 is used.
Have a look in directory dist/bin for a D81 disk image file (mega65.d81) for MEGA65.
- Current code base is chaotic since major parts of it was written by me in 2017 and I only try to revive some unfinished old work of mine here
- So far only i8080 emulation, not Z80: newer CP/M programs written for Z80 won't work
- Even i8080 is not emulated perfectly, no half-carry flag and no DAA and for sure can be many bugs
- Original DR BDOS and CCP. There are open souce GNU/GPL replacements but requires Z80
- In nutshell, I would guess I'll go for Z80 emulation later and don't plan to fix i8080 emulation bugs too much
- No real disk access, CP/M disk image is copied to MEGA65 Attic RAM on startup and all changes later will be lost
- Only "dumb" terminal emulation,
CR,LFandBSis interpreted. For more advanced CP/M programs, some terminal emulation should be done - Need some tool which at least makes possible to easily import files into CP/M from MEGA65 SD card or such (at runtime)
- Porting to other systems: 65816 CPU seems to be a viable option for porting, and machines using that CPU and having enough memory:
- Commodore 64 with SuperCPU
- Commander X16 if CPU is 65816 and it can access RAM in higher bankes freely
- Apple IIgs? Unfortunately I don't know too much about Apple computers, but could be a fun ...
- Porting for non-65xx CPU based platforms: certainly it's not impossible but then you need to rewrite everything, as it's an assembly project.
- Integrated on-the-fly memory monitor and things like that
- No, I don't plan CP/M v3. The problem: it requires extensive bank switching and other difficulties, since I'm emulating i8080 (later hopefully a Z80) emulating bank switching would be impossible or very slow
Tools needed to build:
Various tools (ca65, ld65, cl65) from the CC65 suite to assemble 65xx assembly sources:
Only tested with V2.18 (as of I am writing this), so any older version can cause problems. Currently the C compiler part (cc65) is not used though.
SjASMPlus Z80 Cross-Assembler to assemble i8080 / Z80 assembly sources: https://github.com/z00m128/sjasmplus
Only tested with version v1.20.3 (as of I am writing this), so any older version can cause problems, like certain ASSERT features was introduced in v1.18.1 but there can be other problems as well.