The Disk II single-sided floppy drive used 5.25-inch floppy disks; double-sided disks could be used, one side at a time, by turning them over and notching a hole for the write protect sensor. The first disk operating systems for the were and DOS 3.2, which stored 113.75 KB on each disk, organized into 35 tracks of 13 256-byte sectors each. After about two years, DOS 3.3 was introduced, storing 140 KB thanks to a minor firmware change on the disk controller that allowed it to store 16 sectors per track. (This upgrade was user-installable as two PROMs on older controllers.) After the release of DOS 3.3, the user community discontinued use of except for running legacy software. Programs that required DOS 3.2 were fairly rare; however, as DOS 3.3 was not a major architectural change aside from the number of sectors per track, a program called MUFFIN was provided with DOS 3.3 to allow users to copy files from DOS 3.2 disks to DOS 3.3 disks. It was possible for software developers to create a DOS 3.2 disk which would also boot on a system with firmware.