This is a brief description
of 8", 5.25", and 3.5" floppy disk media. The media characteristics
and physical recording schemes are discussed. Logical recording schemes are not discussed.
This is the original diskette, introduced by IBM as a way to distribute code for their mainframe computers. It is a round flexible "cookie" (the magnetic disk itself) enclosed in a flexible sleeve approximately eight inches by eight inches, and dubbed a "Flexible disk", and commonly called a "Floppy disk". There are hard sector and soft sector disks, with soft sector the most common. Hard sector disks have small holes punched in a circle near the hub of the disk that mark the locations of the sectors on the disk. Soft sector disks write magnetic patterns on each track to indicate the start of a sector.
The original IBM disk was single-side single-density, but the 256KB capacity wasn't enough when it started being used as a data storage medium for personal computers, and "double density" recording was invented. Single density recording uses FM (Frequency Modulation) and double density recording uses MFM (Modified Frequency Modulation), which stores twice as much data in the same space.
Both hard and soft sector disks have an "index hole" punched in the disk to indicate where each track on the disk begins. This hole is near the hub (the large center hole of the disk). You will see a small, quarter-inch hole punched in the jacket. If you rotate the cookie (the magnetic media), you will find one hole in the cookie for soft sector disks, and multiple holes for hard sector disks. Most disks are soft sector; Wang is the largest user of hard sector 8" disks.
The disks can be one or two sided; the index hole in the jacket is punched in different locations for one and two sided disks, so the drive can detect the type of disk. Disks are certified for single or double density, but the magnetic coating is the same for both. A disk certified for "Double Density" can be recorded at Single Density, but disks certified for "Single Density" have not been tested for Double Density and should not normally be recorded in Double Density, as they have not been certified for the more critical Double Density magnetic bit patterns.
Soft sector disks can be recorded with any sector size, but the common sizes are 128, 256, 512, and 1024 bytes of data per sector. Single density, also known as FM (frequency modulation) recording, is usually recorded at 128 bytes per sector, and double density, also known as MFM (modified FM), is usually recorded at 256 to 1024 bytes per sector, but this is not mandatory.
All 8" disks have 77 cylinders recorded at 48 TPI (tracks per inch), and rotate at 360 RPM. Each sector on the disk contains a header which, among other things, contains the "address" of the sector. The address is the side, cylinder, and sector number. This uniquely identifies that sector. Sector, track, and side numbering schemes vary and are determined by the operating system device driver. Capacities range from about 256 KB for single-sided single-density, to 1.2 MB for double-sided double-density.
Although this article does not
discuss the logical disk formats (how data is organized on the disk), we should
mention that there are several thousand
schemes used to write data to these disks. Some of the best known are the
IBM "Exchange" formats, which come in single and double sided, single and
double density formats with both blocked and unblocked records and spanned
or unspanned sectors, with the disk recorded in EBCDIC. DEC (Digital
Equipment Corporation) used 8" disks on their PDP-11 computers, under several
different operating systems, all using ASCII. CP/M computers
often used 8" disks, and there are several hundred different CP/M disk formats, all
recorded in ASCII. Many early dedicated word-processors used 8" floppy disks, such as
the IBM DisplayWriter, DEC DECMate, CPT, NBI, and several Wang systems. Each of these
products wrote a different logical disk format. Most were some variant of ASCII, but
the IBM Displaywriter was EBCDIC. All used their proprietary WP codes.
These are smaller versions of the 8" disks, with similar characteristics. Although there are hard sector 5.25" disks, they are rare today, and virtually all 5.25" disks are soft sector, and nearly all soft sector disks are formatted with 512 byte sectors.
There are three basic types of 5.25" drives:
These three types of drives are discussed below. There was a unique 100 TPI drive manufactured, but it never gained popularity and is seldom seen today.
This was the first type of 5.25" drive made. Some early drives were only single sided, but most were double sided. They could record in single density FM and double density MFM. Most could read hard sector disks. This is the drive used by the original IBM PC, and commonly called 360K.
Capacity ranged from about 80K for single-side, single-density, to 360K for double-side, double-density. They use 300oe media, and the drive rotates at 300 RPM.
