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The MS-DOS Encyclopedia

Section I: The Development of MS-DOS

    To many people who use personal computers, MS-DOS is the key that
    unlocks the power of the machine. It is their most visible connection
    to the hardware hidden inside the cabinet, and it is through MS-DOS
    that they can run applications and manage disks and disk files.

    In the sense that it opens the door to doing work with a personal
    computer, MS-DOS is indeed a key, and the lock it fits is the Intel
    8086 family of microprocessors. MS-DOS and the chips it works with
    are, in fact, closely connected--so closely that the story of MS-DOS
    is really part of a larger history that encompasses not only an
    operating system but also a microprocessor and, in retrospect, part of
    the explosive growth of personal computing itself.

    Chronologically, the history of MS-DOS can be divided into three
    parts. First came the formation of Microsoft and the events preceding
    Microsoft's decision to develop an operating system. Then came the
    creation of the first version of MS-DOS. Finally, there is the
    continuing evolution of MS-DOS since its release in 1981.

    Much of the story is based on technical developments, but dates and
    facts alone do not provide an adequate look at the past. Many people
    have been involved in creating MS-DOS and directing the lines along
    which it continues to grow. To the extent that personal opinions and
    memories are appropriate, they are included here to provide a fuller
    picture of the origin and development of MS-DOS.

Before MS-DOS

    The role of International Business Machines Corporation in Microsoft's
    decision to create MS-DOS has been well publicized. But events, like
    inventions, always build on prior accomplishments, and in this respect
    the roots of MS-DOS reach farther back, to four hardware and software
    developments of the 1970s: Microsoft's disk-based and stand-alone
    versions of BASIC, Digital Research's CP/M-80 operating system, the
    emergence of the 8086 chip, and a disk operating system for the 8086
    developed by Tim Paterson at a hardware company called Seattle
    Computer Products.

Microsoft and BASIC

    On the surface, BASIC and MS-DOS might seem to have little in common,
    but in terms of file management, MS-DOS is a direct descendant of a
    Microsoft version of BASIC called Stand-alone Disk BASIC.

    Before Microsoft even became a company, its founders, Paul Allen and
    Bill Gates, developed a version of BASIC for a revolutionary small
    computer named the Altair, which was introduced in January 1975 by
    Micro Instrumentation Telemetry Systems (MITS) of Albuquerque, New
    Mexico. Though it has long been eclipsed by other, more powerful makes
    and models, the Altair was the first "personal" computer to appear in
    an environment dominated by minicomputers and mainframes. It was,
    simply, a metal box with a panel of switches and lights for input and
    output, a power supply, a motherboard with 18 slots, and two boards.
    One board was the central processing unit, with the 8-bit Intel 8080
    microprocessor at its heart; the other board provided 256 bytes of
    random-access memory. This miniature computer had no keyboard, no
    monitor, and no device for permanent storage, but it did possess one
    great advantage: a price tag of $397.

    Now, given the hindsight of a little more than a decade of
    microcomputing history, it is easy to see that the Altair's
    combination of small size and affordability was the thin edge of a
    wedge that, in just a few years, would move everyday computing power
    away from impersonal monoliths in climate-controlled rooms and onto
    the desks of millions of people. In 1975, however, the computing
    environment was still primarily a matter of data processing for
    specialists rather than personal computing for everyone. Thus when 4
    KB memory expansion boards became available for the Altair, the
    software needed most by its users was not a word processor or a
    spreadsheet, but a programming language--and the language first
    developed for it was a version of BASIC written by Bill Gates and Paul

    Gates and Allen had become friends in their teens, while attending
    Lakeside School in Seattle. They shared an intense interest in
    computers, and by the time Gates was in the tenth grade, they and
    another friend named Paul Gilbert had formed a company called Traf-O-
    Data to produce a machine that automated the reading of 16-channel, 4-
    digit, binary-coded decimal (BCD) tapes generated by traffic-
    monitoring recorders. This machine, built by Gilbert, was based on the
    Intel 8008 microprocessor, the predecessor of the 8080 in the Altair.

    Although it was too limited to serve as the central processor for a
    general-purpose computer, the 8008 was undeniably the ancestor of the
    8080 as far as its architecture and instruction set were concerned.
    Thus Traf-O-Data's work with the 8008 gave Gates and Allen a head
    start when they later developed their version of BASIC for the Altair.

    Paul Allen learned of the Altair from the cover story in the January
    1975 issue of Popular Electronics  magazine. Allen, then an employee
    of Honeywell in Boston, convinced Gates, a student at Harvard
    University, to develop a BASIC for the new computer. The two wrote
    their version of BASIC for the 8080 in six weeks, and Allen flew to
    New Mexico to demonstrate the language for MITS. The developers gave
    themselves the company name of Microsoft and licensed their BASIC to
    MITS as Microsoft's first product.

    Though not a direct forerunner of MS-DOS, Altair BASIC, like the
    machine for which it was developed, was a landmark product in the
    history of personal computing. On another level, Altair BASIC was also
    the first link in a chain that led, somewhat circuitously, to Tim
    Paterson and the disk operating system he developed for Seattle
    Computer Products for the 8086 chip.

From paper tape to disk

    Gates and Allen's early BASIC for the Altair was loaded from paper
    tape after the bootstrap to load the tape was entered into memory by
    flipping switches on the front panel of the computer. In late 1975,
    however, MITS decided to release a floppy-disk system for the Altair--
    the first retail floppy-disk system on the market. As a result, in
    February 1976 Allen, by then Director of Software for MITS, asked
    Gates to write a disk-based version of Altair BASIC. The Altair had no
    operating system and hence no method of managing files, so the disk
    BASIC would have to include some file-management routines. It would,
    in effect, have to function as a rudimentary operating system.

    Gates, still at Harvard University, agreed to write this version of
    BASIC for MITS. He went to Albuquerque and, as has often been
    recounted, checked into the Hilton Hotel with a stack of yellow legal
    pads. Five days later he emerged, yellow pads filled with the code for
    the new version of BASIC. Arriving at MITS with the code and a request
    to be left alone, Gates began typing and debugging and, after another
    five days, had Disk BASIC running on the Altair.

    This disk-based BASIC marked Microsoft's entry into the business of
    languages for personal computers--not only for the MITS Altair, but
    also for such companies as Data Terminals Corporation and General
    Electric. Along the way, Microsoft BASIC took on added features, such
    as enhanced mathematics capabilities, and, more to the point in terms
    of MS-DOS, evolved into Stand-alone Disk BASIC, produced for NCR in

    Designed and coded by Marc McDonald, Stand-alone Disk BASIC included a
    file-management scheme called the FAT, or file allocation table that
    used a linked list for managing disk files. The FAT, born during one
    of a series of discussions between McDonald and Bill Gates, enabled
    disk-allocation information to be kept in one location, with "chained"
    references pointing to the actual storage locations on disk. Fast and
    flexible, this file-management strategy was later used in a stand-
    alone version of BASIC for the 8086 chip and eventually, through an
    operating system named M-DOS, became the basis for the file-handling
    routines in MS-DOS.


    During 1977 and 1978, Microsoft adapted both BASIC and Microsoft
    FORTRAN for an increasingly popular 8-bit operating system called
    CP/M. At the end of 1978, Gates and Allen moved Microsoft from
    Albuquerque to Bellevue, Washington. The company continued to
    concentrate on programming languages, producing versions of BASIC for
    the 6502 and the TI9900.

    During this same period, Marc McDonald also worked on developing an 8-
    bit operating system called M-DOS (usually pronounced "Midas" or "My
    DOS"). Although it never became a real part of the Microsoft product
    line, M-DOS was a true multitasking operating system modeled after the
    DEC TOPS-10 operating system. M-DOS provided good performance and,
    with a more flexible FAT than that built into BASIC, had a better
    file-handling structure than the up-and-coming CP/M operating system.
    At about 30 KB, however, M-DOS was unfortunately too big for an 8-bit
    environment and so ended up being relegated to the back room. As Allen
    describes it, "Trying to do a large, full-blown operating system on
    the 8080 was a lot of work, and it took a lot of memory. The 8080
    addresses only 64 K, so with the success of CP/M, we finally concluded
    that it was best not to press on with that."


    In the volatile microcomputer era of 1976 through 1978, both users and
    developers of personal computers quickly came to recognize the
    limitations of running applications on top of Microsoft's Stand-alone
    Disk BASIC or any other language. MITS, for example, scheduled a July
    1976 release date for an independent operating system for its machine
    that used the code from the Altair's Disk BASIC. In the same year,
    Digital Research, headed by Gary Kildall, released its Control
    Program/Monitor, or CP/M.

