MFX with a difference grows up

[Martin A]

Seeing as CPM3 on other platforms often had incompatible disk formats to CPM2 on the same machine, is there a reason for keeping the old restrictions in the original BIOS?

CPM2 has been left exactly as it was, there's no changes to the bootrom or the BIOS, that's to ensure compatibility. With HexTrain we have the "best" 8 bit game ever. I wasn't going to do anything to break that, or require a "special" version. Similarly NewWord is tied pretty tightly to the BIOS/Bootrom. Plus of course SDX basic and all the games that makes available, is not only tied to the CPM2 BIOS/BDOS, its tied specifcally to the 59k version.

CPM3 more or less required a new partition format. CPM2 expects each track to be 26 sectors of 128 bytes, that works out at 3.25k per track. CPM3 works with the native sector size, for CF and SD that's 512 bytes, meaning there's no way to define a track that is 3.25k long. I possibly could have made the track 6.5K long and made things work. However a 4k track size and a 4K block size makes the disc geometry calculations incredibly simple. The CPM 2 calculation is something like 42 bytes to the CPM3 version's 18 bytes. It's a lot simpler and quicker to multuply by 8 than 26!

As for the current hybrid structure, CPM 2 can only see 4 partitions at once, so restricting it to 4 partitions total is no real hardship. SDX Basic can't ever see the last 4 partitions so there is no loss there either. Putting the CPM3 partitions where the last 4 CPM2 partitions were meant that the HexTrain data that sits immediately after the CPM area isn't affected in any way.

Reducing the size of the CPM3 partitions from 8meg to 4meg, that was just to make file management a bit easier. With no real sub directory support 256 files per drive (or 192 with time stamps) is easier to manage than double that. There's no overall loss of capacity, because CPM3 can cope with 16 drives at once, all 8 partitions are always visible for 32meg of storage. Moving to 16 x 2 meg partitions would require more memory for the drive tables in the BIOS, and therfore consume more TPA space that I was willing to give up, so 8 was a compromise.

With FTP available to both versions, that means there's no requirement for direct file transfer between CPM2 and CPM3 partitions. So keping them totally separate isn't a problem.