Editing Disk formatting (section)

=== Low-level formatting of floppy disks ===

The low-level format of floppy disks (and early hard disks) is performed by the disk drive's controller.

For a standard [[Floppy disk#microfloppy|1.44 MB floppy disk]], low-level formatting normally writes 18 [[Disk sector|sector]]s of 512 [[byte]]s to each of 160 tracks (80 on each side) of the floppy disk, providing 1,474,560 bytes of storage on the disk.

Physical sectors are actually larger than 512 bytes, as in addition to the 512 byte data field they include a sector identifier field, [[Cyclic redundancy check|CRC]] bytes (in some cases [[Error detection and correction|error correction bytes]]) and gaps between the fields. These additional bytes are not normally included in the quoted figure for overall storage capacity of the disk.

Different low-level formats can be used on the same [[Recording medium|media]]; for example, large records can be used to cut down on inter-record gap size.

Several [[freeware]], [[shareware]] and [[free software]] programs (e.g. [[GParted]], [[Fdformat|FDFORMAT]], NFORMAT, [[VGA-Copy]] and 2M) allowed considerably more control over formatting, allowing the formatting of high-density 3.5" disks with a capacity up to 2 MB.

Techniques used include:

* head/track sector skew (moving the sector numbering forward at side change and track stepping to reduce mechanical delay),
* [[interleaving (disk storage)|interleaving]] sectors (to boost throughput by organizing the sectors on the track),
* increasing the number of sectors per track (while a normal 1.44 MB format uses 18 sectors per track, it is possible to increase this to a maximum of 21), and
* increasing the number of tracks (most drives could tolerate extension to 82 tracks: though some could handle more, others could jam).

[[Linux]] supports a variety of sector sizes,<ref>{{Cite web|url=https://tools.ietf.org/doc/fdutils/Fdutils.html#Media-description|title = Fdutils}}</ref> and [[DOS]] and [[Microsoft Windows|Windows]] support a large-record-size [[Distribution Media Format|DMF]]-formatted floppy format.<ref>{{cite web
|url=http://support.microsoft.com/kb/120348
|title=Definition of Distribution Media Format (DMF)
|publisher=[[Microsoft Knowledge Base]]
|date=2007-01-19
|access-date=2011-10-16
|archive-url=https://web.archive.org/web/20110914232540/http://support.microsoft.com/kb/120348
|archive-date=2011-09-14
|url-status=dead}}</ref>

After establishing the structure of tracks, a formatter also needs to fill the entire floppy and look for [[bad sector]]s. Traditionally, the physical sectors were initialized with a fill value of <code>0xF6</code> as per the INT&nbsp;1Eh's [[Disk Parameter Table]] (DPT)<!-- TBD: IIRC this resembles a bit pattern optimized for MFM controllers --> during format on IBM compatible machines. This value is also used on the [[Atari Portfolio]]. [[CP/M]] 8-inch floppies typically came pre-formatted with a value of <code>0xE5</code>,<ref name="Schulman_1994_Undocumented-DOS">{{cite book |author-first1=Andrew |author-last1=Schulman |author-first2=Ralf D. |author-last2=Brown |author-link2=Ralf D. Brown |author-first3=David |author-last3=Maxey |author-first4=Raymond J. |author-last4=Michels |author-first5=Jim |author-last5=Kyle |title=Undocumented DOS: A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1 |publisher=[[Addison Wesley]] |edition=2 |date=1994 |orig-year=November 1993<!-- first printing --> |isbn=0-201-63287-X |url=https://archive.org/details/undocumenteddosp00andr_0 }} (xviii+856+vi pages, 3.5"-floppy) Errata: [https://web.archive.org/web/20190417215556/http://www.cs.cmu.edu/afs/cs/user/ralf/pub/books/UndocumentedDOS/errata.ud2][https://web.archive.org/web/20190417212906/https://www.pcjs.org/pubs/pc/programming/Undocumented_DOS/#errata-2nd-edition]</ref> and by way of [[Digital Research]] this value was also used on [[Atari ST]] and some [[Amstrad]] formatted floppies.<ref group="lower-alpha" name="NB_Magic_E5"/> Amstrad otherwise used <code>0xF4</code> as a fill value.