Physical layout

The original media (a 5.25" disk) has the tracks laid out in circles, with track 1 on the very outside of the disk (closest to the sides) to track 35 being on the inside of the disk (closest to the inner hub ring).

Commodore, varied the number of sectors per track and data densities across the disk to optimize available storage.

Since the outside diameter of a circle is the largest (versus closer to the center), the outside tracks have the largest amount of storage.

Track counting starts at 1, not 0, and goes up to 35.

Track's 36…40 are only valid for 40 track disks, although with low-level programming the 1541 drive could access tracks 36…40.

Some copy-protection schemes did this.

Sector counting starts at 0, not 1, for the first sector, therefore a track with 21 sectors will go from 0 to 20.

* Standard CBM DOS disks had only 35 tracks.