    CP/M was a typical microcomputer software product of the 1970s in that
    it was written by one person, not a group, in response to a specific
    need that had not yet been filled. One of the most interesting aspects
    of CP/M's history is that the software was developed several years
    before its release date--actually, several years before the hardware
    on which it would be a standard became commercially available.

    In 1973, Kildall, a professor of computer science at the Naval
    Postgraduate School in Monterey, California, was working with an 8080-
    based small computer given him by Intel Corporation in return for some
    programming he had done for the company. Kildall's machine, equipped
    with a monitor and paper-tape reader, was certainly advanced for the
    time, but Kildall became convinced that magnetic-disk storage would
    make the machine even more efficient than it was.

    Trading some programming for a disk drive from Shugart, Kildall first
    attempted to build a drive controller on his own. Lacking the
    necessary engineering ability, he contacted a friend, John Torode, who
    agreed to handle the hardware aspects of interfacing the computer and
    the disk drive while Kildall worked on the software portion--the
    refinement of an operating system he had written earlier that year.
    The result was CP/M.

    The version of CP/M developed by Kildall in 1973 underwent several
    refinements. Kildall enhanced the CP/M debugger and assembler, added a
    BASIC interpreter, and did some work on an editor, eventually
    developing the product that, from about 1977 until the appearance of
    the IBM Personal Computer, set the standard for 8-bit microcomputer
    operating systems.

    Digital Research's CP/M included a command interpreter called CCP
    (Console Command Processor), which acted as the interface between the
    user and the operating system itself, and an operations handler called
    BDOS (Basic Disk Operating System), which was responsible for file
    storage, directory maintenance, and other such housekeeping chores.
    For actual input and output--disk I/O, screen display, print requests,
    and so on--CP/M included a BIOS (Basic Input/Output System) tailored
    to the requirements of the hardware on which the operating system ran.

    For file storage, CP/M used a system of eight-sector allocation units.
    For any given file, the allocation units were listed in a directory
    entry that included the filename and a table giving the disk locations
    of 16 allocation units. If a long file required more than 16
    allocation units, CP/M created additional directory entries as
    required. Small files could be accessed rapidly under this system, but
    large files with more than a single directory entry could require
    numerous relatively time-consuming disk reads to find needed

    At the time, however, CP/M was highly regarded and gained the support
    of a broad base of hardware and software developers alike. Quite
    powerful for its size (about 4KB), it was, in all respects, the
    undisputed standard in the 8-bit world, and remained so until, and
    even after, the appearance of the 8086.

The 8086

    When Intel released the 8-bit 8080 chip in 1974, the Altair was still
    a year in the future. The 8080 was designed not to make computing a
    part of everyday life but to make household appliances and industrial
    machines more intelligent. By 1978, when Intel introduced the 16-bit
    8086, the microcomputer was a reality and the new chip represented a
    major step ahead in performance and memory capacity. The 8086's full
    16-bit buses made it faster than the 8080, and its ability to address
    one megabyte of random-access memory was a giant step beyond the
    8080's 64 KB limit. Although the 8086 was not compatible with the
    8080, it was architecturally similar to its predecessor and 8080
    source code could be mechanically translated to run on it. This
    translation capability, in fact, was a major influence on the design
    of Tim Paterson's operating system for the 8086 and, through
    Paterson's work, on the first released version of MS-DOS.

    When the 8086 arrived on the scene, Microsoft, like other developers,
    was confronted with two choices: continue working in the familiar 8-
    bit world or turn to the broader horizons offered by the new 16-bit
    technology. For a time, Microsoft did both. Acting on Paul Allen's
    suggestion, the company developed the SoftCard for the popular Apple
    II, which was based on the 8-bit 6502 microprocessor. The SoftCard
    included a Z80 microprocessor and a copy of CP/M-80 licensed from
    Digital Research. With the SoftCard, Apple II users could run any
    program or language designed to run on a CP/M machine.

    It was 16-bit technology, however, that held the most interest for
    Gates and Allen, who believed that this would soon become the standard
    for microcomputers. Their optimism was not universal--more than one
    voice in the trade press warned that industry investment in 8-bit
    equipment and software was too great to successfully introduce a new
    standard. Microsoft, however, disregarded these forecasts and entered
    the 16-bit arena as it had with the Altair: by developing a stand-
    alone version of BASIC for the 8086.

    At the same time and, coincidentally, a few miles south in Tukwila,
    Washington, a major contribution to MS-DOS was taking place. Tim
    Paterson, working at Seattle Computer Products, a company that built
    memory boards, was developing an 8086 CPU card for use in an S-100 bus


    Paterson was introduced to the 8086 chip at a seminar held by Intel in
    June 1978. He had attended the seminar at the suggestion of his
    employer, Rod Brock of Seattle Computer Products. The new chip sparked
    his interest because, as he recalls, "all its instructions worked on
    both 8 and 16 bits, and you didn't have to do everything through the
    accumulator. It was also real fast--it could do a 16-bit ADD in three

    After the seminar, Paterson--again with Brock's support--began work
    with the 8086. He finished the design of his first 8086 CPU board in
    January 1979 and by late spring had developed a working CPU, as well
    as an assembler and an 8086 monitor. In June, Paterson took his system
    to Microsoft to try it with Stand-alone BASIC, and soon after,
    Microsoft BASIC was running on Seattle Computer's new board.

    During this period, Paterson also received a call from Digital
    Research asking whether they could borrow the new board for developing
    CP/M-86. Though Seattle Computer did not have a board to loan,
    Paterson asked when CP/M-86 would be ready. Digital's representative
    said December 1979, which meant, according to Paterson's diary, "we'll
    have to live with Stand-alone BASIC for a few months after we start
    shipping the CPU, but then we'll be able to switch to a real operating

    Early in June, Microsoft and Tim Paterson attended the National
    Computer Conference in New York. Microsoft had been invited to share
    Lifeboat Associates' ten-by-ten foot booth, and Paterson had been
    invited by Paul Allen to show BASIC running on an S-100 8086 system.
    At that meeting, Paterson was introduced to Microsoft's M-DOS, which
    he found interesting because it used a system for keeping track of
    disk files--the FAT developed for Stand-alone BASIC--that was
    different from anything he had encountered.

    After this meeting, Paterson continued working on the 8086 board, and
    by the end of the year, Seattle Computer Products began shipping the
    CPU with a BASIC option.

    When CP/M-86 had still not become available by April 1980, Seattle
    Computer Products decided to develop a 16-bit operating system of its
    own. Originally, three operating systems were planned: a single-user
    system, a multiuser version, and a small interim product soon
    informally christened QDOS (for Quick and Dirty Operating System) by

    Both Paterson (working on QDOS) and Rod Brock knew that a standard
    operating system for the 8086 was mandatory if users were to be
    assured of a wide range of application software and languages. CP/M
    had become the standard for 8-bit machines, so the ability to
    mechanically translate existing CP/M applications to run on a 16-bit
    system became one of Paterson's major goals for the new operating
    system. To achieve this compatibility, the system he developed
    mimicked CP/M-80's functions and command structure, including its use
    of file control blocks (FCBs) and its approach to executable files.

    At the same time, however, Paterson was dissatisfied with certain
    elements of CP/M, one of them being its file-allocation system, which
    he considered inefficient in the use of disk space and too slow in
    operation. So for fast, efficient file handling, he used a file
    allocation table, as Microsoft had done with Stand-alone Disk BASIC
    and M-DOS. He also wrote a translator to translate 8080 code to 8086
    code, and he then wrote an assembler in Z80 assembly language and used
    the translator to translate it.

    Four months after beginning work, Paterson had a functioning 6 KB
    operating system, officially renamed 86-DOS, and in September 1980 he
    contacted Microsoft again, this time to ask the company to write a
    version of BASIC to run on his system.


    While Paterson was developing 86-DOS, the third major element leading
    to the creation of MS-DOS was gaining force at the opposite end of the
    country. IBM, until then seemingly oblivious to most of the
    developments in the microcomputer world, had turned its attention to
    the possibility of developing a low-end workstation for a market it
    knew well: business and business people.

    On August 21, 1980, a study group of IBM representatives from Boca
    Raton, Florida, visited Microsoft. This group, headed by a man named
    Jack Sams, told Microsoft of IBM's interest in developing a computer
    based on a microprocessor. IBM was, however, unsure of microcomputing
    technology and the microcomputing market. Traditionally, IBM relied on
    long development cycles--typically four or five years--and was aware
    that such lengthy design periods did not fit the rapidly evolving
    microcomputer environment.

