FLOPPY DISK

2/28/98

Floppy Disk: An inexpensive rotating computer storage medium often used for software distribution and storage. Traditionally, floppies have come in three sizes -- 8in, 5.25in and 3.5in. The first IBM PC used 5.25 inch disks. The AT switched to 3.5 inch disks. Several data densities have been used. Confusingly, computer disks in common use come in two densities -- Double Density (lower density) and High Density(higher density). A data density twice that of high density 3.5 inch floppy disk (extra high density) is available, but is rarely used.

5.25 inch floppies have a small notch in the envelope that is covered in order to write protect the disk. 3.5 inch floppies have a plastic slide that accomplishes the same thing. 5.25 inch floppies have a small locating hole hear the center used to mark sector 0. Some older drives used a second set of small holes to mark sectors. These disks may not work properly in a PC floppy drive. High Density 3.5 inch floppies have a second HD marker hole in addition to the write protect hole.

All PC floppies are read/written using MFM at constant angular velocity (constant rotation speed). Some Macintosh floppies were written at constant linear velocity. Floppies must be formatted by writing sector markers on the disk. Almost all floppies use 512 byte sectors although it is possible to format a floppy with larger sectors (and therefore less lost space in sector gaps) and some specialized programs do so. Some floppies are sold preformatted. Apple supports expensive 800 and 1.4 mb floppy drives. 1.4 Mb disks can be compatible with MSDOS disks. MSDOS supports a number of 5.25 inch floppy formats including 40 cylinder 8 sector per track single sided; 40-8-double sided; 40-9-double sided (365K) and 80-15-double sided (1.2mb). MSDOS also supports a number of 3.5 Inch formats 80-9-double sided (720K); 80-18-double sided (1.44Mb) and 160-18-double sided (2.88Mb). Special formats are used in Japan and on Windows 95 distribution disks. Although High Density (1.2/1.44Mb) drives can read and write Double Density formats, they may not be able to overwrite disks written on Double Density drives, and the disks they write may not read on Double Density drives. Microsoft introduced a 1.44Mb DMF high density floppy format for WIN95 distribution disks. DMF disks have 21 sectors per track. They are rarely used.

Floppy disk drives often have a plethora of jumpers identified by two letter codes that no one understands. Most drives use a standard 40 pin cable with a twist in some wires used to invert the ID for Drives A and B. A few vendors -- IBM, Tandy, Compaq have used drives with non-standard cabling and connectors. Japanese computers built for the home market often use 1.2mb floppies spun at 360 rpm with 1024 byte sectors.

PC Floppy drives rotate at 300rpm and typically have 3ms track to track access. If the drive is not spinning, the motor start time is typically 300ms. Sector skewing is not done on floppies. MSDOS defaults to Verify ON which requires a read of each sector written to verify that the CRC matches the data. Typical Verify-OFF floppy access times are about:

*Subtract .3Sec if drive is spinning at time operation starts

Typical transfer specifications of 500KBit/sec (high density) do not include various delays included above, and substantially overestimate practical data transfer rates. Multiplying the maximum rate by 0.4 will come close to the practical rate for large files.

The practical data transfer rate of floppy disks is largely determined by mechanical and rotational delays. The controller must be able to transfer a much higher data rate during the much shorter intervals when data is actually being moved. 8 Bit PCs (PCs and XTs) typically had floppy controllers that transferred only 300KBits/Sec and would not support Double Density drives. AT drives typically support 500KBits/Sec. As of 1998, many controllers will support 1000KBits/Sec.

Power consumption for mid 1990 drives are around 40mw standby, 1.5W normal, 3.2Watt peak during motor pulses. Operating temperature ranges are typically 5-45C. Non-operating ranges are -40C to 60C. MTBFs are typically 10,000 Power On Hours.

See

http://web.archive.org/web/20120410233702/http://www.tux.org/pub/knaff/fdutils/Fdutils.html

for details on formatting variants.

Return To Index Copyright 1994-2002 by Donald Kenney.