Disk operating system
A disk operating system (abbreviated DOS) is a computer operating system that resides on and can use a disk storage device, such as a floppy disk, hard disk drive, or optical disc. A disk operating system must provide a file system for organizing, reading, and writing files on the storage disk. Strictly speaking, this definition does not apply to current generations of operating systems, such as versions of Microsoft Windows in use, and is more appropriately used only for older generations of operating systems.

Disk operating systems were available for mainframes, minicomputers, microprocessors and home computers and were usually loaded from the disks themselves as part of the boot process.

History
In the early days of computers, there were no disk drives, floppy disks or modern flash storage devices. Early storage devices such as delay lines, core memories, punched cards, punched tape, magnetic tape, and magnetic drums were used instead. And in the early days of microcomputers and home computers, paper tape or audio cassette tape (see Kansas City standard) or nothing were used instead. In the latter case, program and data entry was done at front panel switches directly into memory or through a computer terminal / keyboard, sometimes controlled by a BASIC interpreter in ROM; when power was turned off any information was lost.

In the early 1960s, as disk drives became larger and more affordable, various mainframe and minicomputer vendors began introducing disk operating systems and modifying existing operating systems to exploit disks.

Both hard disks and floppy disk drives require software to manage rapid access to block storage of sequential and other data. For most microcomputers, a disk drive of any kind was an optional peripheral; systems could be used with a tape drive or booted without a storage device at all. The disk operating system component of the operating system was only needed when a disk drive was used.

By the time IBM announced the System/360 mainframes, the concept of a disk operating system was well established. Although IBM did offer Basic Programming Support (BPS/360) and TOS/360 for small systems, they were out of the mainstream and most customers used either DOS/360 or OS/360.

Most home and personal computers of the late 1970s and 1980s used a disk operating system, most often with "DOS" in the name and simply referred to as "DOS" within their respective communities: CBM DOS for Commodore 8-bit systems, Atari DOS for the Atari 8-bit family, TRS-DOS for the TRS-80, Apple DOS and ProDOS for the Apple II, AmigaDOS for the Amiga, and MS-DOS for IBM PC compatibles.

Usually, a disk operating system was loaded from a disk. Among the exceptions were Commodore, whose DOS resided on ROM chips in the disk drives. The Lt. Kernal hard disk subsystem for the Commodore 64 and Commodore 128 models stored its DOS on the disk, as is the case with modern systems, and loaded the DOS into RAM at boot time; the British BBC Micro's optional Disc Filing System, DFS, offered as a kit with a disk controller chip, a ROM chip, and a handful of logic chips, to be installed inside the computer.

Disk operating systems that were extensions to the OS
Apple DOS was the primary operating system for the Apple II series of computers, from 1979 with the introduction of the floppy disk drive, until 1983 when it was replaced by ProDOS.
Commodore DOS was used on 8-bit Commodore computers such as the Commodore 64. Unlike most other DOS systems, it was integrated into the disk drives, not loaded into the computer's own memory.
Atari DOS was used by the Atari 8-bit family of computers. The Atari OS only offered low-level disk-access, so an extra layer called DOS was booted from a floppy and offered higher level functions such as filesystems. Third-party replacements for Atari DOS were also available: DOS XL, SpartaDOS, MyDOS, TurboDOS, Top-DOS.
MSX-DOS for the MSX computer standard. Initial version, released in 1984, was nothing but MS-DOS 1.0 ported to Z80; but in 1988 it evolved to version 2, offering facilities such as subdirectories, memory management and environment strings. The MSX-DOS kernel resided in ROM (built-in on the disk controller) so basic file access capacity was available even without the command interpreter, by using BASIC extended commands.
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
Advanced Disc Filing System (ADFS) was a successor to Acorn's DFS.
AMSDOS for the Amstrad CPC computers.
GDOS and G+DOS, for the +D and DISCiPLE disk interfaces for the ZX Spectrum.

Disk operating systems that were the main OSs
Some disk operating systems were the operating systems for the entire computer system.
The Burroughs Master Control Program (MCP) for the B5000 originally ran from a drum, but starting with the B5500 it ran from a disk. It is the basis for the MCP on the B6500, B7500 and their successor systems.
The SIPROS, Chippewa Operating System (COS), SCOPE, MACE and KRONOS operating systems on the Control Data Corporation  (CDC) 6000 series and 7600 were all disk operating systems. KRONOS evolved into NOS and SCOPE evolved into NOS/BE.
The GECOS operating system for the GE 600 family of mainframe computers (it later became GCOS).
The IBM Basic Operating System/360 (BOS/360), Disk Operating System/360 (DOS/360) and Operating System/360 (OS/360) were standard for all but the smallest System/360 installations. BOS is gone, DOS has evolved into z/VSE and OS has evolved into z/OS.
The DOS-11 operating system for DEC PDP-11 minicomputers.
CP/M was a disk operating system--even though it was not named as such--used as the main or alternate operating system for numerous microcomputers of the 1970s and 1980s.
TRSDOS was the operating system for the TRS-80 line of computers from Tandy.
MS-DOS for IBM PC compatibles with Intel x86 CPUs. It started as 86-DOS, which was modeled on CP/M, and then itself served as the basis for Microsoft's MS-DOS, and was rebranded by IBM as IBM PC DOS until 1993. Various compatible systems were later produced by different organizations, starting with DR-DOS in 1988.

See also
 Live CD

References

Notes

Disk operating systems