    One of IBM's solutions--the one outlined by Sams's group--was to base
    the new machine on products from other manufacturers. All the
    necessary hardware was available, but the same could not be said of
    the software. Hence the visit to Microsoft with the question: Given
    the specifications for an 8-bit computer, could Microsoft write a ROM
    BASIC for it by the following April?

    Microsoft responded positively, but added questions of its own: Why
    introduce an 8-bit computer? Why not release a 16-bit machine based on
    Intel's 8086 chip instead? At the end of this meeting--the first of
    many--Sams and his group returned to Boca Raton with a proposal for
    the development of a low-end, 16-bit business workstation. The venture
    was named Project Chess.

    One month later, Sams returned to Microsoft asking whether Gates and
    Allen could, still by April 1981, provide not only BASIC but also
    FORTRAN, Pascal, and COBOL for the new computer. This time the answer
    was no because, though Microsoft's BASIC had been designed to run as a
    stand-alone product, it was unique in that respect--the other
    languages would need an operating system. Gates suggested CP/M-86,
    which was then still under development at Digital Research, and in
    fact made the initial contact for IBM. Digital Research and IBM did
    not come to any agreement, however.

    Microsoft, meanwhile, still wanted to write all the languages for
    IBM--approximately 400 KB of code. But to do this within the allotted
    six-month schedule, the company needed some assurances about the
    operating system IBM was going to use. Further, it needed specific
    information on the internals of the operating system, because the ROM
    BASIC would interact intimately with the BIOS.

The turning point

    That state of indecision, then, was Microsoft's situation on Sunday,
    September 28, 1980, when Bill Gates, Paul Allen, and Kay Nishi, a
    Microsoft vice president and president of ASCII Corporation in Japan,
    sat in Gates's eighth-floor corner office in the Old National Bank
    Building in Bellevue, Washington. Gates recalls, "Kay and I were just
    sitting there at night and Paul was on the couch. Kay said, `Got to do
    it, got to do it.' It was only 20 more K of code at most--actually, it
    turned out to be 12 more K on top of the 400. It wasn't that big a
    deal, and once Kay said it, it was obvious. We'd always wanted to do a
    low-end operating system, we had specs for low-end operating systems,
    and we knew we were going to do one up on 16-bit."

    At that point, Gates and Allen began looking again at Microsoft's
    proposal to IBM. Their estimated 400 KB of code included four
    languages, an assembler, and a linker. To add an operating system
    would require only another 20 KB or so, and they already knew of a
    working model for the 8086: Tim Paterson's 86-DOS. The more Gates,
    Allen, and Nishi talked that night about developing an operating
    system for IBM's new computer, the more possible--even preferable--the
    idea became.

    Allen's first step was to contact Rod Brock at Seattle Computer
    Products to tell him that Microsoft wanted to develop and market SCP's
    operating system and that the company had an OEM customer for it.
    Seattle Computer Products, which was not in the business of marketing
    software, agreed and licensed 86-DOS to Microsoft. Eventually, SCP
    sold the operating system to Microsoft for $50,000, favorable language
    licenses, and a license back from Microsoft to use 86-DOS on its own

    In October 1980, with 86-DOS in hand, Microsoft submitted another
    proposal to IBM. This time the plan included both an operating system
    and the languages for the new computer. Time was short and the
    boundaries between the languages and the operating system were
    unclear, so Microsoft explained that it needed to control the
    development of the operating system in order to guarantee delivery by
    spring of 1981. In November, IBM signed the contract.

Creating MS-DOS

    At Thanksgiving, a prototype of the IBM machine arrived at Microsoft
    and Bill Gates, Paul Allen, and, primarily, Bob O'Rear began a
    schedule of long, sometimes hectic days and total immersion in the
    project. As O'Rear recalls, "If I was awake, I was thinking about the

    The first task handled by the team was bringing up 86-DOS on the new
    machine. This was a challenge because the work had to be done in a
    constantly changing hardware environment while changes were also being
    made to the specifications of the budding operating system itself.

    As part of the process, 86-DOS had to be compiled and integrated with
    the BIOS, which Microsoft was helping IBM to write, and this task was
    complicated by the media. Paterson's 86-DOS--not counting utilities
    such as EDLIN, CHKDSK, and INIT (later named FORMAT)--arrived at
    Microsoft as one large assembly-language program on an 8-inch floppy
    disk. The IBM machine, however, used 5-1/4-inch disks, so Microsoft
    needed to determine the format of the new disk and then find a way to
    get the operating system from the old format to the new.

    This work, handled by O'Rear, fell into a series of steps. First, he
    moved a section of code from the 8-inch disk and compiled it. Then, he
    converted the code to Intel hexadecimal format. Next, he uploaded it
    to a DEC-2020 and from there downloaded it to a large Intel fixed-disk
    development system with an In-Circuit Emulator. The DEC-2020 used for
    this task was also used in developing the BIOS, so there was
    additional work in downloading the BIOS to the Intel machine,
    converting it to hexadecimal format, moving it to an IBM development
    system, and then crossloading it to the IBM prototype.

    Defining and implementing the MS-DOS disk format--different from
    Paterson's 8-inch format--was an added challenge. Paterson's ultimate
    goal for 86-DOS was logical device independence, but during this first
    stage of development, the operating system simply had to be converted
    to handle logical records that were independent of the physical record

    Paterson, still with Seattle Computer Products, continued to work on
    86-DOS and by the end of 1980 had improved its logical device
    independence by adding functions that streamlined reading and writing
    multiple sectors and records, as well as records of variable size. In
    addition to making such refinements of his own, Paterson also worked
    on dozens of changes requested by Microsoft, from modifications to the
    operating system's startup messages to changes in EDLIN, the line
    editor he had written for his own use. Throughout this process, IBM's
    security restrictions meant that Paterson was never told the name of
    the OEM and never shown the prototype machines until he left Seattle
    Computer Products and joined Microsoft in May 1981.

    And of course, throughout the process the developers encountered the
    myriad loose ends, momentary puzzles, bugs, and unforeseen details
    without which no project is complete. There were, for example, the
    serial card interrupts that occurred when they should not and,
    frustratingly, a hardware constraint that the BIOS could not
    accommodate at first and that resulted in sporadic crashes during
    early MS-DOS operations.

    In spite of such difficulties, however, the new operating system ran
    on the prototype for the first time in February 1981. In the six
    months that followed, the system was continually refined and expanded,
    and by the time of its debut in August 1981, MS-DOS, like the IBM
    Personal Computer on which it appeared, had become a functional
    product for home and office use.

Version 1

    The first release of MS-DOS, version 1.0, was not the operating system
    Microsoft envisioned as a final model for 16-bit computer systems.
    According to Bill Gates, "Basically, what we wanted to do was one that
    was more like MS-DOS 2, with the hierarchical file system and
    everything...the key thing [in developing version 1.0] was my saying,
    `Look, we can come out with a subset first and just go upward from

    This first version--Gates's subset of MS-DOS--was actually a good
    compromise between the present and the future in two important
    respects: It enabled Microsoft to meet the development schedule for
    IBM and it maintained program-translation compatibility with CP/M.

    Available only for the IBM Personal Computer, MS-DOS 1.0 consisted of
    4000 lines of assembly-language source code and ran in 8 KB of memory.
    In addition to utilities such as DEBUG, EDLIN, and FORMAT, it was
    organized into three major files. One file, IBMBIO.COM, interfaced
    with the ROM BIOS for the IBM PC and contained the disk and character
    input/output system. A second file, IBMDOS.COM, contained the DOS
    kernel, including the application-program interface and the disk-file
    and memory managers. The third file, COMMAND.COM, was the external
    command processor--the part of MS-DOS most visible to the user.

    To take advantage of the existing base of languages and such popular
    applications as WordStar and dBASE II, MS-DOS was designed to allow
    software developers to mechanically translate source code for the 8080
    to run on the 8086. And because of this link, MS-DOS looked and acted
    like CP/M-80, at that time still the standard among operating systems
    for microcomputers. Like its 8-bit relative, MS-DOS used eight-
    character filenames and three-character extensions, and it had the
    same conventions for identifying disk drives in command prompts. For
    the most part, MS-DOS also used the same command language, offered the
    same file services, and had the same general structure as CP/M. The
    resemblance was even more striking at the programming level, with an
    almost one-to-one correspondence between CP/M and MS-DOS in the system
    calls available to application programs.

New Features

    MS-DOS was not, however, a CP/M twin, nor had Microsoft designed it to
    be inextricably bonded to the IBM PC. Hoping to create a product that
    would be successful over the long term, Microsoft had taken steps to
    make MS-DOS flexible enough to accommodate changes and new directions
    in the hardware technology--disks, memory boards, even
    microprocessors--on which it depended. The first steps toward this
    independence from specific hardware configurations appeared in MS-DOS
    version 1.0 in the form of device-independent input and output,
    variable record lengths, relocatable program files, and a replaceable
    command processor.

