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When a computer is turned on an Operating System must be loaded into the computer's memory before the user can begin using it. IBM compatible machines use an operating system called MS-DOS. MS stands for "MicroSoft" (a trade name), while DOS stands for "Disk Operating System", which tells us that it's original purpose was to provide an interface between a computer and its disk drives. Technically, DOS is a high-level interface between an application program and the computer.

DOS has been extended further, allowing programs to handle the likes of simple memory management, disk operations, assorted system tasks (e.g. date/time), user input commands and managing input/output (i/o) devices (i.e. it provides operating instructions for the computer to manage both hardware and software). Versions 3.1 and up also provide basic networking functions.

Beyond this, DOS provides the user with the important facility of file and disk management (often referred to as disk-housekeeping). This part of DOS is particularly crucial to the user as it was specifically designed for him or her to interact with.

With the advent of installable device drivers and TSR (Terminate but Stay Resident) programs in DOS v2.0, the basic DOS functions could be extended to handle virtually any scale of operations required. This was the first instance of multiple programs being run at once in DOS. Some TSR's can however interfere with the running of programs. When a program is loaded into memory it assumes that it has exclusive use of this memory and will not take into account the fact that another program (the TSR) is also using this area. Thus a conflict can arise causing the program to hang (hang means that the screen freezes and the task being executed stops responding forcing the user to either switch off or reboot the computer).

History:

The development of MS-DOS/PC-DOS began in October 1980, when IBM began searching the market for a suitable operating system to go with their soon to be released IBM PC's (commercial Personal Computers). IBM had originally intended to use Digital Research's industry standard operating system - CP/M (Control Program/Monitor or Control Program for Microcomputer - originally written in 1973 by Gary Kildall in his PL/M language). This was never implemented due to uncertain reasons, the most likely being poor diplomatic relations between the two companies. Later, IBM approached a relatively small company called Microsoft, which specialised in language vending. Bill Gates and Paul Allen had written Microsoft BASIC and were selling it on punched tape or disk to early PC hobbyists.

Earlier, in April 1980, Tim Patterson began writing an operating system for use with Seattle Computer Products' 8086-based (S100 bus micros) computer. Seattle Computer Products decided to come up with their own disk operating system, due to delays by Digital Research in releasing a CP/M-86 operating system. By August 86-DOS or QDOS v0.10 (Quick and Dirty Operating System) was shipped by Seattle Computer Products. It was a 16-bit version of CP/M. Even though it had been created in only two man-months, the DOS worked surprisingly well. A week later, the EDLIN line editor was created. EDLIN was supposed to last only six months, before being replaced, but it endured for longer.

In September Tim Patterson showed Microsoft his 86-DOS, written for the 8086 chip. At this stage Microsoft had no 8086 real operating system to offer, but capitalized when, in October, Microsoft's Paul Allen contacted Patterson, asking for the rights to sell SCP's DOS to an unnamed client (IBM). Microsoft paid less than $100 000 for the rights. Patterson's DOS v1.0 was approximately 4000 lines of assembler source. This code was quickly polished up and presented to IBM for evaluation.

An agreement was reached between the two companies and IBM agreed to accept 86-DOS as the main operating system for their new PC. In February 1981, 86-DOS was run for the first time on IBM's prototype microcomputer. Furthermore, Microsoft purchased all rights to 86-DOS in July 1981.

In August, IBM announced the IBM 5150 PC, featuring a 4.77-MHz Intel 8088 CPU, 64KB RAM, 40KB ROM, one 5.25-inch floppy drive, and PC-DOS 1.0 (Microsoft's MS-DOS), for $3000.

Thus, "IBM Personal Computer DOS v1.0" was available for the introduction of the IBM PC in October 1981. IBM heavily subjected the program to an extensive quality-assurance test and found there to be well over 300 bugs and decided to rewrite the programs. This is why PC-DOS is copyrighted to both Microsoft and IBM.

Some early OEM (Original Equipment Manufacture) versions of DOS had different names such as Compaq-DOS, Z-DOS, Software Bus 86, etc. By version 2.0 Microsoft had succeeded to persuade everyone but IBM to call it MS-DOS.

It is interesting to reflect on the fact that the IBM PC was not originally meant to run MS-DOS. Instead it was supposed to use a (not yet in existence) 8086 version of CP/M. On the other hand, DOS was originally written before the IBM PC was created. CP/M-86 would have been the main operating system except for two things: Digital Research wanted $495 for CP/M-86 (considering PC-DOS was basically free) and many software developers found it easier to port software from CP/M into PC-DOS than into CP/M-86.

The IBM PC was first shipped without an operating system. IBM only started including DOS when the second generation AT/339 came out. A user could order one of three available operating systems: IBM PC-DOS, a version of UCSD p-System (type of integrated Pascal operating system - like the improved BASIC operating systems used by the Commodore 64,) and CP/M-86, which was officially an option even though it was unavailable until later on. Since IBM's $39.95 DOS was much cheaper than anyone else's it soon became the most popular.

An upgrade from DOS v3.3 to v4.0 was solely done by IBM, it was later licenced back to Microsoft. In early 1990 IBM declared that it would be ceasing development of DOS, handing the reigns over to Microsoft from then on.

Microsoft's Press' "MSDOS Encyclopaedia" illustrated an example of a late DOS v1.25 OEM brochure. Microsoft was praising future enhancement to v1.25 including XENIX-compatible pipes, process forks and multitasking, as well as "graphics and cursor positioning, kanji support, multi-user and hard disk support and networking". Despite these large aspirations, Microsoft failed to produce the forks, multitasking and multi-user support (at least in US versions of DOS).

The notice claimed: "MS-DOS has no practical limit on disk size. MS-DOS uses 4-byte XENIX OS compatible pointers for a file and disk capacity of up to 4 gigabytes."

For the record they actually delivered:

kanji support:
DOS 2.01, 2.25 (double-byte character set)
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hard disk support:
DOS 2.0 (subdirectories)
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networking:
DOS 3.1 (file locking support MS Networks)
DOS 6.0 (bundled Interlink in with DOS)
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Microsoft launched an aggressive marketing campaign for MS-DOS. Early Microsoft advertisements promoted DOS' XENIX-like features and promised XENIX functionality in future releases.

Microsoft had announced their intention of building a multi-user, multitasking operating system since as early as 1982. They Shipped beta versions of "DOS 4.0" in 1986/87 before v3.3 was even announced. Microsoft UK had announced that they had licenced v4.0 to Apricot Computers and the French Postal Service was supposed to be running it.

MS-DOS and PC-DOS have been run on more than just the IBM-PC and clones:

Hardware PC Emulation:

Software PC Emulation:

DOS Emulation: