5.25" (inch) floppy disks were the format most popular throughout the 1980's. They followed on from 8" disks and pre-dated 3.5" disks.

Physically, the disk itself was a 5.25" (!) piece of plastic coated with magnetic material. The disk was then enclosed in a thin square plastic package. This package itself was also slightly flexible (unlike with 3.5" disks) which may have contributed to their unreliability. The disk looked vaguely like this.

  |+----------+                    |
  || LABEL    |                    |
  ||          |                    |
  |+----------+                  +-+
  |                              |
  |               OO             +-+
  |             OOOOOO             |
  |            OOOOOOOO            |
  |             OOOOOO             |
  |               OO    O          |
  |               __               |
  |              /  \              |
  |              |   |             |
  |              |   |             |
  |              |   |             |
  |              \__/              |

We can see the following features.

  • The label was stuck in the corner of the disk, although of course it could be stuck anywhere as long as it didn't cover any of the cutouts.
  • In the centre, we have the circular hole where the spindle in the drive gripped the disk.
  • Underneath the hole, we have an oval aperture, through which the disk itself showed (there was no cover, unlike 3.5" disks). This aperture was present on both sides of the cover, even for single sided disks (a single sided disk was usually written to the bottom of the disk with respect to the label). Touching the visible disk was a Bad Idea.
  • The small hole just offset from the centre hole was the timing hole, used by some drives to check they were spinning at the correct speed. Most systems didn't use this, however.
  • The notch in the top-right of the case was the write-protect hole. Covering this prevented the disk being written to.

Some disks were single sided, some double sided. All disks were manufactured as double-sided, but if one side was tested to be not up to standard, it would be placed in the cover with this side face-up, and the disk marketted as single-sided.

True double-sided drives had two heads which could read/write to both sides of the disk together. However, most computers could use a double-sided disk in a single-sided drive by simply flipping it over and putting it in the drive the other way up. This required you to cut a new write-protect notch on the other side of the disk. If the drive used the timing hole, this wouldn't work (although you could buy "flippy disks" which had this hole as well). However, some people said this wasn't a good idea as the inside of the jacket contained material to keep the disk clean. Flipping the disk meant it spun the other way in the jacket, which could cause the material to flake off.

5.25" disks were available for most computers throughout the 1980's, in various formats. On IBM systems, they were anything from 160k (single sided, single density) to 1.2 MB (double sided, high density). BBC Master systems could use them up to 720k, and Atari 8-bit systems could use up to 360k.

The IBM capacities were as follows. All sectors are 512 bytes.

  • 8 sectors / track, 40 tracks, single sided - 160k
  • 8 sectors / track, 40 tracks, double sided - 320k
  • 9 sectors / track, 40 tracks, single sided - 180k
  • 9 sectors / track, 40 tracks, double sided - 360k (Double Density)
  • 15 sectors / track, 80 tracks, double sided - 1.2 Mb (High Density)

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