    MS-DOS made input and output device-independent by treating peripheral
    devices as if they were files. To do this, it assigned a reserved
    filename to each of the three devices it recognized: CON for the
    console (keyboard and display), PRN for the printer, and AUX for the
    auxiliary serial ports. Whenever one of these reserved names appeared
    in the file control block of a file named in a command, all operations
    were directed to the device, rather than to a disk file. (A file
    control block, or FCB, is a 37-byte housekeeping record located in an
    application's portion of the memory space. It includes, among other
    things, the filename, the extension, and information about the size
    and starting location of the file on disk.)

    Such device independence benefited both application developers and
    computer users. On the development side, it meant that applications
    could use one set of read and write calls, rather than a number of
    different calls for different devices, and it meant that an
    application did not have to be modified if new devices were added to
    the system. From the user's point of view, device independence meant
    greater flexibility. For example, even if a program had been designed
    for disk I/O only, the user could still use a file for input or direct
    output to the printer.

    Variable record lengths provided another step toward logical
    independence. In CP/M, logical and physical record lengths were
    identical: 128 bytes. Files could be accessed only in units of 128
    bytes and file sizes were always maintained in multiples of 128 bytes.
    With MS-DOS, however, physical sector sizes were of no concern to the
    user. The operating system maintained file lengths to the exact size
    in bytes and could be relied on to support logical records of any size

    Another new feature in MS-DOS was the relocatable program file. Unlike
    CP/M, MS-DOS had the ability to load two different types of program
    files, identified by the extensions .COM and .EXE. Program files
    ending with .COM mimicked the binary files in CP/M. They were more
    compact than .EXE files and loaded somewhat faster, but the combined
    program code, stack, and data could be no larger than 64 KB. A .EXE
    program, on the other hand, could be much larger, because the file
    could contain multiple segments, each of which could be up to 64 KB.
    Once the segments were in memory, MS-DOS then used part of the file
    header, the relocation table, to automatically set the correct
    addresses for each of the different program segments.

    In addition to supporting .EXE files, MS-DOS made the external command
    processor, COMMAND.COM, more adaptable by making it a separate
    relocatable file just like any other program. It could therefore be
    replaced by a custom command processor, as long as the new file was
    also named COMMAND.COM.


    Everyone familiar with the IBM PC knows that MS-DOS eventually became
    the dominant operating system on 8086-based microcomputers. There were
    several reasons for this, not least of which was acceptance of MS-DOS
    as the operating system for IBM's phenomenally successful line of
    personal computers. But even though MS-DOS was the only operating
    system available when the first IBM PCs were shipped, positioning
    alone would not necessarily have guaranteed its ability to outstrip
    CP/M-86, which appeared six months later. MS-DOS also offered
    significant advantages to the user in a number of areas, including the
    allocation and management of storage space on disk.

    Like CP/M, MS-DOS shared out disk space in allocation units. Unlike
    CP/M, however, MS-DOS mapped the use of these allocation units in a
    central file allocation table--the FAT--that was always in memory.
    Both operating systems used a directory entry for recording
    information about each file, but whereas a CP/M directory entry
    included an allocation map--a list of sixteen 1 KB allocation units
    where successive parts of the file were stored--an MS-DOS directory
    entry pointed only to the first allocation unit in the FAT and each
    entry in the table then pointed to the next unit associated with the
    file. Thus, CP/M might require several directory entries (and more
    than one disk access) to load a file larger than 16 KB, but MS-DOS
    retained a complete in-memory list of all file components and all
    available disk space without having to access the disk at all. As a
    result, MS-DOS's ability to find and load even very long files was
    extremely rapid compared with CP/M's.

    Two other important features--the ability to read and write multiple
    records with one operating-system call and the transient use of memory
    by the MS-DOS command processor--provided further efficiency for both
    users and developers.

    The independence of the logical record from the physical sector laid
    the foundation for the ability to read and write multiple sectors.
    When reading multiple records in CP/M, an application had to issue a
    read function call for each sector, one at a time. With MS-DOS, the
    application could issue one read function call, giving the operating
    system the beginning record and the number of records to read, and MS-
    DOS would then load all of the corresponding sectors automatically.

    Another innovative feature of MS-DOS version 1.0 was the division of
    the command processor, COMMAND.COM, into a resident portion and a
    transient portion. (There is also a third part, an initialization
    portion, which carries out the commands in an AUTOEXEC batch file at
    startup. This part of COMMAND.COM is discarded from memory when its
    work is finished.) The reason for creating resident and transient
    portions of the command processor had to do with maximizing the
    efficiency of MS-DOS for the user: On the one hand, the programmers
    wanted COMMAND.COM to include commonly requested functions, such as
    DIR and COPY, for speed and ease of use; on the other hand, adding
    these commands meant increasing the size of the command processor,
    with a resulting decrease in the memory available to application
    programs. The solution to this trade-off of speed versus utility was
    to include the extra functions in a transient portion of COMMAND.COM
    that could be overwritten by any application requiring more memory. To
    maintain the integrity of the functions for the user, the resident
    part of COMMAND.COM was given the job of checking the transient
    portion for damage when an application terminated. If necessary, this
    resident portion would then load a new copy of its transient partner
    into memory.

Ease of Use

    In addition to its moves toward hardware independence and efficiency,
    MS-DOS included several services and utilities designed to make life
    easier for users and application developers. Among these services were
    improved error handling, automatic logging of disks, date and time
    stamping of files, and batch processing.

    MS-DOS and the IBM PC were targeted at a nontechnical group of users,
    and from the beginning IBM had stressed the importance of data
    integrity. Because data is most likely to be lost when a user responds
    incorrectly to an error message, an effort was made to include concise
    yet unambiguous messages in MS-DOS. To further reduce the risks of
    misinterpretation, Microsoft used these messages consistently across
    all MS-DOS functions and utilities and encouraged developers to use
    the same messages, where appropriate, in their applications.

    In a further attempt to safeguard data, MS-DOS also trapped hard
    errors--such as critical hardware errors--that had previously been
    left to the hardware-dependent logic. Now the hardware logic could
    simply report the nature of the error and the operating system would
    handle the problem in a consistent and systematic way. MS-DOS could
    also trap the Control-C break sequence so that an application could
    either protect against accidental termination by the user or provide a
    graceful exit when appropriate.

    To reduce errors and simplify use of the system, MS-DOS also
    automatically updated memory information about the disk when it was
    changed. In CP/M, users had to log new disks as they changed them--a
    cumbersome procedure on single-disk systems or when data was stored on
    multiple disks. In MS-DOS, new disks were automatically logged as long
    as no file was currently open.

    Another new feature--one visible with the DIR command--was date and
    time stamping of disk files. Even in its earliest forms, MS-DOS
    tracked the system date and displayed it at every startup, and now,
    when it turned out that only the first 16 bytes of a directory entry
    were needed for file-header information, the MS-DOS programmers
    decided to use some of the remaining 16 bytes to record the date and
    time of creation or update (and the size of the file) as well.

    Batch processing was originally added to MS-DOS to help IBM. IBM
    wanted to run scripts--sequences of commands or other operations--one
    after the other to test various functions of the system. To do this,
    the testers needed an automated method of calling routines
    sequentially. The result was the batch processor, which later also
    provided users with the convenience of saving and running MS-DOS
    commands as batch files.

    Finally, MS-DOS increased the options available to a program when it
    terminated. For example, in less sophisticated operating systems,
    applications and other programs remained in memory only as long as
    they were active; when terminated, they were removed from memory. MS-
    DOS, however, added a terminate-and-stay-resident function that
    enabled a program to be locked into memory and, in effect, become part
    of the operating-system environment until the computer system itself
    was shut down or restarted.

The Marketplace

    When IBM announced the Personal Computer, it said that the new machine
    would run three operating systems: MS-DOS, CP/M-86, and SofTech
    Microsystem's p-System. Of the three, only MS-DOS was available when
    the IBM PC shipped. Nevertheless, when MS-DOS was released, nine out
    of ten programs on the InfoWorld bestseller list for 1981 ran under
    CP/M-80, and CP/M-86, which became available about six months later,
    was the operating system of choice to most writers and reviewers in
    the trade press.

