MS-DOS, the most commonly used member of the DOS family of operating systems, turned 30-year-old on Wednesday.

Way back on July 27, 1981, Microsoft purchased DOS operating from Seattle Computer Products for $50,000. The OS was the backbone of most PCs for the following decade or longer until it was gradually superseded by operating systems offering a graphical user interface (GUI), in particular by various generations of the Microsoft Windows operating system.

MS-DOS was a renamed form of 86-DOS — informally known as the Quick-and-Dirty Operating System or Q-DOS owned by Seattle Computer Products (SCP) written by Tim Paterson.

When IBM came to Microsoft asking for a 16-bit operating system rather than writing a new one from the ground up, it turned to Seattle Computer Products and its existing 86-DOS (aka QDOS), and bought a non-exclusive licence for $US25,000. But he did not tell SCP that he had a deal with IBM to supply the operating system for its first personal computer.

However, he never told SCP and Paterson that he had a deal in the works with Big Blue until it acquired the OS on July 27. It paid another $US50, 000 for all rights, which eventually turned Bill Gates and Paul Allen into two of the world’s richest men.

During its life MS-DOS itself would go through eight versions, until development ceased in 2000. Ultimately it was the key product in Microsoft's growth from a programming languages company to a diverse software development firm, providing the company with essential revenue and marketing resources. It was also the underlying basic operating system on which early versions of Windows ran as a GUI.

Three decades later, DOS is still present on Windows systems, often hidden away. You can use it by type CMD in the Run diag box.