Introduction

The official "Standard Floppy Disk Formats Supported by MS-DOS" are listed in the Microsoft Knowledge Base (KB) Article Q75131.

Note: If a format is supported by MS-DOS, this does not automatically mean that it is also supported by IBM PC-DOS. MS-DOS already existed (long) before the first IBM PC (with PC-DOS) came onto the market! MS-DOS 2.0 ran, for example, on the NEC APC, which has two 8 inch drives.

In KB-Q75131 three 8 inch formats are mentioned, but these formats were never used in IBM PC, XT, AT or compatible computers and were not supported by IBM PC compatible computers. The FORMAT, DISKCOPY, ... commands do not support 8-inch formats because 77 tracks and 128 or 1.024 bytes/sector are NOT supported, only 512 bytes/sector!

And here is a pdf version of the DOS floppy disk formats. There is a bit of floppy maths in lines A to I.

In this table you can also see very well why 8 inch and 1.2 MB (high density) drives are very comparable. Clock rate and revolutions per minute are the same. For this reason you can replace an 8" drive with an 1.2MB 5.25" HD drive.

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A few fundamental things

8 inch and 5.25" 1.2 MB (high density) drives always spin with 360 revolutions per minute (rpm, r/m) or 60 revolutions per second (rps, r/s) or 166,67 milliseconds per revolution (ms/r).

The clock rate (frequency) on 8" (and 5.25" 1.2 MB high density) floppies is always 500 kHz. In other words, the cell timing is 2µs(period) MFM or 4µs(period) FM.

The clock rate (frequency) on 5.25" 160/180/320/360 KB floppies is always 250 kHz. In other words, the cell timing is 4µs(period) MFM or 8µs(period) FM.

Clock rate (kHz or k) has nothing to do with the data rate or transfer rate (kBit/s or kbps). Why do I emphasise this? Because the „same“ values "250" and "500" sometimes appear in both the data rate and the clock rate. This may lead to confusion.

Data rate or transfer rate (kBit/s or kbps) have something to do with data encoding (modulation) on floppy disks. The keywords here are: single density (SD), double density (DD), frequency modulation (FM) and modified frequency modulation (MFM). SD is synonymous with FM and DD with MFM.

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The Increase in Data Capacity

Pre-DOS era: FM to MFM / SD to DD

The change from FM to MFM does not concern the MS-DOS or PC-DOS world (from 1981 onwards) but the era before that and here in particular the CP/M operating system.

The terms "single" and "double" are derived from the fact that the MFM encoding requires only „half“ the storage space for the same amount of data. You can therefore store „twice“ as much data on a diskette. The R/W speed is also doubled.

With FM, you still need a clock bit for a data bit (a total of 4 µs per cell bit). The clock bit is only used for synchronisation and is simply discarded later when reading. With MFM, this special synchronisation clock bit is omitted (only 2 µs per cell bit). That's where the factor 2 comes from, the doubling.

MS/PC-DOS era: 8 to 9 Sector per Track

MS/PC-DOS era: Single to Double Sided

In the above figure "More Formats" is a mistake! The 9-sector format was introduced with PC-DOS 2.0; not 1.1! You can easily check this with an original PC-DOS 1.1 system diskette. Only two parameters are supported: /S and /1. Basically, the disk is always formatted double-sided with 8 sectors/track, unless the parameter /1 is specified. See below: Microsoft KB Article Q75131, 5.25 Inch

MS/PC-DOS era: Double to High Density

MS/PC-DOS era: High to Extra Density

When switching from HD to ED, the data rate was doubled again (500 kbps to 1,000 kbps). Furthermore, the number of sectors per track has been doubled. The recording technique has also changed from longitudinal to perpendicular.

For ED floppy disks you need a special 3.5" drive and also other floppy disks. These have a barium coating instead of the previous cobalt or iron coating.

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External Links

• Microsoft KB Article: Q75131
• Tim Patterson: A brief abstract about "All Those Floppy Disk Formats ..." ... the "Father of DOS". This article is a must read!
• Tim Patterson: The First DOS Machine
• Some history: The IBM Diskette General Information Manual