Floppy Disk - Sizes - Sizes, Performance and Capacity

Sizes, Performance and Capacity

Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3 1⁄₂-inch disks is entitled in part "90 mm (3.5 in)" though 90 mm is closer to 3.54 inches. Formatted capacities are generally set in terms of kilobytes and megabytes.

Data is generally written to floppy disks in sectors (angular blocks) and tracks (concentric rings at a constant radius). For example, the HD format of 3 1⁄₂-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. Some disk controllers can vary these parameters at the user's request, increasing storage on the disk, although they may not be able to be read on machines with other controllers. For example, Microsoft applications were often distributed on 3 1⁄₂-inch 1.68 MB DMF disks formatted with 21 sectors instead of 18; they could still be recognized by a standard controller. On the IBM PC, MSX and most other microcomputer platforms, disks were written using a Constant Angular Velocity (CAV) format, with the disk spinning at a constant speed and the sectors hold the same amount of information on each track regardless of radial location.

This was not the most efficient way to use the disk surface with available drive electronics; because the sectors have constant angular size, the 512 bytes in each sector are compressed more near the disk's center. A more space-efficient technique would be to increase the number of sectors per track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping constant the amount of physical disk space used for storing each sector; an example is zone bit recording. Apple implemented this in early Macintosh computers by spinning the disk slower when the head was at the edge, while maintaining the data rate, allowing 400 kB of storage per side and an extra 160 kB on a double-sided disk. This higher capacity came with a disadvantage: the format used a unique drive mechanism and control circuitry, meaning that Mac disks could not be read on other computers. Apple eventually reverted to constant angular velocity on HD floppy disks with their later machines, still unique to Apple as they supported the older variable-speed formats.

Disk formatting is usually done by a utility program supplied by the computer OS manufacturer; generally, it sets up a file storage directory system on the disk, and initializes its sectors and tracks. Areas of the disk unusable for storage due to flaws can be locked (marked as "bad sectors") so that the operating system does not attempt to use them. This was time consuming so many environments had quick formatting which skipped the error checking process. When floppy disks were often used, disks pre-formatted for popular computers were sold. A formatted floppy disk does not include the sector and track headings of an unformatted disk; the difference in storage between them depends on the drive's application. Floppy disk drive and media manufacturers specify the unformatted capacity (for example, 2 MB for a standard 3 1⁄₂-inch HD floppy). It is implied that this should not be exceeded, since doing so will most likely result in performance problems. DMF was introduced permitting 1.68 MB to fit onto an otherwise standard 3 1⁄₂-inch disk; utilities then appeared allowing disks to be formatted as such.

Mixtures of decimal prefixes and binary sector sizes require care to properly calculate total capacity. Whereas semiconductor memory naturally favors powers of two (size doubles each time an address pin is added to the integrated circuit), the capacity of a disk drive is the product of sector size, sectors per track, tracks per side and sides (which in hard disk drives can be greater than 2). Although other sector sizes have been known in the past, formatted sector sizes are now almost always set to powers of two (256 bytes, 512 bytes, etc.), and in some cases, disk capacity is calculated as multiples of the sector size rather than in just bytes, leading to a combination of decimal multiples of sectors and binary sector sizes. For example, 1.44 MB 3 1⁄₂-inch HD disks have the "M" prefix peculiar to their context, coming from their capacity of 2,880 512-byte sectors (1,440 KiB), inconsistent with either a decimal megabyte nor a binary mebibyte (MiB). Hence, these disks hold 1.47 MB or 1.41 MiB. Usable data capacity is a function of the disk format used, which in turn is determined by the FDD controller and its settings. Differences between such formats can result in capacities ranging from approximately 1,300 to 1760 KiB (1.80 MB) on a "standard" 3 1⁄₂-inch high density floppy (and up to nearly 2 MB with utilities such as 2MGUI). The highest capacity techniques require much tighter matching of drive head geometry between drives, something not always possible and unreliable. For example, the LS-240 drive supports a 32 MB capacity on standard 3 1⁄₂-inch HD disks, but it is, however, a write-once technique, and requires its own drive.

The raw maximum transfer rate of 3 1⁄₂-inch HD floppy drives and interfaces, disregarding overheads, is as much as 1000 kilobit/s, or approximately 83% that of single-speed CDROM (71% of audio CD). This represents the speed of raw data bits moving under the read head; however, because of the very high amount of overhead in the system (use of soft sectors with headers, sync issues preventing sequential reads of an entire 18-sector track in a single rotation, etc.), the actual user data read/write speed is much lower. In fact, a DSHD diskette formatted with an efficient non-sequential (interleaved or "twist") sector layout could sync and read an average of only slightly more than three double-sided pairs of 512-byte sectors per 0.2s revolution, or a little over 15 sectors/second, for an effective data rate of approximately 125 kbit/s. At this speed, a single, disk-filling file would take a good 90 seconds to transfer; smaller and/or fragmented files further reduced transfer speed because of the slow head seek speed and the requirement to re-read the FAT from Track 0 along with any folder data, as removable media is rarely cached. Unusually, when compared to hard disks, optical drives and archive tapes, the floppy disk standard proper did not receive any further successful speed or capacity upgrades throughout its period of relevance, from the mid-80s introduction of DSHD through to its eventual death more than 20 years later.

However, some developments did seek to improve this, but with limited success. Double-sided extended-density (DSED) 3 1⁄₂-inch floppy disks, introduced by Toshiba in 1987 and adopted by IBM on the PS/2 in 1994, doubled the number of sectors per track, thereby providing double the data rate and capacity of conventional DSHD 3 1⁄₂-inch drives. Although it was not enabled by default, both the MSDOS/Windows 3.1 "Smartdrive" caching TSR and the system cache of later Windows versions can be configured to cache removable drives, including floppy disks. Similarly, some USB floppy drives use caching to increase performance while being built from standard speed drives; alternatively, the X10 accelerated floppy drive was an attempt to physically increase floppy performance by increasing spindle RPM.

More successfully, a number of (typically QIC-standard) tape-based backup drives that interfaced via the floppy drive controller were developed and sold by manufacturers such as Travan and Iomega. These made better use of the available bandwidth, and eventually pushed the 500/1000 kbit/s limits of standard (DD/HD) motherboard floppy disk controllers; higher end models could make use of the 2000 kbit/s bandwidth of DSED controllers, and plug-in "high speed" adapter cards were offered for PCs lacking this capability. Though inadequate by modern standards, their speed was competitive with early CD recorders and Zip drives, and was sufficient for overnight backups of a contemporary home or small office users' hard drive.