    Understandably, MS-DOS was compared with CP/M-80 and, later, CP/M-86.
    The main concern was compatibility: To what extent was Microsoft's new
    operating system compatible with the existing standard? No one could
    have foreseen that MS-DOS would not only catch up with but supersede
    CP/M. Even Bill Gates now recalls that "our most optimistic view of
    the number of machines using MS-DOS wouldn't have matched what really
    ended up happening."

    To begin with, the success of the IBM PC itself surprised many
    industry watchers. Within a year, IBM was selling 30,000 PCs per
    month, thanks in large part to a business community that was already
    comfortable with IBM's name and reputation and, at least in
    retrospect, was ready for the leap to personal computing. MS-DOS, of
    course, benefited enormously from the success of the IBM PC--in large
    part because IBM supplied all its languages and applications in MS-DOS

    But, at first, writers in the trade press still believed in CP/M and
    questioned the viability of a new operating system in a world
    dominated by CP/M-80. Many assumed, incorrectly, that a CP/M-86
    machine could run CP/M-80 applications. Even before CP/M-86 was
    available, Future Computing  referred to the IBM PC as the "CP/M
    Record Player"--presumably in anticipation of a vast inventory of CP/M
    applications for the new computer--and led its readers to assume that
    the PC was actually a CP/M machine.

    Microsoft, meanwhile, held to the belief that the success of IBM's
    machine or any other 16-bit microcomputer depended ultimately on the
    emergence of an industry standard for a 16-bit operating system.
    Software developers could not afford to develop software for even two
    or three different operating systems, and users could (or would) not
    pay the prices the developers would have to charge if they did.
    Furthermore, users would almost certainly rebel against the
    inconvenience of sharing data stored under different operating-system
    formats. There had to be one operating system, and Microsoft wanted
    MS-DOS to be the one.

    The company had already taken the first step toward a standard by
    choosing hardware independent designs wherever possible. Machine
    independence meant portability, and portability meant that Microsoft
    could sell one version of MS-DOS to different hardware manufacturers
    who, in turn, could adapt it to their own equipment. Portability
    alone, however, was no guarantee of industry-wide acceptance. To make
    MS-DOS the standard, Microsoft needed to convince software developers
    to write programs for MS-DOS. And in 1981, these developers were a
    little confused about IBM's new operating system.

An operating system by any other name...

    A tangle of names gave rise to one point of confusion about MS-DOS.
    Tim Paterson's "Quick and Dirty Operating System" for the 8086 was
    originally shipped by Seattle Computer Products as 86-DOS. After
    Microsoft purchased 86-DOS, the name remained for a while, but by the
    time the PC was ready for release, the new system was known as MS-DOS.
    Then, after the IBM PC reached the market, IBM began to refer to the
    operating system as the IBM Personal Computer DOS, which the trade
    press soon shortened to PC-DOS. IBM's version contained some
    utilities, such as DISKCOPY and DISKCOMP, that were not included in
    MS-DOS, the generic version available for license by other
    manufacturers. By calling attention to these differences, publications
    added to the confusion about the distinction between the Microsoft and
    IBM releases of MS-DOS.

    Further complications arose when Lifeboat Associates agreed to help
    promote MS-DOS but decided to call the operating system Software Bus
    86. MS-DOS thus became one of a line of trademarked Software Bus
    products, another of which was a product called SB-80, Lifeboat's
    version of CP/M-80.

    Finally, some of the first hardware companies to license MS-DOS also
    wanted to use their own names for the operating system. Out of this
    situation came such additional names as COMPAQ-DOS and Zenith's Z-DOS.

    Given this confusing host of names for a product it believed could
    become the industry standard, Microsoft finally took the lead and, as
    developer, insisted that the operating system was to be called MS-DOS.
    Eventually, everyone but IBM complied.

Developers and MS-DOS

    Early in its career, MS-DOS represented just a small fraction of
    Microsoft's business--much larger revenues were generated by BASIC and
    other languages. In addition, in the first two years after the
    introduction of the IBM PC, the growth of CP/M-86 and other
    environments nearly paralleled that of MS-DOS. So Microsoft found
    itself in the unenviable position of giving its support to MS-DOS
    while also selling languages to run on CP/M-86, thereby contributing
    to the growth of software for MS-DOS's biggest competitor.

    Given the uncertain outcome of this two-horse race, some other
    software developers chose to wait and see which way the hardware
    manufacturers would jump. For their part, the hardware manufacturers
    were confronting the issue of compatibility between operating systems.
    Specifically, they needed to be convinced that MS-DOS was not a
    maverick--that it could perform as well as CP/M-86 as a base for
    applications that had been ported from the CP/M-80 environment for use
    on 16-bit computers.

    Microsoft approached the problem by emphasizing four related points in
    its discussions with hardware manufacturers:

    ■  First, one of Microsoft's goals in developing the first version of
        MS-DOS had always been translation compatibility from CP/M-80 to
        MS-DOS software.

    ■  Second, translation was possible only for software written in 8080
        or Z80 assembly language; thus, neither MS-DOS nor CP/M-86 could
        run programs written for other 8-bit processors, such as the 6800
        or the 6502.

    ■  Third, many applications were written in a high-level language,
        rather than in assembly language.

    ■  Fourth, most of those high-level languages were Microsoft products
        and ran on MS-DOS.

    Thus, even though some people had originally believed that only CP/M-
    86 would automatically make the installed base of CP/M-80 software
    available to the IBM PC and other 16-bit computers, Microsoft
    convinced the hardware manufacturers that MS-DOS was, in actuality, as
    flexible as CP/M-86 in its compatibility with existing--and
    appropriate--CP/M-80 software.

    MS-DOS was put at a disadvantage in one area, however, when Digital
    Research convinced several manufacturers to include both 8080 and 8086
    chips in their machines. With 8-bit and 16-bit software used on the
    same machine, the user could rely on the same disk format for both
    types of software. Because MS-DOS used a different disk format, CP/M
    had the edge in these dual-processor machines--although, in fact, it
    did not seem to have much effect on the survival of CP/M-86 after the
    first year or so.

    Although making MS-DOS the operating system of obvious preference was
    not as easy as simply convincing hardware manufacturers to offer it,
    Microsoft's list of MS-DOS customers grew steadily from the time the
    operating system was introduced. Many manufacturers continued to offer
    CP/M-86 along with MS-DOS, but by the end of 1983 the technical
    superiority of MS-DOS (bolstered by the introduction of such products
    as Lotus 1-2-3) carried the market. For example, when DEC, a longtime
    holdout, decided to make MS-DOS the primary operating system for its
    Rainbow computer, the company mentioned the richer set of commands and
    "dramatically" better disk performance of MS-DOS as reasons for its
    choice over CP/M-86.

Version 2

    After the release of PC-specific version 1.0 of MS-DOS, Microsoft
    worked on an update that contained some bug fixes. Version 1.1 was
    provided to IBM to run on the upgraded PC released in 1982 and enabled
    MS-DOS to work with double-sided, 320 KB floppy disks. This version,
    referred to as 1.25 by all but IBM, was the first version of MS-DOS
    shipped by other OEMs, including COMPAQ and Zenith.

    Even before these intermediate releases were available, however,
    Microsoft began planning for future versions of MS-DOS. In developing
    the first version, the programmers had had two primary goals: running
    translated CP/M-80 software and keeping MS-DOS small. They had neither
    the time nor the room to include more sophisticated features, such as
    those typical of Microsoft's UNIX-based multiuser, multitasking
    operating system, XENIX. But when IBM informed Microsoft that the next
    major edition of the PC would be the Personal Computer XT with a 10-
    megabyte fixed disk, a larger, more powerful version of MS-DOS--one
    closer to the operating system Microsoft had envisioned from the
    start--became feasible.

    There were three particular areas that interested Microsoft: a new,
    hierarchical file system, installable device drivers, and some type of
    multitasking. Each of these features contributed to version 2.0, and
    together they represented a major change in MS-DOS while still
    maintaining compatibility with version 1.0.

The File System

    Primary responsibility for version 2.0 fell to Paul Allen, Mark
    Zbikowski, and Aaron Reynolds, who wrote (and rewrote) most of the
    version 2.0 code. The major design issue confronting the developers,
    as well as the most visible example of its difference from versions
    1.0, 1.1, and 1.25, was the introduction of a hierarchical file system
    to handle the file-management needs of the XT's fixed disk.

    Version 1.0 had a single directory for all the files on a floppy disk.
    That system worked well enough on a disk of limited capacity, but on a
    10-megabyte fixed disk a single directory could easily become
    unmanageably large and cumbersome.

