- Reverse engineering by Ranger Harke
The FocusDrive is an IDE disk controller for the Apple II, sold in the early 2000s. It was designed by Parsons Engineering.
Block 0 has an identifier string and the partition table. Blocks 1 and 2 hold partition names. Up to 30 partitions can be defined.
The partition map layout is:
+$00 /14: signature, "Parsons Engin." in ASCII
+$0e / 1: (unknown, always zero?)
+$0f / 1: number of partitions (1-30)
+$10 /16: (unknown; serial number?)
+$20 /nn: array of partition entries:
+$00 / 4: start LBA
+$04 / 4: size, in blocks
+$08 / 4: (unknown, always zero?)
+$0c / 4: (unknown)
This completely fills block 0. All multi-byte values are little-endian.
Blocks 1 and 2 hold the partition names. Each name is 32 bytes of ASCII, padded with zeroes at the end.
The area where you'd expect to find the first entry is actually used to hold other data. A count of the number of blocks not included in the partition map appears is a 32-bit value at +$04. The name of the first partition is found at +$20. Since there are only 30 entries, the last slot, in block 2 at +$1e0, is always blank.