Introduction to DOS
DOS, short for "Disk Operating System", is a family of similarly designed computer operating systems originally popularized by Microsoft in the early 1980s which remained in popular use through about the year 2000 as the technology underlying some versions of the Windows graphical user interface. From nearly the beginning there were competing versions of DOS available, the most popular being MS-DOS (by Microsoft) and IBM PC DOS (also originally by Microsoft). Other versions of DOS include DR-DOS, FreeDOS, PTS-DOS, ROM-DOS, JM-OS, and several others. The most popular version of DOS still under active development and maintenance is FreeDOS, an open source, MS-DOS compatible version of DOS. Despite the variety of DOS vendors the systems are generally compatible in that a program written on one version of DOS (usually MS-DOS) will usually run on any other vendor's version. One notable exception to this rule is ProDOS written for Apple II based computers which was incompatible because of underlying differences in the hardware, such as the CPU architecture.
Although DOS has largely fallen out of favor as a mainstream operating system in favor of Windows NT based operating systems it remains a useful tool for any PC technician and at least a basic understanding of DOS is still useful. In addition Windows NT based operating systems still retain a "command line interface" (CLI) which uses familiar DOS commands and conventions to accomplish many system maintenance and repair functions.
History
Early hobbyist personal computers of the late 1970s and early 1980s ran a variety of, often incompatible, operating systems. One of the most popular at the time IBM was developing their original PC was the CP/M operating system by Digital Research which, at the time, ran on the 8-bit Intel 8080 and Zilog Z80 processors. During the development of the IBM PC an operating systems was needed for the Intel 8088 processor around which the system was based. IBM initially approached then Microsoft CEO Bill Gates about developing an 8088 version of CP/M but was directed to Digital Reseach, the owner of CP/M.
For obvious cost reasons IBM was looking for a blanket license to relabel and sell CP/M as IBM PC-DOS with their new computer. Digital Research founder and owner Gary Kildall refused, holding out for a royalty based deal at which time IBM withdrew their offer and returned to Microsoft looking for an alternative operating system. This time Gates approached Seattle Computer Products about a new CP/M like operating system code named "QDOS" which stood for "Quick and Dirty Operating System" developed by Tim Paterson to test Intel 8086 hardware made by Seattle Computer Products. Gates was able to secure a nonexclusive license to QDOS in December of 1980 for $25,000 and soon after hired designer Tim Paterson to port the operating system to the IBM PC which used the less expensive Intel 8088 CPU.
Shortly before the launch of the IBM PC, in the summer of 1981, Microsoft purchased all rights for QDOS (now known as the commercial product 86-DOS) from Seattle Computer Products for $50,000. Microsoft, in-turn, licensed the IBM PC version of 86-DOS to IBM to be sold as IBM PC DOS 1.0. In a stroke of genius the license agreement between Microsoft and IBM was non-exclusive, allowing Microsoft to license the operating system to other PC manufacturers when they arrived some time later.
The original DOS was designed to look and act much like the competing CP/M operating system, and was even compatible with CP/M programs to some extent, but was much less expensive and soon overtook CP/M as the primary operating system of the personal computer.