List of floppy disk formats
A list of floppy disk formats documents the diverse physical and logical structures employed for storing data on flexible magnetic disks, primarily from the 1970s through the 1990s, including variations in size, density, capacity, and encoding methods across different computer systems.[1][2] These formats evolved to meet growing storage needs, starting with IBM's pioneering 8-inch single-sided single-density (SSSD) disk in 1971, which held approximately 80 KB of data and used 77 tracks with 26 sectors per track at 128 bytes per sector.[1][2] Subsequent developments introduced smaller form factors and higher densities for personal computing. The 5.25-inch format, commercialized by Shugart Associates in 1976, became standard for early microcomputers like the Apple II and IBM PC, with common variants including double-sided double-density (DSDD) at 360 KB (40 tracks per side, 9 sectors per track, 512 bytes per sector) and high-density (HD) at 1.2 MB (80 tracks per side, 15 sectors per track, 512 bytes per sector).[1][3] The 3.5-inch format, introduced by Sony in 1981 and popularized by Apple in 1984, offered greater durability with its rigid shell and sliding metal door; key MS-DOS-supported variants included double-sided double-density (DSDD) at 720 KB (80 tracks, 9 sectors per track, 512 bytes per sector) and high-density (HD) at 1.44 MB (80 tracks, 18 sectors per track, 512 bytes per sector), with an extra-density (ED) option at 2.88 MB (80 tracks, 36 sectors per track, 512 bytes per sector).[1][2][3] Beyond these mainstream standards, proprietary formats emerged for specific platforms, such as Apple's Macintosh using group-coded recording (GCR) for 400 KB and 800 KB disks, or Amiga systems employing modified frequency modulation (MFM) with interleaved sectors for 880 KB capacity.[2] Overall, floppy disk formats standardized data transfer rates (typically 250–500 kbit/s), sector organizations, and file systems like FAT, enabling interoperability but also leading to compatibility challenges across vendors until optical and solid-state media supplanted them by the early 2000s.[3][1]Introduction
Historical Development
The floppy disk was invented by IBM engineers in 1971 as a read-only 8-inch single-sided single-density (SSSD) format for loading microcode and software into IBM System/370 mainframe disk controllers, such as the 23FD “Minnow” system, offering 80 KB capacity and serving as an alternative to punched cards.[4] This format quickly found adoption in mainframe computing environments during the 1970s, where it served as a reliable, low-cost alternative for data loading and program distribution in enterprise settings.[1] It was later adapted for read-write use in the 1973 IBM System/370 Model 3740 data entry system, with the standardized SSSD format achieving approximately 256 KB formatted capacity using 77 tracks, 26 sectors per track, and 128 bytes per sector.[1] In 1976, Shugart Associates, founded by former IBM engineer Alan Shugart, introduced the 5.25-inch single-sided single-density format through its SA-400 drive, with unformatted capacity of up to 110 KB per disk (formatted approximately 98 KB) and paving the way for broader accessibility beyond mainframes.[5] The 1980s marked a period of rapid proliferation driven by the personal computing boom; the IBM PC, released in 1981, standardized 5.25-inch double-sided double-density (DSDD) disks at 360 KB, while Sony unveiled the 3.5-inch format in the same year, offering improved durability with a rigid plastic shell.[1][6] These developments were influenced by escalating demands for greater capacity, reduced manufacturing costs, and cross-vendor compatibility, though varying standards often created interoperability challenges.[7] By the 1990s, formats evolved to high-density (HD) variants, such as the 1.2 MB 5.25-inch in 1984 and 1.44 MB 3.5-inch in 1987, followed by extended-density (ED) 2.88 MB 3.5-inch drives adopted by IBM in 1991, alongside experimental portable options like the 120 MB LS-120 Superdisk introduced in 1997.[4][8][9] However, the rise of optical media like CD-ROMs in the mid-1990s and solid-state USB drives in the early 2000s rendered floppy disks obsolete for most applications by the decade's end, confining them to niche uses such as industrial boot disks and legacy systems.[6] Sony, the last major manufacturer, ceased production of 3.5-inch disks in March 2011 due to negligible demand.[10]Key Concepts and Terminology
