The Late Arrival of 16-Bit CP/M

The story of how MS-DOS became the industry standard for the 16-bit business computers at the expense of Digital Research’s CP/M is somewhat of a legend among fans of computing history. The event has been described in books, articles and documentaries, with most of them focusing on entertaining details such as corporate drama, back-stabbing and nepotism 1.

Some of the more serious accounts mention one important detail of the story: DRI was late with the port of the CP/M to the Intel 8086. When IBM representatives visited in August of 1980, looking for an operating system for their upcoming PC, only the 8-bit version of CP/M could be demonstrated. IBM eventually turned to Microsoft and selected what became MS‑DOS.

In this article, we will examine why CP/M‑86 was delayed and, in the end, speculate how much that delay actually mattered.

PL/I Development

To understand why the CP/M port to the Intel 8086 was delayed, we need to go back to 1978. The current version of CP/M was 1.4, and it was already the industry standard as an operating system for 8-bit business microcomputers.

At the time, Digital Research founder Dr. Gary Kildall felt that microcomputers needed a high-level application language - something better suited to application development than his PL/M, which was essentially a systems language 2. At first, he considered Pascal. Then he received a call from Boston‑based computer scientist Dr. Robert Freiburghouse, who invited him to join an ANSI standards committee working on the PL/I‑G dialect. Kildall was familiar with PL/I and did not have a good opinion of it, but after attending several committee meetings he realized he liked the “Subset G” proposal and decided to implement a compiler.

The plan was to complete the new compiler in nine months. Instead, development stretched to more than two years; the compiler was finally released in late 1980 under the name PL/I‑80.

DRI’s PL/I received praises for its quality 3 - after all compilers were Gary Kildal…