Disk operating system

Disk operating system

Disk Operating System (specifically) and disk operating system (generically), most often reveal themselves in abbreviated form as DOS, refer to an files of all sorts). Such software is referred to as a disk operating system when the storage devices it manages are made of rotating platters, such as floppy disks or hard disks.

In the early days of microcomputers, computer memory space was often limited, so the disk operating system was an extension of the operating system. This component was only loaded if needed. Otherwise, disk access would be limited to low-level operations such as reading and writing disks at the sector-level.

In some cases, the disk operating system component (or even the operating system) was known as DOS.

Sometimes, a disk operating system can refer to the entire operating system if it is loaded off a disk and supports the abstraction and management of disk devices. Examples include DOS/360. On the PC compatible platform, an entire family of operating systems was called DOS.


In the early days of computers, there were no disk drives, floppies 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, 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 were many such microcomputer systems.

Mature versions of the Commodore, SWTPC, Atari and Apple home computer systems all featured a disk operating system (actually called 'DOS' in the case of the Commodore 64 (CBM DOS), Atari 800 (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). Most home computer DOS'es were stored on a floppy disk always to be booted at start-up, with the notable exception of 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.

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 TRS-DOS, their Color Computer used OS-9, and most of the Intel 8080 based machines from IMSAI, MITS (makers of the legendary 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 floppy disks; among these exceptions were 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; and Commodore's CBM DOS, located in a ROM chip in each disk drive.

Disk operating systems that were extensions to the OS

  • The DOS operating system was the primary operating system for the Apple Computer's Apple II family of computers, from 1979 with the introduction of the floppy disk drive, until 1983 with the introduction of ProDOS; many people continued using it long after that date. Usually, it was called Apple DOS to distinguish it from MS-DOS.
  • Commodore DOS, which was used by 8-bit Commodore computers. Unlike most other DOS systems, it was integrated into the disk drives, not loaded into the computer's own memory.
  • Atari DOS, which 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 off a floppy that offered higher level functions such as filesystems.
  • 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 OS

Some disk operating systems were the operating system for the entire computer system.

The best known family of operating systems named "DOS" is that running on IBM PCs type hardware using Intel x86 CPUs or their compatible cousins from other makers. Any DOS in this family is usually just referred to as DOS. The original was 86-DOS, which would later become Microsoft MS-DOS. It was also licensed to IBM by Microsoft, and marketed by them as PC DOS. Digital Research produced a compatible variant known as DR DOS, which was eventually taken over (after a buyout of Digital Research) by Novell, then by Caldera. This became Novell DOS, then the open source OpenDOS, before being changed back to DR-DOS. There is also a free and open source version named FreeDOS.