Floppy disks consist of several core physical components that enable data storage and interaction with drive hardware. The disk media is a thin, flexible sheet of Mylar (polyethylene terephthalate) coated with a magnetic layer, typically iron oxide particles in a binder, allowing data to be recorded as magnetic domains.[11][12] At the center is a metal hub, often stainless steel, which engages the drive's spindle to rotate the disk at a constant speed.[11] A write-protect notch on the disk jacket, when uncovered, prevents writing by interrupting a sensor in the drive, safeguarding data from accidental erasure.[12] An index hole in the jacket aligns with a hole in the media to signal the start of each disk rotation, providing timing synchronization for read and write operations.[12] In the drive, read/write heads—spring-loaded electromagnetic components—hover over the disk surface to detect or alter magnetic fields, converting them to binary data via voltage changes.[13] A stepper motor positions these heads radially across the disk using discrete steps, ensuring precise alignment over specific storage areas.[13] Key physical parameters define the geometry of data storage on the disk surface. Disks vary in diameter, such as the common 8-inch size developed by IBM in the early 1970s.[14] The surface is organized into concentric tracks, narrow circular bands typically numbering 35 to 80 per side, where data is magnetically encoded.[15] Each track is subdivided into sectors, pie-shaped divisions holding fixed-size data blocks, usually 8 to 36 sectors per track depending on the format.[16] A cylinder refers to the set of tracks at the same radial position across both sides of a double-sided disk, allowing simultaneous access without head movement.[17] Density metrics quantify the storage efficiency of the media and drive. Linear density is measured in bits per inch (bpi) along a track, with frequency modulation (FM) encoding achieving around 4,000 to 5,876 bpi for single-density formats, while modified frequency modulation (MFM) reaches 8,717 to 10,000 bpi for double-density.[18] Track density, or tracks per inch (tpi), indicates radial spacing, commonly 48 tpi for early drives or 96 tpi for higher-capacity ones.[18] Media coercivity, the resistance to magnetic demagnetization measured in oersteds, is low (around 290 Oe) for single-density (SD) disks to suit weaker write fields, but higher (660 to 720 Oe) for high-density (HD) disks to support denser recording without signal degradation.[18][11] Floppy disks can be single-sided, using one read/write head to access only the top surface, or double-sided, employing two heads for both surfaces to double capacity.[19][17] Sectoring determines how sectors are demarcated: hard-sectored disks use physical holes near the hub (one per sector plus the index hole) for hardware detection, while soft-sectored disks rely on software-defined markers recorded magnetically during formatting for greater flexibility.[16] Basic capacity is calculated from physical geometry, with unformatted capacity approximating the raw storage before overhead: \text{Unformatted capacity} = (\text{tracks per side}) \times (\text{sectors per track}) \times (\text{bytes per sector}) \times (\text{sides}) For example, a single-sided single-density (SSSD) 8-inch disk with 77 tracks, 26 sectors per track, and 128 bytes per sector has an unformatted capacity of 256 KB; the formatted capacity is approximately 242 KB after accounting for gaps, headers, and other overhead.[20] Physical formats describe the hardware geometry and media characteristics, such as track count and density, while logical formats govern data layout, including sector organization and access protocols imposed by the operating system or controller.[21] This distinction allows the same physical disk to support varied logical structures across platforms.Physical Formats
8-inch Formats
The 8-inch floppy disk formats originated in the early 1970s as the first standardized removable magnetic storage medium, primarily for mainframe and minicomputer applications such as data entry and software distribution. These disks featured a 8-inch (200 mm) diameter flexible Mylar substrate coated with iron oxide particles, housed in a square protective envelope measuring approximately 8 by 8 inches. Recording occurred at a linear velocity of 360 RPM, with track spacing of 48 tracks per inch (tpi) and low coercivity media at 300 oersteds (Oe) to allow reliable writing using low-current heads. While some variants were hard-sectored with physical holes (typically 26 or 32 sector holes plus one index hole) to define fixed sector boundaries, the dominant IBM standard employed soft-sectoring, where sectors were defined by data patterns written during formatting. These formats evolved from single-density FM encoding to higher-density MFM, enabling increased capacities while maintaining compatibility with early systems like the IBM System/370.