Right: low-down on FAT and APT, etc. If you want to have a FAT partition on the HDD which you can use with the FAT16.SYS driver, the partition entry in the APT partition table needs to point to the FAT partition as described in the MBR of the hard disk. This is the only way the partition will be visible to the PC AND to the Atari, since the Atari PBI / soft driver doesn't look in the MBR for partitions: it looks in the MBR for the APT entry, which is a pointer to the APT partition table. What's needed is a facility to LINK an APT partition entry to one of the MBR FAT partitions. I've done this with a hex editor before and it worked well, but I need to code something up in FDISK. It's not the easiest proposition, however, since I bascially need to add an MBR partition table browser to the software. You'd then select "Create external FAT partition", browse entries in the MBR, and press enter when you find the one you want. The Atari and PC would then both see this entry as - say - drive D:, and you could load the FAT driver on the A8 read files from it. If you put the FAT driver on CAR, you could even make it the boot partition. :)
Really, I'd like to add to FDISK the ability to create more than one FAT area on the disk, and this means a mini MBR partition table editor. So it's a question of finding the opportunity to get it done. I realize the FAT/APT size selection is about the weakest area of the software, and this is partly (though not entirely) because of the need to be concise in the 8KB CAR: version of FDISK.
The XEX loader does its own FAT housekeeping, and (in the final version) will simply "register" FAT partition information with the PBI driver. This relieves the PBI BIOS of having to parse MBRs and extended partition entries. The XEX loader will (when mounting an ATR, for example), simply pass the cluster number of the ATR to the PBI BIOS, along with the number of the registered FAT partition (which the XEX loader previously registered) the ATR resides in.