Disk operating system

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.

Disk operating systems were available for mainframes, 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, 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; and Commodore's CBM DOS, located in a ROM chip in each disk drive.

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 renamed ProDOS.
• Commodore DOS, which was used by 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, 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 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 OS

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

• The DOS/360 initial/simple operating system for the IBM System/360 family of mainframe computers (it later became DOS/VSE, and was eventually just called VSE).
• The DOS operating system for DEC PDP-11 minicomputers.
• 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 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.