Groundbreaking Operating System Is Named an IEEE Milestone

Gary Kildall’s CP/M paved the way for affordable personal computers

14 July 2014

The greatest technological innovations often happen in the most modest of places. In his backyard toolshed in Pacific Grove, Calif., computer programmer Gary Kildall built an operating system in 1974 that, along with the microprocessor and disk drive, would become one of the three fundamental building blocks of the personal computer revolution. His OS, Control Program for Microprocessors (CP/M), was the first commercial system to allow a microprocessor-based computer to interface with a disk storage unit. It paved the way for low-cost computers to be used in business, industry, academia and, eventually, the home.

To recognize the breakthrough, Kildall’s invention was named an IEEE Milestone in Electrical Engineering and Computing. Administered by the IEEE History Center, the Milestone program recognizes important developments with a ceremony and a plaque. I worked with IEEE Life Senior Member Dick Ahrons, the IEEE Santa Clara Valley Section Milestone coordinator, to propose the achievement for recognition.


Before CP/M, application software developed by computer manufacturers would work only on their own hardware. The programs were written from scratch to operate on each machine’s unique configuration. Kildall’s OS, on the other hand, was designed to work with the popular Intel 8080 microprocessor so that computer systems built by any manufacturer around that chip could for the first time run application programs written by third-party suppliers.

IEEE_6%20[072_Laws_Michel_unveiling] David Laws [left] and IEEE President-Elect Howard Michel unveil the Milestone plaque honoring Gary Kildall and his CP/M operating system. Photo: David Laws

By 1980, Kildall’s CP/M was the preferred OS for pioneering computer hobbyists. It resonated among members of the Homebrew Computer Club (an early computer hobbyist group in Silicon Valley) and its associated start-up companies including IMSAI and Osborne. In 1982, Digital Research Inc. (DRI), the company Kildall founded with his wife, Dorothy, reported annual sales in excess of US $20 million. More than a million people were using CP/M-controlled systems.

Microsoft licensed CP/M for sale with its BASIC computer program, the company’s foundational product. When IBM approached Microsoft to acquire BASIC for its first personal computer, introduced in 1981, founder Bill Gates referred IBM to DRI for an operating system. When IBM and DRI were unable to agree on financial terms, Microsoft acquired another operating system, QDOS. Designed by Seattle Computer Products, QDOS replicated the look and functionality of CP/M by copying its application-programming interface. IBM subsequently promoted QDOS, renamed PC-DOS, as its preferred operating system, and offered CP/M as an optional upgrade.

IBM made CP/M prohibitively more expensive than PC-DOS, though: $240, compared with $40. CP/M rapidly lost market share as IBM’s personal computers took off, and it never regained its former popularity. Kildall continued to oversee DRI and pursued computing-related projects outside the company until he died in 1994.


On 25 April, Kildall’s gift to the world of personal computing was celebrated in Pacific Grove when IEEE President-Elect Howard E. Michel cut a “ribbon”—a length of punched paper tape secured with a floppy disk—to unveil the Milestone plaque, which reads:

“Dr. Gary A. Kildall demonstrated the first working prototype of CP/M (Control Program for Microcomputers) in Pacific Grove in 1974. Together with his invention of the BIOS (Basic Input Output System), Kildall’s operating system allowed a microprocessor-based computer to communicate with a disk drive storage unit and provided an important foundation for the personal computer revolution.”

The ceremony took place in front of the former DRI headquarters and was presided over by Pacific Grove’s mayor, Bill Kampe. Afterward, guests attended a reception in the Victorian house that once served as the DRI engineering building. With several vintage rack-mounted servers still in the basement, the building today houses the publishing offices of the weekly Carmel Pine Cone newspaper. The event attracted the attention of other local media and was featured in a broadcast by the National Public Radio station in Monterey, Calif.

Close to 150 people, including members and friends of the Kildall family, DRI employees, IEEE members, attendees of the 40th annual Asilomar Microcomputer Workshop, and officers and faculty from the Naval Postgraduate School, in Monterey (where Kildall was an instructor), braved stormy weather to observe the important computing history breakthrough.

Several notable figures from the computing world came to speak that afternoon of Kildall’s contributions, which extended far beyond his development of CP/M.

IEEE Senior Member Brian Berg, a vice-chair of the IEEE Santa Clara Section History Committee and an organizer of the Asilomar Microcomputer Workshop, recalled Kildall’s frequent appearances at the workshop and mentioned the debt that many of the attendees owed to his work. Tom Rolander, former vice president of operating systems at DRI, cited Kildall’s concept of the Basic Input Output System. BIOS allowed CP/M to be easily adapted to operate with a wide variety of machine designs—which stimulated, Rolander pointed out, the rise of independent applications software developers.

Brian Halla, who worked with Kildall at Intel when Halla was a microprocessor software consultant to the company, stressed the importance of his former colleague’s development of the PL/M programming language and other tools used by Intel in the early battle for microprocessor market share. Halla also noted Kildall’s pioneering work in consumer CD-ROM systems, smartphones, graphics, and networking technologies. John Wharton, who served as Intel’s technical liaison to DRI and coauthored several technical papers with Kildall, described his extraordinary gifts as a programmer and visionary of the PC’s potential to change the nature of computing. Kildall’s son, Scott, spoke as well.

David Laws is the semiconductor curator at the Computer History Museum, in Mountain View, Calif.

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