ftp://ftp.funet.fi/pub/cbm/firmware/drives/

Firmware for Commodore disk drives.
According to William Levak, Commodore used six different native disk formats:
2040/3040 - This is slightly different from later formats: tracks 18 - 24 have 20 sectors instead of 19.
2031/4040/1540/1541/1551/1570 - single sided, single density, 48tpi
1571 - A double sided version.
8050 - single sided, 96tpi
1001/8250 - double sided, 96tpi
1581, 3½" DD with MFM
While we're on the subject, Commodore's single density drives are actually double density, and the double density drives are actually quad density. Quad density disks have the same capacity of high density disks, but the magnetic film is compatible with double density disks.