    CP/M had approached the problem of high-capacity storage media by
    using a partitioning scheme that divided the fixed disk into 10 user
    areas equivalent to 10 separate floppy-disk drives. On the other hand,
    UNIX, which had traditionally dealt with larger systems, used a
    branching, hierarchical file structure in which the user could create
    directories and subdirectories to organize files and make them readily
    accessible. This was the file-management system implemented in XENIX,
    and it was the MS-DOS team's choice for handling files on the XT's
    fixed disk.

    Partitioning, IBM's initial choice, had the advantages of familiarity,
    size, and ease of implementation. Many small-system users--
    particularly software developers--were already familiar with
    partitioning, if not overly fond of it, from their experience with
    CP/M. Development time was also a major concern, and the code needed
    to develop a partitioning scheme would be minimal compared with the
    code required to manage a hierarchical file system. Such a scheme
    would also take less time to implement.

    However, partitioning had two inherent disadvantages. First, its
    functionality would decrease as storage capacity increased, and even
    in 1982, Microsoft was anticipating substantial growth in the storage
    capacity of disk-based media. Second, partitioning depended on the
    physical device. If the size of the disk changed, either the number or
    the size of the partitions must also be changed in the code for both
    the operating system and the application programs. For Microsoft, with
    its commitment to hardware independence, partitioning would have
    represented a step in the wrong direction.

    A hierarchical file structure, on the other hand, could be independent
    of the physical device. A disk could be partitioned logically, rather
    than physically. And because these partitions (directories) were
    controlled by the user, they were open-ended and enabled the
    individual to determine the best way of organizing a disk.

    Ultimately, it was a hierarchical file system that found its way into
    MS-DOS 2.0 and eventually convinced everyone that it was, indeed, the
    better and more flexible solution to the problem of supporting a fixed
    disk. The file system was logically consistent with the XENIX file
    structure, yet physically consistent with the file access incorporated
    in versions 1.x, and was based on a root, or main, directory under
    which the user could create a system of subdirectories and sub-
    subdirectories to hold files. Each file in the system was identified
    by the directory path leading to it, and the number of subdirectories
    was limited only by the length of the pathname, which could not exceed
    64 characters.

    In this file structure, all the subdirectories and the filename in a
    path were separated from one another by backslash characters, which
    represented the only anomaly in the XENIX/MS-DOS system of
    hierarchical files. XENIX used a forward slash as a separator, but
    versions 1.x of MS-DOS, borrowing from the tradition of DEC operating
    systems, already used the forward slash for switches in the command
    line, so Microsoft, at IBM's request, decided to use the backslash as
    the separator instead. Although the backslash character created no
    practical problems, except on keyboards that lacked a backslash, this
    decision did introduce inconsistency between MS-DOS and existing UNIX-
    like operating systems. And although Microsoft solved the keyboard
    problem by enabling the user to change the switch character from a
    slash to a hyphen, the solution itself created compatibility problems
    for people who wished to exchange batch files.

    Another major change in the file-management system was related to the
    new directory structure: In order to fully exploit a hierarchical file
    system, Microsoft had to add a new way of calling file services.

    Versions 1.x of MS-DOS used CP/M-like structures called file control
    blocks, or FCBs, to maintain compatibility with older CP/M-80
    programs. The FCBs contained all pertinent information about the size
    and location of a file but did not allow the user to specify a file in
    a different directory. Therefore, version 2.0 of MS-DOS needed the
    added ability to access files by means of handles, or descriptors,
    that could operate across directory lines.

    In this added step toward logical device independence, MS-DOS returned
    a handle whenever an MS-DOS program opened a file. All further
    interaction with the file involved only this handle. MS-DOS made all
    necessary adjustments to an internal structure--different from an FCB-
    -so that the program never had to deal directly with information about
    the file's location in memory. Furthermore, even if future versions of
    MS-DOS were to change the structure of the internal control units,
    program code would not need to be rewritten--the file handle would be
    the only referent needed, and this would not change.

    Putting the internal control units under the supervision of MS-DOS and
    substituting handles for FCBs also made it possible for MS-DOS to
    redirect a program's input and output. A system function was provided
    that enabled MS-DOS to divert the reads or writes directed to one
    handle to the file or device assigned to another handle. This
    capability was used by COMMAND.COM to allow output from a file to be
    redirected to a device, such as a printer, or to be piped to another
    program. It also allowed system cleanup on program terminations.

Installable Device Drivers

    At the time Microsoft began developing version 2.0 of MS-DOS, the
    company also realized that many third-party peripheral devices were
    not working well with one another. Each manufacturer had its own way
    of hooking its hardware into MS-DOS and if two third-party devices
    were plugged into a computer at the same time, they would often
    conflict or fail.

    One of the hallmarks of IBM's approach to the PC was open
    architecture, meaning that users could simply slide new cards into the
    computer whenever new input/output devices, such as fixed disks or
    printers, were added to the system. Unfortunately, version 1.0 of MS-
    DOS did not have a corresponding open architecture built into it--the
    BIOS contained all the code that permitted the operating system to run
    the hardware. If independent hardware manufacturers wanted to develop
    equipment for use with a computer manufacturer's operating system,
    they would have to either completely rewrite the device drivers or
    write a complicated utility to read the existing drivers, alter them,
    add the code to support the new device, and produce a working set of
    drivers. If the user installed more than one device, these patches
    would often conflict with one another. Furthermore, they would have to
    be revised each time the computer manufacturer updated its version of

    By the time work began on version 2.0, the MS-DOS team knew that the
    ability to install any device driver at run time was vital. They
    implemented installable device drivers by making the drivers more
    modular. Like the FAT, IO.SYS (IBMBIO.COM in PC-DOS) became, in
    effect, a linked list--this time, of device drivers--that could be
    expanded through commands in the CONFIG.SYS file on the system boot
    disk. Manufacturers could now write a device driver that the user
    could install at run time by including it in the CONFIG.SYS file. MS-
    DOS could then add the device driver to the linked list.

    By extension, this ability to install device drivers also added the
    ability to supersede a previously installed driver--for example, the
    ANSI.SYS console driver that supports the ANSI standard escape codes
    for cursor positioning and screen control.

Print Spooling

    At IBM's request, version 2.0 of MS-DOS also possessed the
    undocumented ability to perform rudimentary background processing--an
    interim solution to a growing awareness of the potentials of

    Background print spooling was sufficient to meet the needs of most
    people in most situations, so the print spooler, PRINT.COM, was
    designed to run whenever MS-DOS had nothing else to do. When the
    parent application became active, PRINT.COM would be interrupted until
    the next lull. This type of background processing, though both limited
    and extremely complex, was exploited by a number of applications, such
    as SideKick.

Loose Ends and a New MS-DOS

    Hierarchical files, installable device drivers, and print spooling
    were the major design decisions in version 2.0. But there were dozens
    of smaller changes, too.

    For example, with the fixed disk it was necessary to modify the code
    for automatic logging of disks. This modification meant that MS-DOS
    had to access the disk more often, and file access became much slower
    as a result. In trying to find a solution to this problem, Chris
    Peters reasoned that, if MS-DOS had just checked the disk, there was
    some minimum time a user would need to physically change disks. If
    that minimum time had not elapsed, the current disk information in
    RAM--whether for a fixed disk or a floppy--was probably still good.

    Peters found that the fastest anyone could physically change disks,
    even if the disks were damaged in the process, was about two seconds.
    Reasoning from this observation, he had MS-DOS check to see how much
    time had gone by since the last disk access. If less than two seconds
    had elapsed, he had MS-DOS assume that a new disk had not been
    inserted and that the disk information in RAM was still valid. With
    this little trick, the speed of file handling in MS-DOS version 2.0
    increased considerably.

    Version 2.0 was released in March 1983, the product of a surprisingly
    small team of six developers, including Peters, Mani Ulloa, and Nancy
    Panners in addition to Allen, Zbikowski, and Reynolds. Despite its
    complex new features, version 2.0 was only 24 KB of code. Though it
    maintained its compatibility with versions 1.x, it was in reality a
    vastly different operating system. Within six months of its release,
    version 2.0 gained widespread public acceptance. In addition, popular
    application programs such as Lotus 1-2-3 took advantage of the
    features of this new version of MS-DOS and thus helped secure its
    future as the industry standard for 8086 processors.

