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