CP/M

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CP/M is an operating system for Intel 8080/85 and Zilog Z80-based machines that reached great popularity in the 1980s. Its name was an acronym for Control Program/Monitor or Control Program for Microcomputers, in the usual naming tradition of its age. CP/M had an extremely important contribution to the adoption of computers and software in the mainstream usage. It was initially developed by Gary Kildall, an employee of Digital Research, Inc.

Although it was quite limited (the file system supported only the traditional 8.3 file names, no subdirectory support, little security and a limited single-user environment), it was technologically competent at its time.

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Hardware requirements

Digital Research only developed CP/M for Intel 8080-based systems. However, other companies had the right to develop custom CP/M solutions for their own hardware. Nevertheless, there were some hardware requirements that CP/M required from all platforms. Any system needed an ASCII-based terminal with at least 16 kilobytes of RAM. At least one diskette drive should have been present, although at lest two were recommended for optimal use.

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Components

8-bit CP/M versions contained three main components:

  • The Command Processor (CCP). The CCP was quite similar to the command line interface in DEC's operating systems like the RSTS/E. It would accept ASCII commands from the keyboard, and was the model of most operating systems that followed until the GUI explosion.
  • The basic Disk Operating System (BDOS), which provided the basic disk-related functions, like file opening, console input and output etc.
  • The Basic Input/Output System (BIOS), which provided the lowest level functions, like disk input/output.

CP/M was one of the pioneers of this layered structure. At the time, computer operating systems were destined to a single platform, and porting was very hard to do. In fact, most computer manufacturers preferred to rewrite any software from scratch instead of porting it from another platform.

Most of CP/M's complex functions were gathered in the BDOS and the CCP, making the BIOS the only portion that required substantial porting efforts. This made CP/M one of the most popular platforms, since almost any new platform could run CP/M with very few modifications, avoiding the effort of writing an entire operating system from scratch. This applied to software as well: if the software was written in a compiled or interpreted language, a simple recompilation was often enough to run it on another system.

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