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.
Here is a link for you: https://raw.githubusercontent.com/lgblgblgb/mega80/refs/heads/master/dist/bin/mega65.d81
- 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 and heavily based on 65xx assembly (even to port to 65816 would require major modifications)
- Integrated on-the-fly memory monitor and things like that
- No, I don't plan CP/M v3 (as far as I can see now). 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
- Native BDOS? Maybe adopoting Hjalfi's CPM65 with added Z80/i8080 CPU emulation to allow execute both ordinary
CP/M (CP/M-80) and CP/M-65 applications.
- Also a native BDOS-replacement can be written which does not use CP/M filesystem internally but normal FAT32, thus MEGA65's SD-card would be seen from CP/M as-is without major issue to import/export data/programs between MEGA65 and CP/M environment all the time.
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.
Using Linux or other kind of UNIX-class OS (including Mac) should be enough.