Versions 2.1 and 2.25

    The world into which version 2.0 of MS-DOS emerged was considerably
    different from the one in which version 1.0 made its debut. When IBM
    released its original PC, the business market for microcomputers was
    as yet undefined--if not in scope, at least in terms of who and what
    would dominate the field. A year and a half later, when the PC/XT came
    on the scene, the market was much better known. It had, in fact, been
    heavily influenced by IBM itself. There were still many MS-DOS
    machines, such as the Tandy 2000 and the Hewlett Packard HP150, that
    were hardware incompatible with the IBM, but manufacturers of new
    computers knew that IBM was a force to consider and many chose to
    compete with the IBM PC by emulating it. Software developers, too, had
    gained an understanding of business computing and were confident they
    could position their software accurately in the enormous MS-DOS

    In such an environment, concerns about the existing base of CP/M
    software faded as developers focused their attention on the fast-
    growing business market and MS-DOS quickly secured its position as an
    industry standard. Now, with the obstacles to MS-DOS diminished,
    Microsoft found itself with a new concern: maintaining the standard it
    had created. Henceforth, MS-DOS had to be many things to many people.
    IBM had requirements; other OEMs had requirements. And sometimes these
    requirements conflicted.

Hardware Developers

    When version 2.0 was released, IBM was already planning to introduce
    its PCjr. The PCjr would have the ability to run programs from ROM
    cartridges and, in addition to using half-height 5-1/4-inch drives,
    would employ a slightly different disk-controller architecture.
    Because of these differences from the standard PC line, IBM's
    immediate concern was for a version 2.1 of MS-DOS modified for the new

    For the longer term, IBM was also planning a faster, more powerful PC
    with a 20-megabyte fixed disk. This prospect meant Microsoft needed to
    look again at its file-management system, because the larger storage
    capacity of the 20-megabyte disk stretched the size limitations for
    the file allocation table as it worked in version 2.0.

    However, IBM's primary interest for the next major release of MS-DOS
    was networking. Microsoft would have preferred to pursue multitasking
    as the next stage in the development of MS-DOS, but IBM was already
    developing its IBM PC Network Adapter, a plug-in card with an 80188
    chip to handle communications. So as soon as version 2.0 was released,
    the MS-DOS team, again headed by Zbikowski and Reynolds, began work on
    a networking version (3.0) of the operating system.


    The international market for MS-DOS was not significant in the first
    few years after the release of the IBM PC and version 1.0 of MS-DOS.
    IBM did not, at first, ship its Personal Computer to Europe, so
    Microsoft was on its own there in promoting MS-DOS. In 1982, the
    company gained a significant advantage over CP/M-86 in Europe by
    concluding an agreement with Victor, a software company that was very
    successful in Europe and had already licensed CP/M-86. Working closely
    with Victor, Microsoft provided special development support for its
    graphics adaptors and eventually convinced the company to offer its
    products only on MS-DOS. In Japan, the most popular computers were Z80
    machines, and given the country's huge installed base of 8-bit
    machines, 16-bit computers were not taking hold. Mitsubishi, however,
    offered a 16-bit computer. Although CP/M-86 was Mitsubishi's original
    choice for an operating system, Microsoft helped get Multiplan and
    FORTRAN running on the CP/M-86 system, and eventually won the
    manufacturer's support for MS-DOS.

    In the software arena, by the time development was underway on the 2.x
    releases of MS-DOS, Microsoft's other customers were becoming more
    vocal about their own needs. Several wanted a networking capability,
    adding weight to IBM's request, but a more urgent need for many--a
    need not  shared by IBM at the time--was support for international
    products. Specifically, these manufacturers needed a version of MS-DOS
    that could be sold in other countries--a version of MS-DOS that could
    display messages in other languages and adapt to country-specific
    conventions, such as date and time formats.

    Microsoft, too, wanted to internationalize MS-DOS, so the MS-DOS team,
    while modifying the operating system to support the PCjr, also added
    functions and a COUNTRY command that allowed users to set the date and
    time formats and other country-dependent variables in the CONFIG.SYS

    At about the same time, another international requirement appeared.
    The Japanese market for MS-DOS was growing, and the question of
    supporting 7000 Kanji characters (ideograms) arose. The difficulty
    with Kanji is that it requires dual-byte characters. For English and
    most European character sets, one byte corresponds to one character.
    Japanese characters, however, sometimes use one byte, sometimes two.
    This variability creates problems in parsing, and as a result MS-DOS
    had to be modified to parse a string from the beginning, rather than
    back up one character at a time.

    This support for individual country formats and Kanji appeared in
    version 2.01 of MS-DOS. IBM did not want this version, so support for
    the PCjr, developed by Zbikowski, Reynolds, Ulloa, and Eric Evans,
    appeared separately in version 2.1, which went only to IBM and did not
    include the modifications for international MS-DOS.

Different customers, different versions

    As early as version 1.25, Microsoft faced the problem of trying to
    satisfy those OEM customers that wanted to have the same version of
    MS-DOS as IBM. Some, such as COMPAQ, were in the business of selling
    100-percent compatibility with IBM. For them, any difference between
    their version of the operating system and IBM's introduced the
    possibility of incompatibility. Satisfying these requests was
    difficult, however, and it was not until version 3.1 that Microsoft
    was able to supply a system that other OEMs agreed was identical with

    Before then, to satisfy the OEM customers, Microsoft combined versions
    2.1 and 2.01 to create version 2.11. Although IBM did not accept this
    because of the internationalization code, version 2.11 became the
    standard version for all non-IBM customers running any form of MS-DOS
    in the 2.x series. Version 2.11 was sold worldwide and translated into
    about 10 different languages. Two other intermediate versions provided
    support for Hangeul (the Korean character set) and Chinese Kanji.

Software Concerns

    After the release of version 2.0, Microsoft also gained an
    appreciation of the importance--and difficulty--of supporting the
    people who were developing software for MS-DOS.

    Software developers worried about downward compatibility. They also
    worried about upward compatibility. But despite these concerns, they
    sometimes used programming practices that could guarantee neither.
    When this happened and the resulting programs were successful, it was
    up to Microsoft to ensure compatibility.

    For example, because the information about the internals of the BIOS
    and the ROM interface had been published, software developers could,
    and often did, work directly with the hardware in order to get more
    speed. This meant sidestepping the operating system for some
    operations. However, by choosing to work at the lower levels, these
    developers lost the protection provided by the operating system
    against hardware changes. Thus, when low-level changes were made in
    the hardware, their programs either did not work or did not run
    cooperatively with other applications.

    Another software problem was the continuing need for compatibility
    with CP/M. For example, in CP/M, programmers would call a fixed
    address in low memory in order to request a function; in MS-DOS, they
    would request operating-system services by executing a software
    interrupt. To support older software, the first version of MS-DOS
    allowed a program to request functions by either method. One of the
    CP/M-based programs supported in this fashion was the very popular
    WordStar. Since Microsoft could not make changes in MS-DOS that would
    make it impossible to run such a widely used program, each new version
    of MS-DOS had to continue supporting CP/M-style calls.

    A more pervasive CP/M-related issue was the use of FCB-style calls for
    file and record management. The version 1.x releases of MS-DOS had
    used FCB-style calls exclusively, as had CP/M. Version 2.0 introduced
    the more efficient and flexible handle calls, but Microsoft could not
    simply abolish the old FCB-style calls, because so many popular
    programs used them. In fact, some of Microsoft's own languages used
    them. So, MS-DOS had to support both types of calls in the version 2.x
    series. To encourage the use of the new handle calls, however,
    Microsoft made it easy for MS-DOS users to upgrade to version 2.0. In
    addition, the company convinced IBM to require version 2.0 for the
    PC/XT and also encouraged software developers to require 2.0 for their

    At first, both software developers and OEM customers were reluctant to
    require 2.0 because they were concerned about problems with the
    installed user base of 1.0 systems--requiring version 2.0 meant
    supporting both sets of calls. Applications also needed to be able to
    detect which version of the operating system the user was running. For
    versions 1.x, the programs would have to use FCB calls; for versions
    2.x, they would use the file handles to exploit the flexibility of MS-
    DOS more fully.

    All told, it was an awkward period of transition, but by the time
    Microsoft began work on version 3.0 and the support for IBM's upcoming
    20-megabyte fixed disk, it had become apparent that the change had
    been in everyone's best interest.

Version 3

    The types of issues that began to emerge as Microsoft worked toward
    version 3.0, MS-DOS for networks, exaggerated the problems of
    compatibility that had been encountered before.

    First, networking, with or without a multitasking capability, requires
    a level of cooperation and compatibility among programs that had never
    been an issue in earlier versions of MS-DOS. As described by Mark
    Zbikowski, one of the principals involved in the project, "there was a
    very long period of time between 2.1 and 3.0--almost a year and a
    half. During that time, we believed we understood all the problems
    involved in making DOS a networking product. [But] as time progressed,
    we realized that we didn't fully understand it, either from a
    compatibility standpoint or from an operating-system standpoint. We
    knew very well how it [DOS] ran in a single-tasking environment, but
    we started going to this new environment and found places where it
    came up short."

