The FocusDrive3 device driver supports the Focus IDE interface card in the Apple /// as part of the SOS operating system. SOS is very much like ProDOS in terms of filesystem layout; however, most Apple /// interpreters use a signed integer to represent blocks, so the standard maximum size of partition for ProDOS (65535 blocks, ~32MB) is halved in SOS as 32767 blocks (~16MB).
There's no ///-based partitioning software avaialble, so you will want to have a pre-partitioned CF card or IDE drive with partitions of size 32767 blocks (~16MB) or less for best results. As of this writing, the partitioning software only runs on the IIgs.
The driver is built via the ca65 assembler (part of the [cc65 toolchain](https://cc65.github.io/)). [Rob Justice](https://github.com/robjustice)'s excellent [a3driverutil](https://github.com/robjustice/a3driverutil) enables rapid development by not requiring the developer to ever run SCP! It's a modern marvel.
