Development of MSDOS/PCDOS began in October 1980, when IBM began searching the market for an operating system for the yet-to-be-introduced IBM PC. Microsoft had no real operating system to sell, but after some research licensed Seattle Computer Products' 86-DOS operating system, which had been written by a man named Tim Paterson earlier in 1980 for use on that company's line of 8086, S100 bus micros. 86-DOS (also called QDOS, for Quick and Dirty Operating System) had been written as more or less a 16-bit version of CP/M, since Digital Research was showing no hurry in introducing CP/M-86.

This code was hurriedly polished up and presented to IBM for evaluation. IBM had originally intended to use Digital Research's CP/M operating system, which was the industry standard at the time. Folklore reports everything from obscure legal entanglements to outright snubbing of the IBM representatives by Digital, irregardless, IBM found itself left with Microsoft's offering of "Microsoft Disk Operating System 1.0". An agreement was reached between the two, and IBM agreed to accept 86-DOS as the main operating system for thir new PC. Microsoft purchased all rights to 86-DOS in July 1981, and "IBM PC-DOS 1.0" was ready for the introduction of the IBM PC in October 1981. IBM subjected the operating system to an extensive quality-assurance program, found well over 300 bugs, and decided to rewrite the programs. This is why PC-DOS is copyrighted by both IBM and Microsoft.

It is sometimes amusing to reflect on the fact that the IBM PC was not originally intended to run MSDOS. The target operating system at the end of the development was for a (not yet in existence) 8086 version of CP/M. On the other hand, when DOS was originally written the IBM PC did not yet exist! Although PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably have been the main operating system for the PC except for two things - Digital Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and many software developers found it easier to port existing CP/M software to DOS than to the new version of CP/M.

After eight years the wheel has turned a full circle. Digital Research aided IBM in writing DOS 4.0, which was subsequently licensed back to Microsoft, which has dropped further development of the operating system to tilt at the windmills of OS/2.

MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. There was an expansion board for the Apple ][ that allowed one to run (some) well - behaved DOS programs. There are expansion boards for the Commodore Amiga 2000, the Apple MacIntosh II, and the IBM RT PC allowing them to run DOS, the Atari STs can run an emulator program called PC-Ditto and boot any version of MSDOS with full functionality. Microsoft's OS/2 operating system incorporates the so-called "DOS Compatibility Box" to run DOS software; QNX will run DOS as a task, and a few Unix systems offer "DOS Merge" or control programs that allow DOS to run as a task.

DOS 1.x is essentially 86-DOS. DOS 2.x kept the multiple file layout (the two hidden files and COMMAND.COM) but for all practical purposes is an entirely different operating system with backwards compatibility with 1.x. I seriously doubt there has been much code from 1.x retained in 2.x. DOS 3.x is merely an enhancement of 2.x; there seems little justification for jumping a whole version number. DOS 4.0, originating as it did from outside Microsoft, can justify a version jump. Unfortunately, 4.x seems to have very little to reason justify its existence - virtually all of its core features can be found in one version or another of DOS 3.x.

DOS version nomenclature: major.minor.minor. The digit to the left of the decimal point indicates a major DOS version change. 1.0 was the first version. 2.0 added support for subdirectorites, 3.0 added support for networking, 4.0 added support for Lotus-Intel-Microsoft EMS.

The first minor version indicates customization for a major application. For example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not seem to have any particular meaning.

IBM's PC-DOS is considered to be the "standard" version of DOS; Microsoft has sold MS-DOS over the counter only since version 3.2 (previously, Microsoft sold its versions only to OEMs). Most versions of DOS functionally duplicate the external DOS commands such as DISKCOPY, etc. Microsoft has announced that they will sell MS-DOS 4.0 only to OEMs, so it looks like the "generic" MS-DOS was short-lived.

Some versions of MS-DOS varied from PC-DOS in the availible external commands. Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves or contracted them from outside software houses like Phoenix. Most of the external programs for DOS 3.x and 4.x are written in "C" while the 1.x and 2.x utilities were written in assembly language. Other OEMs required customized versions of DOS for their specific hardware configurations, such as Sanyo 55x and early Tandy computers, which were unable to exchange their DOS with the IBM version.

At least two versions of DOS have been modified to be run entirely out of ROM. The Sharp PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1000 and some Tandy models have MSDOS 2.11 in ROM.

