Floppy Disk Formats, Standards and Geometry

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This is an effort to provide information on the format and geometry of actual floppy disk media and to discuss the evolution of standard formats for the different types of disks. A lot of material on the older technologies seems to be falling off the Web lately.

My definitions for various terms are here.

36+ Years Ago

8-inch Systems

These were the first floppy drives available in the mid 70s. They were initially designed by IBM as a method to load microcode into their large DASD disk controllers. Other manufacturers started to manufacture them as storage devices for the then beginning small computer market. Many 8080/8085/Z80 CPM-80 systems used these drives. These drives were large, heavy, and required +5V, -5V and 24V to operate. The media spun continuously although the heads were unloaded when not reading or writing. This introduced the head load and unload time to the FDCs. These drives were available in soft- and hard-sectored models. Media was not interchangeable between them soft- and hard-sectored models.

The first standard disk was the IBM 3740 format 8-inch SSSD disk 241Kb (77 tracks, one head, 26 each 128 byte sectors per track with a skew of six) disk. This was the disk format used for exchange between all the CPM-80 8-inch systems and software providers.

5.25-inch Systems

These were the second generation of drives available in the late 70s. They were sometimes called minidisks. They were smaller, lighter, more reliable, used less power, and only required 5V and 12V. They were small enough to integrate into the computer cabinet. The media only spun when the drive was reading or writing. Heads unloaded when the drive was idle. This introduced motor spin up time to the FDCs.

The first drives were SSSD and were only 35 tracks. Soon, they moved to 40 tracks. Early IBM PCs came with 160 Kb drives (40 tracks, single sided, 8 each 512b sectors). DS drives quickly became available. As people messed with the gaps in the format of disk, another 512b sector was added to each track giving 180Kb. With DSDD drives we got to 360Kb (40 cylinders, 2 sides, 9 each 512b sectors per track).

These 4 media, 160k, 180k, 320k and 360k were completely interchangeable between all the IBM and clone computers. They were the standard 5.25-inch disk format. And at this point most everyone seemed to standardize on the 512b sector. This is with us in the PATA and SATA hard drives today.

As media got better, a few manufacturers narrowed the cylinder width and doubled the number of tracks to 80. These disk drives were called DSQD drives and could store 720Kb (80 cylinders, 2 heads, 9 each 512b sectors per track). These drives were used by Intel in their iPDS systems. They were overcome by the HD drives.

As media improved more and the electronics on the drives improved, the HD drives began to appear. These used 80 cylinders and higher density read/writes to create the 1.2Mb floppies (80 cylinders, 2 heads, 15 each 512b sectors per track). This was the final evolution of the 5.25-inch drives. They were overcome by the 3.5-inch drives. And they had compatibility problems with the standard 5.25-inch disks because of the narrow cylinders.

3.5-inch Systems

These were the final generation of drives made available in the 80's. They were even smaller, lighter, even more reliable, used even less power and only required 5V. They were small enough to integrate into notebook computers when they appeared. Again, the media only spun when the drive was reading or writing. Heads unloaded when the drive was idle.

These appeared as DSDD drives from the beginning. At the start, the format was 720Kb (80 cylinders, 2 heads, 18 each 512b sectors per track). But with the advances in media and electronics, these drives went HD as well. The standard DSHD 3.5-inch floppy held 1.44Mb (80 cylinders, 2 heads, 18 each 512b sectors per track). This was the final evolution of the floppy drive. They were replaced by CDs. Today you won't find a floppy disk on any new machine.


During most of my career as a Computer Engineer, I dealt with networking. And doing this, I dealt with standards. I used standards and wrote standards. Standards are a good thing. They make interoperability possible. They made my life easier so whenever possible, I try to standardize hardware and software that I work upon.

While I was fiddling with machines, I standardized my 5.25- and 3.5-inch FDDs with a DC-37 connector and an 3.5 x 6 x 10 BUD aluminum box to hold them. I placed 2 identical disk drives in each box. Now I have a pair of 360Kb 5.25-inch DSDD, 720Kb 5.25-inch DSQD , 1.2Mb 5.25-inch and 1.44Mb 3.5-inch DSHD drives all self contained. Data and power come through the DC-37 connector and a single ribbon cable from the host system. This is discussed here. This has made it easier to attach FDDs to the system hardware on which I am currently working. 8-inch drives use a Centronics 50-pin IDC connector and contain their own power supplies.


Some Lessons Learned

1 -- The 512b sector size is here to stay. It is the standard on all hard drives and final versions of the 5.25 and 3.5-inch floppy drives.

2 -- In the early days, many manufacturers of systems used a SD track on cylinder 0, head 0. Their disks contained an identity sector somewhere on cylinder 0, head 0. The identity sector allowed the host operating system to log on the disk or mount the disk meaning it configured the software to match the characteristics of the media. This was a good thing and a bad thing. People played with sector sizes and gaps to stuff more data onto a given media. I have CPM-80 8-inch 1.2Mb floppies that were standard IBM 34 format. But as time went on, the capability to read SD media disappeared from the FDCs and data separators. Today's machines that have floppies at all cannot read a SD track. The old media won't work on the new hardware, if you had drives the proper size for it.

