[kragen]

I just found Tim Paterson's article at http://dosmandrivel.blogspot.com/2007/09/design-of-dos.html?... which says my comment above got an important thing wrong: the sector size on 8" disks was typically 128 bytes, so CP/M was reading per sector. He mentions that North Star DOS used 256-byte sectors. I'm pretty sure the logical HDOS sectors on my H-89 5¼" single-density floppies were 512 bytes, but I've never interacted with the low-level format of the disks. They were 100K per side, 10 sectors per track (with 11 holes punched in the disk to indicate their positions), which I guess would imply 20 tracks, which sounds too low! Maybe the physical sectors were 256 bytes.

https://heathkit.garlanger.com/diskformats/HDOS_Disk.pdf confirms: 256 bytes, 10 sectors, 40 tracks.

Paterson's article explains why he copied the FAT filesystem from Microsoft BASIC but extended the cluster numbers to 12 bits. He also has some pretty damning criticisms of both the CP/M filesystem design and its BIOS interface.

[musicale]

Great reference. I didn't realize the FAT filesystem was from Microsoft Disk BASIC (though I knew it was different from CP/M's) so I guess it came back full-circle with DOS. (Though perhaps not directly compatible with Disk BASIC's FAT8 filesystems.)