PC-DOS 3.0 was extremely buggy on release. It does not handle the DOS environment correctly and there are numerous documented problems with the batch file parser. The network support code is also nonfunctional in this DOS version. It is recommended that users upgrade to at least version 3.1.

DEC MSDOS versions 2.11 for the Rainbow had the device\driver\ANSI.SYS built into the main code. The Rainbow also supported a unique quad density, single sided floppy drive.

IBM had a version 1.85 of PC-DOS in April 1983, after the introduction of DOS 2.0. It was evidently for internal use only, supported multiple drive file searches (a primitive form of PATH), builtin MODE sommands for screen support, a /P parameter for TYPE for paused screens, an editable command stack like the public domain DOSEDIT.COM utility, and could be set up to remain completely resident in RAM instead of a resident/transient part like normal DOS. It is a pity some of the neat enhancements didn't make it into DOS 2.0. IBM also had an internal use only version 3.4, evidently used while developing DOS 4.0.

Some versions of DOS used in compatibles do not maintain the 1.x, 2.x, ..... numbering system. Columbia Data Products computers labeled DOS 1.25 as DOS 2.0. Early Compaqs labeled DOS 2.0 as DOS 1.x. Other versions incorporated special features - Compaq DOS 3.31 and Wyse DOS 3.21 both support 32-bit File Allocation Tables.

AT&T DOS 3.1 differs from generic MSDOS 3.10 in its use of cluster-size and file allocation table structures. AT&T DOS appears to use rules not from version 3, but rather those from version 2.

Epson Equity III and ComputerLand 3.10 DOS's appear to use cluster techniques that are a cross between versions 2 and 3. On type DOS partitions, these DOS's use 3.x rules if the partition is larger than 32,680 sectors in total size. This implies 16 bit FAT entries as well. On partitions below this size, they will use 2.x rules, including the 12 bit FAT entries.

Zenith DOS 3.x and Wyse DOS 3.2 have built in internal device\drivers to handle up to 4 32Mb DOS partitions on a single hard disk. Wyse DOS 3.31 will handle single partitions up to 512Mb with a 32-bit FAT.

According to PC Week Magazine, July 4, 1988, Arabic versions of MSDOS are shipping with a hardware copy-protection system from Rainbow Technologies. This is similar to the short-lived system used by AutoCAD 2.52 and a very few other MSDOS programs, where an adapter block is plugged into the parallel port and software makes use of coded bytes within the block. This type of copy protection has been common on Commodore products for several years, where it is called a "dongle."

The AutoCAD dongle was defeated by a small program written within weeks of version 2.52's debut. Version 2.62 was released 3 months later, without the dongle. The DOS dongle will, however, prevent the system from booting at all unless it is found.

This makes the Arabic version of MSDOS the first copy-protected operating system, a dubious distinction at best. The modifications to the operating system to support the dongle are not known at this time.

Versions of DOS sold in Great Britain are either newer than those sold in the US or use a different numbering system. DOS 3.4, 4.0, 4.1, 4.2, and 4.3 had been released there between the US releases of 3.3 and 4.0.

MSDOS 4.0 was introduced in mid-1987 in Europe (at SICOB in Paris and sometime earlier by Apricot Computer in the UK). It apparently offers multitasking PROVIDED applications are specially written for it.

David Fraser (UK MD of Microsoft) is on record saying that DOS 4 is unlikely to set the world alight and is of interest only to specific OEM's who want its features for networking/comms etc. Standard DOS applications will run under DOS 4.x as a foreground task according to uncertain information. It differs from earlier versions only in allowing background tasks to run. No info about how many tasks/memory requirements etc. at this time. It will run an MS-DOS 3.2 or earlier type task in a foreground partition, and a specially written task in the background.

Microsoft changed their OEM licensing agreements between DOS versions 2.x and 3.x. OEM versions of DOS 3.x must maintain certain data areas and undocumented functions in order to provide compatibility with the networking features of the operating system. For this reason, resident programs will be much more reliable when operating under DOS 3.x.

IBM's release of DOS 4.0 (and the immediate subsequent release of a bugfix) is a dubious step "forward." DOS 4.0 is the first version of DOS to come with a warranty; the catch is that IBM warrants it only for a very slim list of IBM-packaged software. 4.0 has some minor EMS support, support for large hard disks, and not much else. With its voracious RAM requirements and lack of compatibility with previous versions of DOS (many major software packages crash under DOS 4.0), plus the increase in price to a cool $150, there has been no great rush to go to the newest DOS.

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