3 -- Data does not last forever. While I was in the Army we conducted an experiment to figure out what happened to our cassettes we used to do language testing when we shipped them by air. We knew it was NOT fields from the aircraft wiring. We took several recorded cassettes and bounced them off the floor over a 100 times each. When we tried to play them, there was no audio left on them. The vibration of the flight was the problem, rather than any electrical wires near the bags in the hold. We have problems with magnetic disk media as well. Remember that as the drives got smaller, the power requirement was reduced? And the media will take more data per inch than the previous media? Well, that comes back to bite us in longevity on the data on the media. I stored all my old CPM 8-inch floppies in the garage here in Arizona without any cooling or heating, no humidity control, nothing. After about 20 years I decided to bring up my S-100 system and the 8-inch drives. The media had retained the data for over 20 years under horrible conditions. I find the 5.25-inch media does equally well maintaining data over the same period of time. I have Kaypro floppies that work just fine 25 years after they were written. Not so with the 3.5-inch media. Some of it holds data and some of it has been lost. And be aware that CDs and DVDs will lose data over time, as well. They may take longer, but the data will be gone.


Standard Floppy Formats

Here I will describe the standard formats that have evolved and that I use with both hardware and simulators. All of this is driven by the lessons learned and systems on which I am currently working. I have given the idea of the "standard" 8-inch floppy disk format a great deal of thought over the past few years.

What I need is a format that will work on different systems and most importantly, different Floppy Disk Controller chips. There are two main families - Intel 8272/NEC 765 and the WD 179X. They will read some disks interchangably, but some of the denser WD formats will NOT be read/written byt the Intel/NEC controllers. So when I chose the formats I discuss below, I took these two families of controllers and took the minimum number of sectors for a particular N (sector size) from the two families.

8-Inch Disk Formats

With all this thought, I believe I need to change my current 8-inch disk format. The SD cylinder 0 was great in the day, but it has caused me many problems since I began restoring these machines. So my current thought is to detect an IBM 3740 disk by trying to read head 0, cylinder 2, sector 1 (the CPM Directory) in SD. If that works, the disk is SD IBM 3740 format.

For IBM 34 formats, I could take a page out of the CompuPro BIOS software. I would read an ID field off head 0, cylinder 2. A ID read of head 1, cylinder 2 will identify double sided disks. This would provide me with the N value giving me sector size and the number of sides to determine the final disk parameters for the BIOS at disk logon.

For the system tracks I would standardize on a DD format, possibly 42 each 128 byte sectors. Whatever will hold the complete system.

The other possibility for IBM 34 formats would be to use cylinder 0, sector 1 for the ID sector. The ID sector contains the BIOS disk parameters for that particular disk format, and BIOS does not have to hold disk parameters for each and every disk format. Just the IBM 3740 format. I think I will go this way first since it reduces BIOS size considerably. The VF II formats are different than I got with the JadeDD. Both controllers use the same chip, so I believe I can get use the old JadeDD formats.

I checked the CompuPro and SDSystems VF II format to finalize the disk formats. I found the CompuPro allowed fewer sectors for some sector sizes than the SDSystems. This is bacause the WD chips will allow one to control ALL the values controlling the format during format. You actually write out the track image byte by byte. The Intel/NEC controllers only allow you to provide the ID Field. With that, you can only control hard sector skew. Gap 1, 2, 3, 4A and 4B are all fixed for each density.

So I have derrived the following formats: SD IBM 3740 - 128 byte 26 sectors, 1 side, 64 directory entries, 241 KB total size DD IBM 34 - 128 byte 42 sectors, 1 side, 64 directory entries, 391 KB total size DD IBM 34 - 256 byte 26 sectors, 1 side, 64 directory entries, 483 KB total size DD IBM 34 - 512 byte 15 sectors, 1 side, 64 directory entries, 558 KB total size DD IBM 34 - 1024 byte 8 sectors, 1 side, 64 directory entries, 596 KB total size DD IBM 34 - 128 byte 42 sectors, 2 side, 64 directory entries, 785 KB total size DD IBM 34 - 256 byte 26 sectors, 2 side, 64 directory entries, 971 KB total size DD IBM 34 - 512 byte 15 sectors, 2 side, 64 directory entries, 1121 KB total size DD IBM 34 - 1024 byte 8 sectors, 2 side, 64 directory entries, 1196 KB total size

I can also format with 128 and 256 directory entries, which means I actually have 25 different formats that the BIOS will have handle. So this means I need to use the ID sector to minimize the BIOS size!

So these disk formats will be compatible with the Disk 1A and the VF II on all systems. They will also work with the JadeDD if it comes back to life! This is important in the long run.

A horizontal ribbon.

Last Modified: 20 Septmber 2015