The Double-Sided Disk: Cylinders and Heads

In Sectors and Tracks and Bytes (Oh my!), I talked in some detail about how a computer might read and access data on a disc-shaped magnetic storage medium (in our case, a floppy disk), going into detail about how a disk is divided up into sectors and tracks. Tracks and sectors alone don't tell the whole story, though, when it comes to double-sided disks! You also need to know about cylinders and heads! (Note that this doesn't really apply to 'flippy disks'*)

Before I said that a track is the circular area around the circumference of the disk where the head is positioned, and generally this is true but with one extra bit: the track is specifically the area under the head (on the surface of the magnetic disk). The line around the disk where the track is located is called a cylinder, which includes all of the tracks contained top to bottom (or front to back , depending how you're looking at it) in that circle. On a double-sided disk, there are two heads - one on the top side (side 0) and one on the bottom side (side 1), and as such there can be two tracks around the same cylindrical area (cylinder) of the disk. Hard disks, which have multiple magnetic platters instead of simply one sheet of magnetic film, and as such a cylinder might contain several tracks - two for each platter - of data.

For the sake of efficiency (and probably peace of mind), all of the sectors of a track are on the same side and tracks are striped across each head. So for a double-sided floppy disk like our 3.5" friend, track 0 will be on head 0, on the top side of the disk, track 1 will be on head 1 on the bottom side, track 2 will be back on head 0 on the top side, track 3 back on head 1, and so forth. If a disk had instead 8 heads, then a cylinder would consist of 8 tracks on heads 0-7, where they would be similarly striped.

A 3.5" DS/DD disk has 2 heads, 80 cylinders, and 18 sectors per track, which means that in total it has 160 tracks and 2880 sectors. Knowing this, we can easily figure out exactly where a sector is on a given disk, and that's actually available on every sector page on the Sector Disk site!

*A flippy disk is a disk where the user would modify the casing of a 5.25" disk so that it could be flipped over and used as two distinct disks depending on which side was up and under the drive head. The hack was actually popular enough that some disk manufacturers actually produced ready-made flippy disks before true double-sided disks gained popularity.