    In fact, the great variability in programs and programming approaches
    that MS-DOS supported eventually proved to be one of the biggest
    obstacles to the development of a sophisticated networking system and,
    in the longer term, to the addition of true multitasking.

    Further, by the time Microsoft began work on version 3.0, the
    programming style of the MS-DOS team had changed considerably. The
    team was still small, with a core group of just five people:
    Zbikowski, Reynolds, Peters, Evans, and Mark Bebic. But the concerns
    for maintainability that had dominated programming in larger systems
    had percolated down to the MS-DOS environment. Now, the desire to use
    tricks to optimize for speed had to be tempered by the need for
    clarity and maintainability, and the small package of tightly written
    code that was the early MS-DOS had to be sacrificed for the same

Version 3.0

    All told, the work on version 3.0 of MS-DOS proved to be long and
    difficult. For a year and a half, Microsoft grappled with problems of
    software incompatibility, remote file management, and logical device
    independence at the network level. Even so, when IBM was ready to
    announce its new Personal Computer AT, the network software for MS-DOS
    was not quite ready, so in August 1984, Microsoft released version 3.0
    to IBM without network software.

    Version 3.0 supported the AT's larger fixed disk, its new CMOS clock,
    and its high-capacity 1.2-megabyte floppy disks. It also provided the
    same international support included earlier in versions 2.01 and 2.11.
    These features were made available to Microsoft's other OEM customers
    as version 3.05.

    But version 3.0 was not a simple extension of version 2.0. In laying
    the foundation for networking, the MS-DOS team had completely
    redesigned and rewritten the DOS kernel.

    Different as it was from version 1.0, version 2.0 had been built on
    top of the same structure. For example, whereas file requests in MS-
    DOS 1.0 used FCBs, requests in version 2.0 used file handles. However,
    the version 2.0 handle calls would simply parse the pathname and then
    use the underlying FCB calls in the same way as version 1.0. The
    redirected input and output in version 2.0 further complicated the
    file-system requests. When a program used one of the CP/M-compatible
    calls for character input or output, MS-DOS 2.0 first opened a handle
    and then turned it back into an FCB call at a lower level. Version 3.0
    eliminated this redundancy by eliminating the old FCB input/output
    code of versions 1 and 2, replacing it with a standard set of I/O
    calls that could be called directly by both FCB calls and handle
    calls. The look-alike calls for CP/M-compatible character I/O were
    included as part of the set of handle calls. As a result of this
    restructuring, these calls were distinctly faster in version 3.0 than
    in version 2.0.

    More important than the elimination of inefficiencies, however, was
    the fact that this new structure made it easier to handle network
    requests under the ISO Open System Interconnect model Microsoft was
    using for networking. The ISO model describes a number of protocol
    layers, ranging from the application-to-application interface at the
    top level down to the physical link--plugging into the network--at the
    lowest level. In the middle is the transport layer, which manages the
    actual transfer of data. The layers above the transport layer belong
    to the realm of the operating system; the layers below the transport
    layer are traditionally the domain of the network software or

    On the IBM PC network, the transport layer and the server functions
    were handled by IBM's Network Adapter card and the task of MS-DOS was
    to support this hardware. For its other OEM customers, however,
    Microsoft needed to supply both the transport and the server functions
    as software. Although version 3.0 did not provide this general-purpose
    networking software, it did provide the basic support for IBM's
    networking hardware.

    The support for IBM consisted of redirector and sharer software. MS-
    DOS used an approach to networking in which remote requests were
    routed by a redirector that was able to interact with the transport
    layer of the network. The transport layer was composed of the device
    drivers that could reliably transfer data from one part of the network
    to another. Just before a call was sent to the newly designed low-
    level file I/O code, the operating system determined whether the call
    was local or remote. A local call would be allowed to fall through to
    the local file I/O code; a remote call would be passed to the
    redirector which, working with the operating system, would make the
    resources on a remote machine appear as if they were local.

Version 3.1

    Both the redirector and the sharer interfaces for IBM's Network
    Adapter card were in place in version 3.0 when it was delivered to
    IBM, but the redirector itself wasn't ready. Version 3.1, completed by
    Zbikowski and Reynolds and released three months later, completed this
    network support and made it available in the form of Microsoft
    Networks for use on non-IBM network cards.

    Microsoft Networks was built on the concept of "services" and
    "consumers." Services were provided by a file server, which was part
    of the Networks application and ran on a computer dedicated to the
    task. Consumers were programs on various network machines. Requests
    for information were passed at a high level to the file server; it was
    then the responsibility of the file server to determine where to find
    the information on the disk. The requesting programs--the consumers--
    did not need any knowledge of the remote machine, not even what type
    of file system it had.

    This ability to pass a high-level request to a remote server without
    having to know the details of the server's file structure allowed
    another level of generalization of the system. In MS-DOS 3.1,
    different types of file systems could be accessed on the same network.
    It was possible, for example, to access a XENIX machine across the
    network from an MS-DOS machine and to read data from XENIX files.

    Microsoft Networks was designed to be hardware independent. Yet the
    variability of the classes of programs that would be using its
    structures was a major problem in developing a networking system that
    would be transparent to the user. In evaluating this variability,
    Microsoft identified three types of programs:

    ■  First were the MS-DOS-compatible programs. These used only the
        documented software-interrupt method of requesting services from
        the operating system and would run on any MS-DOS machine without

    ■  Second were the MS-DOS-based programs. These would run on IBM-
        compatible computers but not necessarily on all MS-DOS machines.

    ■  Third were the programs that used undocumented features of MS-DOS
        or that addressed the hardware directly. These programs tended to
        have the best performance but were also the most difficult to

    Of these, Microsoft officially encouraged the writing of MS-DOS
    compatible programs for use on the network.

Network concerns

    The file-access module was changed in version 3.0 to simplify file
    management on the network, but this did not solve all the problems.
    For instance, MS-DOS still needed to handle FCB requests from programs
    that used them, but many programs would open an FCB and never close
    it. One of the functions of the server was to keep track of all open
    files on the network, and it ran into difficulties when an FCB was
    opened 50 or 100 times and never closed. To solve this problem,
    Microsoft introduced an FCB cache in version 3.1 that allowed only
    four FCBs to be open at any one time. If a fifth FCB was opened, the
    least recently used one was closed automatically and released. In
    addition, an FCBS command was added in the CONFIG.SYS file to allow
    the user or network manager to change the maximum number of FCBs that
    could be open at any one time and to protect some of the FCBs from
    automatic closure.

    In general, the logical device independence that had been a goal of
    MS-DOS acquired new meaning--and generated new problems--with
    networking. One problem concerned printers on the network. Commonly,
    networks are used to allow several people to share a printer. The
    network could easily accommodate a program that would open the
    printer, write to it, and close it again. Some programs, however,
    would try to use the direct IBM BIOS interface to access the printer.
    To handle this situation, Microsoft's designers had to develop a way
    for MS-DOS to intercept these BIOS requests and filter out the ones
    the server could not handle. Once this was accomplished, version 3.1
    was able to handle most types of printer output on the network in a
    transparent manner.

Version 3.2

    In January 1986, Microsoft released another revision of MS-DOS,
    version 3.2, which supported 3-1/2-inch floppy disks. Version 3.2 also
    moved the formatting function for a device out of the FORMAT utility
    routine and into the device driver, eliminating the need for a special
    hardware-dependent program in addition to the device driver. It
    included a sample installable-block-device driver and, finally,
    benefited the users and manufacturers of IBM-compatible computers by
    including major rewrites of the MS-DOS utilities to increase
    compatibility with those of IBM.

The Future

    Since its appearance in 1981, MS-DOS has taken and held an enviable
    position in the microcomputer environment. Not only has it "taught"
    millions of personal computers "how to think," it has taught equal
    millions of people how to use computers. Many highly sophisticated
    computer users can trace their first encounter with these machines to
    the original IBM PC and version 1.0 of MS-DOS. The MS-DOS command
    interface is the one with which they are comfortable and it is the MS-
    DOS file structure that, in one way or another, they wander through
    with familiarity.

    Microsoft has stated its commitment to ensuring that, for the
    foreseeable future, MS-DOS will continue to evolve and grow, changing
    as it has done in the past to satisfy the needs of its millions of
    users. In the long term, MS-DOS, the product of a surprisingly small
    group of gifted people, will undoubtedly remain the industry standard
    for as long as 8086-based (and to some extent, 80286-based)
    microcomputers exist in the business world. The story of MS-DOS will,
    of course, remain even longer. For this operating system has earned
    its place in microcomputing history.

                                                JoAnne Woodcock

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