This was the second type of 5.25" drive made, and the least popular (and known) of the three types of drives. These double the capacity of the original drive by doubling the number of cylinders (tracks) from 40 to 80. They use the same media as the the 40 cylinder 48 TPI drives, but it is certified (tested) on all 80 tracks, as opposed to the standard disks which were only certified at 40 tracks.
These drives were never common on PCs, although DEC used a single sided version called the RX-50, in the DECMate word-processor, the DEC Rainbow and several other DEC computers, including the PDP-11 and the VAX.
Other than the DEC RX-50, these drives were almost always double sided, and recorded in double density MFM. They had a capacity of around 720K. Like the 40 track drives, they used 300oe media, and the drive rotates at 300 RPM
This is the familiar "1.2 MB High Density" drive found on the XT and later PCs, until the 3.5" drives took over the market. This drive is also 80 cylinders recorded at 96 TPI, but the drive uses a higher coercivity 600oe disk which allows a higher bit density, and hence greater capacity. The drive rotates at 360 RPM, and records at a higher data rate. The rotational speed and data rate match those of the 8" disks, so an 8" floppy decoder circuit can be used.
The types of media correspond roughly to the types of drives, so there are 48 TPI 40 cylinder, 96 TPI 80 cylinger, and 1.2 MB high density diskettes.
Early diskettes were sold as certified for either single-density or double-density, but as the product lines matured all disks were certified for double-density (which can be used at either density). The 300oe media was generally sold in both single-side and double-side products, but the high density media was always double-sided.
Ignoring the single/double density certification, and hard sector disks, there are then 5 basic types of media:
When the 80 cylinder 96 TPI drives were introduced, disk vendors needed a way to distinguish the 80 cylinder media from the 40 cylinder media. The media was normally referred to as 48 TPI or 96 TPI. The 40 cylinder drives were sometimes referred to as "double density", and the new 80 cylinder drives were referred to as "quad density". When the 80 cylinder high density drives became available they were called "1.2 MB", "High Density", or "1.2 MB High Density"
The table below summarizes the range of drive and diskette combinations and parameters. Not all combinations are listed.
| Cylinders | Sides | Density | oe | RPM | Capacity* |
|---|---|---|---|---|---|
| 40 | 1 | SD | 300 | 300 | 80 KB |
| 40 | 1 | DD | 300 | 300 | 180 KB |
| 40 | 2 | DD | 300 | 300 | 360 KB |
| 80 | 1 | DD | 300 | 300 | 360 KB |
| 80 | 2 | DD | 300 | 300 | 720 KB |
| 80 | 2 | HD | 600 | 360 | 1200 KB |
As with 8" disks, there are
thousands of recording schemes in use. MSDOS 360K and 1.2 MB are
the most common 5.25" disk formats.
These are the most popular today. The media is flexible, just like the 5.25 and 8" disks, but is enclosed in a hard plastic shell. A metal hub is attached to the cookie (disk) and engages the drive motor in the disk drive. This solves a common problem of center hole damage found with 8" and 5.25" disks. A shutter provides further protection from damage.
There are both single and double sided and single and double density, as well as high density (HD) and "extended density" (ED), but only the high density are commonly found today. Single and double density disks are 600 Oe, high density are 750 Oe, and extended density are 900 Oe. Most disks are recorded with 80 tracks at 135 TPI, but a few early drives had 40 tracks at 67.5 TPI. Most drives are now double sided.
Most drives rotate at 300 RPM, but some, Sony/HP in particular, rotate at 600 RPM. Either speed records the same thing on the disk. Although any sector size can be recorded, 512 byte sectors with MFM recording is almost universal, with the notable exception of the Macintosh 400K and 800K formats which vary the speed of the disk as the track radius changes, and record in a GCR (Group Code Recording) format. Macintosh 1.4 Meg disks use MFM recording and a fixed rotational speed, the same as IBM and all the others.
Capacity ranges from about 90K (for the 40 track single side, single density) to 2.88 MB for the ED disk, but the 1.44 MB are the vast majority of disks in use today.
For more articles on data conversion, see our TechTalk Index.
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