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Episode 25 Notes - Hard Sectoring

Mar 11, 2020

Diagram of hard sector floppy disk

The floppy disk has changed very little since it's original development way back at IBM in the late 1960s. That being said there have been incremental improvements, and I'd like to look at one of those early changes. As it turns out early floppy disks came with a pattern of holes punched along their outer edge. Many 8 inch disks, and some 5 ¼ inch disks, had this feature. These types of disks were called hard sectored, and even though this approach didn't last for long I think it's useful to examine as it can give us a better understanding of how floppy drives worked.

So what exactly is a sector to begin with? To explain that let's take a look at how data is structured on a floppy disk. Data on a floppy drive is laid out in a similar way to a table, but instead of rows and columns you have tracks and sectors. Each of these sections of the disk can store a small chunk of data, so a read head has to be able to move to any location on the floppy disk. The read/write head of a floppy drive is positioned on a sled ad gear mechanism that can move it in and out over the disk, this allows for track selection. Sector selection is where the holes of hard sector disks come into play.

When in use the disk of a floppy disk spins at a constant velocity. In order to select a specific sector the drive has to wait for that sector to pass under the head. In order to do this the drive has to know which sector it's on, and that's done by reading the s on the disk's perimeter. An optical sensor in the floppy drive trips every time a hole passes so, with a little bit of programming, it's able to keep track of what part of the disk is currently under the head. To finish things off another index hole is usually punched on the inner edge of the disk to mark the first sector, that way the drive knows where to start the sector count.

Programming the controller for a hard sector disk drive is relatively simple. You don't need all that much code or power to count holes passing by. However, this style of floppy disk would pretty quickly fall out of use. I can't find a definitive answer as to why, but it's easy to speculate at a reason. One of the large driving forces behind the development of the floppy disk was price. Disks were cheap, as were disk drives. Punching holes in hard sector disks would have added an extra step and just a little more overhead to manufacturing costs. Another contributing factor cold have simply been the development of more advanced disk controller circuitry. With a little tinkering, and some more complicated software, it was possible to do away with sector holes all together.

These newer disks, called soft sectored disks, replaced the physical sector holes with magnetic markers. Instead of using a seperate optical detector the same read/write head that read data would pull double-duty, also registering sector markers. By looking for passing magnetic markets, and a little more complicated code, these soft sector capable drives were made to function just as well as their hard sector counterparts. Once 5 ¼ inch disks became the norm hard sectroing fell out of favor. Punched 5 ¼ inch disks did exist, but they were not nearly as common as soft sector disks.

To learn more you can listen to my series on the floppy disk here:

Part 1: Website // iTunes

Part 2: Website // iTunes