[14][22][23] The single-sided single-density (SSSD) format, introduced by IBM in 1971 for the System/370 mainframe, used FM encoding at approximately 400 bits per inch (bpi) linear density and was optimized for read-only or simple data loading tasks. It supported 77 tracks, 26 sectors per track, and 128 bytes per sector, delivering 256 KB unformatted capacity per side (approximately 246 KB formatted after overhead). This design equated to the storage of about 2,000 punched cards, facilitating the transition from card-based systems to magnetic media in enterprise environments.[23][4][22] Advancements in the mid-1970s introduced the double-sided double-density (DSDD) format, pioneered by Shugart Associates with the SA850 drive in 1976, which doubled both the number of recordable sides and the linear recording density via MFM encoding at around 800 bpi. Retaining 77 tracks per side, it increased to 26 sectors per track with 256 bytes per sector, achieving a total unformatted capacity of 1 MB (roughly 985 KB formatted). This format became widely adopted in minicomputers for its balance of capacity and compatibility, supporting both single- and double-density operation on the same media.[24][25] Less common variants included quad-density (QDD) formats, which appeared in specialized systems like those from Pertec Computer Corporation in the late 1970s. These rare configurations expanded to 80 tracks per side with variable sector counts and MFM encoding, reaching up to 1.6 MB unformatted capacity, often in double-sided implementations for enhanced storage in OEM peripherals. High-density (HD) upgrades emerged in the 1980s for minicomputer applications, boosting linear density to 2400 bpi using improved media and heads at 96 tpi, with unformatted capacities up to 9.6 MB to meet growing demands for larger datasets in systems like those from DEC and Wang.[26][27] Proprietary variants deviated from industry standards to optimize for specific hardware. The Memorex 650, a single-sided drive released in 1972, offered approximately 175 KB capacity using custom edge-indexed hard-sectoring, while the Memorex 651 extended this to around 312 KB in a similar single-sided design. Burroughs' B-800 system employed a double-sided proprietary format with custom coercivity media exceeding 300 Oe for improved signal stability, integrated as the industry-compatible mini-disk (ICMD) subsystem. These adaptations highlighted the flexibility of 8-inch media before standardization dominated.[25][28][29]| Format | Sides | Density | Tracks/Side | Sectors/Track | Bytes/Sector | Unformatted Capacity | Key System/Introduction | Encoding |
|---|---|---|---|---|---|---|---|---|
| SSSD | Single | Single | 77 | 26 | 128 | 256 KB/side | IBM System/370 (1971)[23] | FM |
| DSDD | Double | Double | 77 | 26 | 256 | 1 MB total | Shugart SA850 (1976)[24] | MFM |
| QDD | Double | Quad | 80 | Variable | Variable | 1.6 MB total | Pertec systems (late 1970s) | MFM |
| HD | Double | High (enhanced MFM) | 154 | 78 | 256 | 9.6 MB total | Minicomputer upgrades (1980s)[27] | MFM, 96 tpi |
| Memorex 650 | Single | Single | Variable | Variable | Variable | ~175 KB | Memorex (1972)[28] | FM |
| Memorex 651 | Single | Double | Variable | Variable | Variable | ~312 KB | Memorex (1970s)[28] | MFM |
| Burroughs B-800 | Double | Double | 77 | 26 | 256 | ~1 MB total | Burroughs B-800 (1970s), custom coercivity[29] | MFM |
5.25-inch Formats
The 5.25-inch floppy disk emerged as the primary removable storage medium for personal computers during the 1980s, offering a compact alternative to the 8-inch format while supporting capacities suitable for early operating systems and applications. These disks consist of a flexible magnetic medium housed in a square jacket measuring approximately 5.25 inches on each side, with the actual recording surface being a circular disk of the same diameter. They are typically soft-sectored, meaning sector boundaries are defined by data patterns rather than physical holes on the disk, allowing flexible formatting. Track densities vary by format, with double-density variants using 48 tracks per inch (tpi) and high-density using 96 tpi, enabling more precise head positioning for increased storage. A key physical feature is the write-protect notch on the disk's edge; covering it with adhesive tape prevents writing, while an uncovered notch allows modification.