Floppy Disk Definitions

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Floppy Disk Controller/Formatter

Floppy Disk Controllers (FDCs) are manufactured by several companies. The most popular are the i8272As and its clones, the N765As. There are derivatives of these chips in the ASIC's that put many peripherals into one chip. Also popular were the Western Digital 199X and 279X chips. The Intel and WD chips had completely incompatible programming interfaces and with the advent of the IBM PC the Western Digital chips fell out of favor.

The FDCs were the chips the controlled the floppy disk drives and interfaced to the host processor. They hide a lot of the complexity of reading from and writing to an FDD from the application programmer.

Floppy Disk Drives

The floppy disk drives (FDDs) are the mechanical devices that allow the FDC to read from and write to the physical media. The data to/from the FDC consists of control lines (like head and drive select and double side detect) and a bit stream (the data to/from the FDD).

Media Size

FDDs come mostly in three different sizes, 8-inch, 5.25-inch and 3.5-inch. There have been others but these are the sizes you will find today. The 8-inch came first, then the 5.25-inch and then the 3.5-inch. Today’s machines have no FDDs.


Tracks refer to one circular segment under one head. A cylinder is the same circular segment under two heads. Cylinders form concentric circles on the surface of the drive media. The length of a track gets longer as the head moves out from the center. Tracks are numbered from 0 to the maximum, going from the outside to the inside.


Sectors are the data areas spread around the cylinder on the surface of the media.

Sector Size

Sector size is specified with the variable N where N is 0 to 7. The sector size is always a power of 2. The formula is: SECSIZ = 128 * N. Thus N=0 is 128b, n=2 is 512b, etc.

Hard Sectored

On hard sectored media, there is a special index mark for the beginning of the track, and a regular index mark for each physical sector. This makes design of FDCs easier but makes the disk format fixed. Tandy TRS-80 and Heathkit H-8 machines used hard sectored FDDs. This document will be concerned with soft sectored formats.

Soft Sectored

On soft sectored media, there is only one index hole in the media. When the FDC senses the index hole, after the head load delay and motor spin up time (if used), it begins reading address (AM) marks to find the required sector number. When the sector number is found, the read/write operation begins.


Skew is the ordering of sectors on the cylinder. Most (FDCs can write the sectors to the track in any order desired. If the FDC writes the sectors in order then the operating system (OS) can implement the skew by mapping sectors between the OS and the physical media. CPM-80 does sector mapping in the BIOS.

Skew was developed to allow the processor time to handle the previous sector before the next sector was under the read head. This greatly speeded up read/write operations between the FDDs and the FDC and thus, the host processor.


Gaps are spaces in the format of a track that allow the FDC time to change modes between read and write and handle the location information recorded on the track.

Single Sided

Single sided (SS) drives only wrote on one side of the media. Enterprising souls created 5.25-inch flippies by placing index holes so they floppy could be inserted upside down and the other side used. This was done with both the early 5.25 and 8-inch SS media which was enclosed in soft mylar envelopes.

Double Sided

Double sided (DS) drives evolved by placing heads on both sides of the media. The concept of head select had to be handled by the FDCs. Two heads doubled the SS size of a particular media.

Single Density

Single density (SD) was the first type of media available. Data was transferred at 250 Kbps on 8-inch drives and at 125 Kbps on 5.25-inch drives. This was referred to as FM.

Double Density

Double density (DD) was the second type of media available. This media could hold twice as much data as SD media. Data was transferred at 500 Kbps on 8-inch drives, at 250 Kbps on 5.25-inch drives and at 250 Kbps on 3.5-inch drives. This was referred to as MFM.

Quad Density

Quad density (QD) was the third type of media available. This media could hold twice as much data as DD media. This was gained by writing a narrower cylinder and writing twice as many cylinders on the media. Data was transferred at 125 (FM) or 250 (MFM) Kbps on 5.25-inch drives. As far as I know, this was only produced as 5.25-inch disk drives

High Density

High density (HD) was the fourth type of media available. This media could hold twice as much data as DD media. This was only produced as 5.25- and 3.5-inch drives. Data was transferred at 500 kbps on 5.25-inch drives. Data was transferred at 500 Kbps on 3.5-inch drives.

Disk Speed

Disk rotational speed varied between the different disk drives. 8-inch drives spin at 360 rpm. 5.25-inch SD, DD and QD drives spin at 300 rpm. 3.5-inch drives spin at 300 and 360 rpm.

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Last Modified: 12 July 2015