A long time ago, I made a NextZXOS based CP/M emulator, called CPemu. It had a number of failings, one of which was it was closed source, another was that it didn't support CCP replacements, and was the CLI as well as the kernel. This turned out to be quite the bottleneck for later enhancement.
This time I want to tackle it differntly, while still keeping the bits and ideas from the former code that I like.
So, bits from the emulator that I want to keep:
-
The filesystem - CPemu, and the reason that DP/M is called DP/M is that NextZXOS should still remain in control of the computer. So, for filesystems that means using the NextZXOS file system layer. And FAT32 native file support. So, that also means BDOS level compatability with CP/M, not BIOS - so anything that expected to do direct sector or low level hardware access is out of scope. CPemu did this with some complex bank switching, and hoop jumping.
-
The technology - CPemu was a dot command - and the whole of the CP/M environment ran within that dot command. I still want to do the entire operation as a continiously running dot command, this means that we can cleanly exit back to NextZXOS and have 100% state preservation, as is the "user expectation" with a dot command.
-
The memory management - CPemu did this with the Next's MMU to keep most code out of the linear memory space, and I plan to keep that idea
- for maximum TPA. But using the NextZXOS APIs for all bank allocation and deallocation faciliating the exiting to NextZXOS.
-
The display - CPemu used the tilemap, we're still doing that - and by using extra memory from NextZXOS, returned when done, we'll be banking up both ULA areas (one for display, one for tiles) so we can even restore the display once finished.