[30][25][31][32] Early 5.25-inch formats focused on single-sided single-density (SSDD) configurations for affordability in systems like the Apple II and TRS-80. These used 35 tracks, with sector counts varying by system: Apple II typically 16 sectors per track at 256 bytes (approximately 140 KB formatted), while TRS-80 Model I used 10 sectors per track at 256 bytes (approximately 87 KB formatted), resulting in formatted capacities of 87-140 KB. The Apple II's Disk II drive, for instance, employed a single-sided setup with 35 tracks and 16 sectors of 256 bytes to achieve around 140 KB usable space, prioritizing reliability over maximum density. Similarly, the TRS-80 Model I supported SSDD with 35 tracks and 10 sectors for basic program and data storage.[33][34][35] As personal computing advanced, double-sided double-density (DSDD) became the de facto standard, exemplified by the IBM PC introduced in 1981. This format doubled usable surface area and density, featuring 40 tracks per side, 9 sectors per track, and 512 bytes per sector for a formatted capacity of 360 KB. The drive rotated at 300 rpm using modified frequency modulation (MFM) encoding, with a data transfer rate of 250 Kbits/sec and support for up to two internal drives via a controller card.[36][37] Higher capacities followed with quad-density (QDD) and high-density (HD) variants. QDD, operating at 96 tpi like HD but with lower sector counts, provided 80 tracks per side, 9 sectors per track, and 512 bytes per sector for 720 KB formatted, as seen in external 5.25-inch drives for the Atari ST and Amiga systems designed for PC compatibility.[38][39] The HD format, introduced with the IBM PC AT in 1984, increased sectors to 15 per track while maintaining 80 tracks per side and 512 bytes per sector, yielding 1.2 MB and requiring high-coercivity media rated at approximately 720 oersteds (Oe) to prevent data remanence from adjacent tracks.[37][40]| Format | Tracks per Side | Sectors per Track | Bytes per Sector | Formatted Capacity | Key Systems | Notes |
|---|---|---|---|---|---|---|
| SSDD | 35 | 10 (TRS-80); 16 (Apple II) | 256 | 87-140 KB | Apple II, TRS-80 | Single-sided, 48 tpi, FM encoding typical.[33][41][35] |
| DSDD | 40 | 9 | 512 | 360 KB | IBM PC (1981) | Double-sided, 48 tpi, MFM encoding, 300 rpm.[36][37] |
| QDD | 80 | 9 | 512 | 720 KB | Atari ST, Amiga | Double-sided, 96 tpi, MFM, PC-compatible external drives.[38][39] |
| HD | 80 | 15 | 512 | 1.2 MB | IBM PC AT (1984) | Double-sided, 96 tpi, MFM, requires 720 Oe media.[37][40] |
3.5-inch Formats
The 3.5-inch floppy disk format emerged as a compact successor to the 5.25-inch design, particularly suited for portable computers due to its rigid enclosure and reduced vulnerability to damage. Developed initially by Sony in 1981, it standardized around a 90 mm × 94 mm × 3.3 mm cartridge housing a 85.8 mm diameter flexible magnetic disk, featuring a central metal hub for drive engagement and a sliding metal shutter to shield the recording surface from dust and wear. The format supports double-sided recording with 80 tracks per side at a linear density of 135 tracks per inch (tpi), enabling higher data packing than prior generations while using modified frequency modulation (MFM) encoding for high-density variants. Key physical characteristics include the disk's cobalt-doped oxide or barium ferrite coating, with coercivity ratings varying by density: approximately 600 oersteds (Oe) for double-density media and up to 900 Oe for high- and extended-density types to accommodate stronger magnetic fields without data loss. Drives rotate the disk at 300 revolutions per minute (RPM), providing an average access time of around 100 milliseconds. These traits contributed to the format's widespread adoption in systems from the mid-1980s onward, balancing portability with reliable data storage up to several megabytes.| Format | Tracks per Side | Sectors per Track | Bytes per Sector | Formatted Capacity | Notable Use Cases |
|---|---|---|---|---|---|
| DSDD (Double-Sided Double-Density) | 80 | 9 | 512 | 720 KB | Early Macintosh and Amiga systems |
| HD (High-Density) | 80 | 18 | 512 | 1.44 MB | IBM PS/2 standard (introduced 1987) |
| ED (Extended-Density) | 80 | 36 | 512 | 2.88 MB | Acorn and NeXT systems |
| 2HD (Japanese High-Density Variant) | 80 | 16 | 512 | 1.6 MB | Japanese PC-98 and compatible systems |