question: Why are main disks in our computer called C: by default and not A:?
comment: The a: and c: nomenclature come from IBM PC and MSDOS. In those early PC, there was one or two drives: floppy drives. They were the primary persistent memory. The first floppy drive was a:. The second one was b: Later on hard drives became an option and were thought of as an accessory after the main floppy drives, thus they were c:. Eventually the hard drive became the main drive in a PC, thus you’re most familiar with the c drive. Cd rom drives came later and are often d:. Dvd drives replaced them later and were still usually d:
passages: The initial design for the DFS was based around an Intel Corporation FDC 8271 disc drive controller, the immediate predecessor of the 8272 design found in the IBM Personal Computer. The 8271 controller was of limited functionality and obsolete, and later versions of the file system from various vendors including Solidisk, Acorn and Watford Electronics were based on the later WD1770 and WD1772 drive controllers. The 1770 controller quickly became the standard controller in the Acorn range following the advent of ADFS.
   Early operating systems used a command line interface in which the user typed short commands, often followed by various parameters and options. For operations on disks, one commonly specified the disk name or drive number (or letter). On early versions of MS-DOS, the codice_2 was usually the first floppy drive (DRIVE A:); on later versions that supported hard disk drives, the option was changed to the hard disk (DRIVE C:).
   Drive letters: In CP/M, the drive booted from is drive A, whether it's a floppy disk or a hard disk. In addition, double-sided Eagles addressed single-sided floppies as drive I or J. Which drive letter applied to which device did not change on a given system, but modifying systems could be confusing:
   Windows uses a "drive letter" abstraction at the user level to distinguish one disk or partition from another. For example, the path C:\WINDOWS represents a directory WINDOWS on the partition represented by the letter C. Drive C: is most commonly used for the primary hard disk drive partition, on which Windows is usually installed and from which it boots. This "tradition" has become so firmly ingrained that bugs exist in many applications which make assumptions that the drive that the operating system is installed on is C. The use of drive letters, and the tradition of using "C" as the drive letter for the primary hard disk drive partition, can be traced to MS-DOS, where the letters A and B were reserved for up to two floppy disk drives. This in turn derived from CP/M in the 1970s, and ultimately from IBM's CP/CMS of 1967.
   From a programming point of view, desk accessories were implemented, like other GEM applications, as DOS .EXE files, with names ending with .ACC (Accessory) rather than .APP (Application). Each .ACC file could support multiple accessories; all three of the standard GEM accessories (Calculator, Clock and Print Spooler) were provided by codice_1. Installation was simply a matter of placing the .ACC in the correct directory - codice_2 in earlier versions, and codice_3 in GEM/3 and later.
   BULLET::::- A: — Floppy disk drives, ″ or ″, and possibly other types of disk drives, if present.
  
  BULLET::::- B: — Reserved for a second floppy drive (that was present on many PCs).
  
  BULLET::::- C: — First hard disk partition.
   Writing at different densities than disks were intended for, sometimes by altering or drilling holes, was possible but not supported by manufacturers. A hole on one side of a ‑inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons. Some computers, such as the PS/2 and Acorn Archimedes, ignored these holes altogether.
   From RAMAC until the early 1960s most hard disk drive data were addressed in the form of a three number block addressing scheme Cylinder, Head & Sector (CHS); the cylinder number, which positioned the head access mechanism; the head number, which selected the read-write head; and the sector number, which specified the rotational position of a fixed size block. On June 2, 1961, IBM introduced the 1301, which had variable length records, and the market for sector-oriented disks was eclipsed for decades.
   MS-DOS/PC DOS versions 4.0 and earlier assign letters to all of the floppy drives before considering hard drives, so a system with four floppy drives would call the first hard drive E:. Starting with DOS 5.0, the system ensures that drive C: is always a hard disk, even if the system has more than two physical floppy drives.
   where "A" is the LBA address, "N" is the number of heads on the disk, "N" is the maximum number of sectors per track, and ("c", "h", "s") is the CHS address.
   By the 20th century, the "k" spelling was more popular in the United States, while the "c" variant was preferred in the UK. In the 1950s, when the American company IBM pioneered hard disk drive storage devices, it used the "k" spelling. Consequently, in computer terminology today it is common for the "k" word to refer mainly to magnetic storage devices (particularly in British English, where the term "disk" is sometimes regarded as a contraction of "diskette", a much later word and actually a diminutive of "disk").
  
  Section::::Computer discs.
   After floppy disks became obsolete in the early 2000s, the letters A and B became unused. But for 25 years, virtually all DOS-based PC software assumed the program installation drive was C, so the primary HDD continues to be "the C drive" even today.
  
  Other operating system families (e.g. Unix) are not bound to these designations.
  
