CP/M (Control Program for Microcomputers) was the operating system Gary Kildall created in 1974. The Computer History Museum’s account of the early Digital Research source code states that Kildall “developed CP/M in 1974 for Intel’s Intellec-8 microcomputer” and that it was unusual among small-computer systems because it “was written in PL/M, a portable higher-level language.”
The museum describes how the system was structured into separable parts. “In version 1.3 the BIOS and BDOS were separated and separately compiled. The BIOS was rewritten in assembly-language so that it has easy access to hardware I/O registers.” The BIOS handled hardware-specific input and output, while the BDOS managed disk operations. That split is what made CP/M portable across the many incompatible 8080-based machines of the late 1970s, and it became the dominant operating system for microcomputers in that period.
CP/M’s influence outlived its market dominance. The museum records that “Tim Paterson at Seattle Computer Products used DRI’s 1976 CP/M Interface Guide and other information to guide the development of QDOS,” the system Microsoft later acquired and licensed as PC DOS and MS-DOS. So even after CP/M lost the IBM PC, its application interface lived on in the operating system that replaced it.