DOS is the abbreviation of Disk Operating System. It is an operating system in personal computer. From 1981 to 1995, DOS played an important role in the market of IBM compatible PC. And, if all Microsoft Windows editions including Windows 95, Windows 98 and Windows ME based on DOS are accumulated, the commercial lifespan of DOS is 2000 years at least.

Basic Situation of DOS
DOS clan includes MS-DOS, PC-DOS, DR-DOS, FreeDOS, PTS-DOS, ROM-DOS,JM-OS and so on, among them MS-DOS is the most famous. Although these systems are called DOS for short, there is no system simply named DOS (except the irrelevant main frame operating system of IBM in 1960s). Besides, names of several disk operating systems that are based on non-x86 microcomputer system also contain characters "DOS". And these systems( for example, AmigaDOS, AMSDOS, ANDOS, Apple DOS, Atari DOS, Commodore DOS, CSI-DOS, ProDOS, TRS-DOS and so on) are simply called "DOS" in the dedicated symposia of these machines. But these operating systems are irrelevant to DOS and they are not compatible with DOS EXE file and MS-DOS API.

Detailed Annotation of DOS
Dos is mainly disk-oriented system software. Simply speaking, DOS is a bridge between human and computer. It is a "shell" of machine hardware. With DOS, user needs not to know hardware structure in depth and cram up the boring machine commands. Most usual operations can be accomplished by using DOS commands which are similar to natural language. In addition, DOS can efficiently manage various kinds of software and hardware resources and schedul them reasonably. Under the control of DOS, all softwares and hardwares work orderly.
Along with the development of computer technology, network is rapidly spreading and developing. When users enjoy the convenient brought by network, they are also attacked by hackers. Among all attacks, the attack named DoS (Denial of Service) is a common and efficient network attack technology. It takes advantage of treaty or system defects, adopts a fraudulent and simulates strategy to attack network, and eventually makes the system paralyzed because of resources exhaustion or failure in response, then the legitimate users can not be provided with normal service. DoS looks unromantic, but its attacks is in wide range, covert and simple and efficient, which make it a strong attack technology in network. And it tremendously affects the effective service of network and host machine system. And DDoS (Distributed Denial of Service) is most famous for its larger scale, higher disguised and it is more difficult to defend

Components of DOS
The rudimentary DOS system is composed of bootstrap based on MBR and three file modules which are IO.SYS, MSDOS.SYS and COMMAND.COM. In addition, Microsoft also adds several standard external programs (external commands) to DOS system package retailed. Together with internal commands (commands executed by COMMAND.COM), they build a relative completed HCI (human-computer interaction) environment in the era of disk operation. List of DOS commands is referrence for related DOS commands

Development History of DOS
DOS1.0: In July,1981, the copyright of 86-DOS（QDOS）is bought by Microsoft, and it became the first operating system in IBM PC history. Meanwhile, Microsoft developed an exclusive edition PC-DOS for IBM PC. Except for system file names and partial kernels designed for IBM machines and external commands and utility programs, the other mobile codes in DOS and PC-DOS have small difference.
DOS 1.25: In July, 1982, it began to support double-sided floppy and correct mistakes. Not only IBM, but OEM also applied it widely.
DOS 2.0: In March, 1983, UNIX type of dendriform file system and hard disk support are added,. Seen from a lower level, it is the first significant revision of DOS. DOS 2.11: In March, 1984, the errors of edition 2.01 were amended for primary OEM edition, and international support for Non-English language and file format was added.
MS-DOS 2.25: In October, 1985, additional expanded character sets (Japanese and Korean) were added and the error of the old edition was amended.
MS-DOS 3.0: In August, 1984, PC/AT supporting for floppy of 1.2MB was introduced and hard disk of larger capacity was added.
MS-DOS 3.1: In November, 1984, support for Microsoft network was added.
MS-DOS 3.2: In January, 1986, enhanced support for floppy was added, and the hardware EMS can be applied through driver.
MS-DOS 3.3: In August, 1987, partial internal errors were amended, external commands related to file handle were added and it began to support floppy. DOS 3.3 was the most stable and salable edition. It was the second significant revision of DOS. And the 4.0 edition released one year later was not as popular as 3.3 edition.
MS-DOS 4.0: In July, 1988, MS-DOS 4.0 was based on Code Base of IBM rather than that of Microsoft itself.
MS-DOS 4.01: In September, 1988, it amended a series of errors of the old edition.
MS-DOS 5.0: In June, 1991, it was the third revision of DOS; maybe it was the most significant one in DOS history.
MS-DOS 6.0: In March, 1993, many gaudy external commands were added.
MS-DOS 6.2: In November, 1993, DoubleSpace was renamed as DrvSpace.
MS-DOS 6.22: In June, 1994, it was the last sale edition. KC got the authorities of Microsoft and IBM and it released its own DOS 7.0 collocated with KC Chinese System.
MS-DOS 7.0: In August, 1995, it is a version built in Windows 95, and it supported long file name.
MS-DOS 7.1: In August, 1996, supports for large hard disk and FAT32 partition were added.
MS-DOS 8.0: In September, 2000, it is the last edition of DOS. Partial functions were canceled. For example, it did not support SYS commands and printer operation.

Software Running in DOS
DOS is the main platform of compatible PC. There are many well-known soft ware designed for DOS. However, when Windows operating system can not started, most users chose DOS system to detect and solve the problems. The following softwares are included.
Lotus 1-2-3: a trial table software applied widely in enterprise market, thought to be the main factor for IBM PC success.

MiniTool Partition Wizard Bootable CD: a professional partition magic can recover partitions when Windows operating system can not be started normally.
WordPerfect: a edit-plus base on Windows platform
dBase: one of the earliest data base software
Telix: modulator-demodulator communication soft ware
Arachne: internet explorer of DOS edition
DJGPP: DOS transplantation version of GCC
4DOS: a software replacing the original command line interface
IDE (integrated development environment) of Borland includes Turbo Pascal, Turbo BASIC, Turbo C, and Turbo Assembler
Dial-up bulletin board system (BBS)
Site-built software: RemoteAccess, Spitfire, Maximus, McBBS and TAG
BASIC language tool program: BASICA and GW-BASIC
Many FPS( first person shooting ) games: the 3D Wolfenstein developed jointly by Id Software and Apogee Software,;Doom and Quake of Id Software; Duke Nukem 3D, Shadow Warrior and Rise of the Triad of 3D Realms. All of these 3 games developed by 3D Realms apply Build Engine of DOS. And the designer of Build Engine is Ken Silverman, so it is also used in the game Ken's Labyrinth of Silverman