A disk operating system (abbreviated DOS) is a computer operating system that 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.
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, punched cards, paper 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 read-only memory (ROM) BASIC interpreter; when power was turned off after running the program, the information so entered vanished.
Both hard disks and floppy disk drives require software to manage rapid access to block storage of sequential and other data. When microcomputers rarely had expensive disk drives of any kind, the need to have software to manage such devices (the disks) carried much status. To have one or the other was a mark of distinction and prestige, and so was having the disk sort of an operating system. As prices for both disk hardware and operating system software decreased, there came to be many such microcomputer systems.
Mature versions of the Commodore, SWTPC, Atari 8-bit, and Apple II home computer systems all featured a disk operating system (actually called DOS in the case of the Commodore 64 (CBM DOS), Atari 8-bit family (Atari DOS), and Apple II machines (Apple DOS)), as did (at the other end of the hardware spectrum, and much earlier) IBM's System/360, 370 and (later) 390 series of mainframes (e.g., DOS/360: Disk Operating System / 360 and DOS/VSE: Disk Operating System / Virtual Storage Extended).
In large machines there were other disk operating systems, such as IBM's VM, DEC's RSTS / RT-11 / VMS / TOPS-10 / TWENEX, MIT's ITS / CTSS, Control Data's assorted NOS variants, Harris's Vulcan, Bell Labs' Unix, and so on. In microcomputers, SWTPC's 6800 and 6809 machines used TSC's FLEX disk operating system, Radio Shack's TRS-80 machines used TRSDOS, their Color Computer used OS-9, and most of the Intel 8080 based machines from IMSAI, MITS (makers of the Altair 8800), Cromemco, North Star, etc., used the CP/M-80 disk operating system. See list of operating systems.
Usually, a disk operating system was loaded from a disk. Only a very few comparable DOSes were stored elsewhere than on floppy disks; among these exceptions were Commodore, whose DOS resided on ROM chips in the disk drives themselves (the computer itself had no DOS, just a form of a BIOS for communicating with peripherals). 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.
Some disk operating systems were the operating system for the entire computer system.