BBC multiple drive support

[ivop]

Hi,

I briefly looked into adding multiple drive support to the BBC port, but I can't seem to find any documentation on how to open a file on the second drive. As you are more knowledgeable about the BBC, could you either point me in the right direction, or add it yourself? It's something I need for when I turn the 8080 emulator into a "bootable" virtual machine that boots from another drive that contains the 8080 CP/M BDOS, CCP, and other utilities, so we can turn that boot disk into a separate project like we discussed earlier and keep it out of the CP/M-65 repository.

Regards,
Ivo

[davidgiven]

On MOS you specify the drive in the path --- the syntax is :0.$.Filename where 0 is the drive, $ is the path (in this case the root directory), and the path separator is .. For another CP/M drive the BIOS would need to open a second image file and have a second DPH. One problem is that the entire BIOS has to fit within the 1kB block of fixed memory at 0x0400, and there are only 11 bytes free. There's another 0x1b which could be salvaged from the startup code but it's still not a lot.

Really the entire MOS BIOS wants to be reworked to use the BDOS emulation layer from neo6502 so that CP/M files are stored as native MOS files. This is a little problematic because Acorn DFS has eight-character filenames and ADFS has 10, but CP/M has 11.

[ivop]

How about !BOOT chainloading a larger BIOS from file and relocate it to wherever LOMEM is? Or part of the BIOS in &400-&7ff and the rest from LOMEM onwards.

The problem with the BDOS approach is that BIOS raw sector reads won't work anymore, and that's what the 8080 emulator relies on. It only maps CP/M 8080 BIOS calls to CP/M-65 BIOS calls.