  Section::::Technology.:Storage media.:OS support.
   In 1981, the Education Departments of Western Australia and South Australia announced joint tenders calling for the supply of personal computers to their schools. Acorn's Australian computer distributor, Barson Computers, convinced Joint Managing Directors Hermann Hauser and Chris Curry to allow the soon to be released Acorn BBC Microcomputer to be offered with disk storage as part of the bundle. They agreed on condition that Barson adapted the Acorn DFS from the System 2 without assistance from Acorn as they had no resources available. This required some minor hardware and software changes to make the DFS compatible with the BBC Micro.
   Some versions of DOS do not assign the drive letter, beginning with C:, to the first active primary partition recognized upon the first physical hard disk, but on the first primary partition recognized of the first hard disk, even if it is not set active.
  
  If there is more than one extended partition in a partition table, only the logical drives in the first recognized extended partition type are processed.
   Section::::History.:Prevalence.
  
  Floppy disks became commonplace during the 1980s and 1990s in their use with personal computers to distribute software, transfer data, and create backups. Before hard disks became affordable to the general population, floppy disks were often used to store a computer's operating system (OS). Most home computers from that period have an elementary OS and BASIC stored in ROM, with the option of loading a more advanced operating system from a floppy disk.
   Section::::Alignment.
  
  Disk drives with exchangeable disk packs or disk cartridges generally required the data heads to be aligned to allow packs formatted on one drive to be read and written on another compatible drive. Alignment required a special "alignment pack", an oscilloscope, an alignment tool that moved the read/write heads, and patience. The pattern generated on the scope looks like a row of alternating "C" and "E" characters on their backs. Head alignment needed to be performed after head replacement, and in any case on a periodic basis as part of the routine maintenance required by the drives.
   BULLET::::- Disc Filing System (DFS) This was an optional component for the Acorn BBC Micro, offered as a kit with a disk controller chip, a ROM chip, and a handful of logic chips, to be installed inside the computer
  
  BULLET::::- Advanced Disc Filing System (ADFS) was a successor to Acorn's DFS.
  
  BULLET::::- AMSDOS for the Amstrad CPC computers.
  
  BULLET::::- GDOS and G+DOS, for the +D and DISCiPLE disk interfaces for the ZX Spectrum.
  
  Section::::Disk operating systems that were the main OS.
  
  Some disk operating systems were the operating system for the entire computer system.
   Under Windows XP, a USB SuperDisk drive will appear as a 3.5" floppy disk drive, receiving either the drive letter A: (if there is no floppy in the machine) or B: (if there already is one). This enables use by software that expects a floppy drive when 1.44 MB or 720 KB disks are inserted. 120 MB and 240 MB disks are also accessed via A: or B:.
  
  Section::::Practicality.
   Section::::History.:Hardware versions.
  
  Sharp released the Twin Famicom, a Famicom model that features a built-in Disk System.
  
  Section::::History.:Disk Writer and Disk Fax kiosks.
   The new device first shipped in 1971 as the 23FD, the program load component of the 2835 Storage Control Unit. and then as a standard part of most System 370 processing units and other IBM products. Internally IBM used another device, code named "Mackerel", to write boot disks for distribution to the field.
   Since it could not connect to director type storage control the 2319-A1 was not an "A-unit" as that term came to be used by IBM.
  
  Section::::Integrated controls attaching A-units.
  
  Section::::Integrated controls attaching A-units.:Direct disk attachments.
  
  Section::::Integrated controls attaching A-units.:Direct disk attachments.:S/370 125-0 and -2.
   BULLET::::- For Disk Operating Systems that do not use the index hole in the disk to mark the beginnings of tracks, the "flippy" modification required only a new write-enable notch to be cut if the disk was designed to be written to. For this purpose, specially designed single-rectangular-hole punchers, commonly known as disk doublers, were produced and sold by third-party computer accessory manufacturers. Many users, however, made do with a standard (round) hole puncher and/or an ordinary pair of scissors for this job.
   While without deliberate remapping, the drive letter assignments are typically fixed until the next reboot, however, Zenith MS-DOS 3.21 will update the drive letter assignments when resetting a drive. This may cause drive letters to change without reboot if the partitioning of the harddisk was changed.
  
  MS-DOS on the Apricot PC assigns letters to hard drives, starting with A:, before considering floppy drives. A system with two of each drive would call the hard drives A: and B:, and the floppies C: and D:.
   Inductive heads use the same element for both reading and writing.
  
  Section::::Inductive heads.:Traditional head.
   Floppy disks, initially as media and later in and inch (90 mm) sizes, were a ubiquitous form of data storage and exchange from the mid-1970s into the first years of the 21st century. By 2006 computers were rarely manufactured with installed floppy disk drives; -inch floppy disks can be used with an external USB floppy disk drive, but USB drives for -inch, 8-inch, and non-standard diskettes are rare to non-existent. These formats are usually handled by older equipment.
  