Lexikon's History of Computing

CP/M Operating System

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CP/M was an operating system for microcomputers developed in 1973 by Dr. Gary Kildall, a brilliant software engineer. In 1976, Dr. Kildall started selling CP/M for $75 a copy through "Dr. Dobb's Journal" (published by Jim Warren), and it sold well.

Dr. Kildall and his wife founded Digital Research (1976) and sales of CP/M continued to increase. By the end of the 1970's, Digital Research had over 900 different firms as clients. CP/M became the most widely used operating program for non-Apple microcomputers in the early 1980's.

By the mid 1980's, CP/M was running on over 300 different models of microcomputers.

When IBM planned to introduce its personal computer, they had some initial talks with Digital Research regarding CP/M. As it turned out, however, IBM entered into an agreement with Microsoft and the IBM PC came out in 1981 with PC DOS as its operating system. DOS gradually took the lead over CP/M, and became pretty much the standard by the end of the 1980's.

It was estimated by Kildall that by 1987, there were over 200 million copies of CP/M in existence, and over 3,000 programs available for CP/M machines, with many still in use today.

(Note: Some sources define CP/M as "Control Program for Microcomputers.")


"User Friendly," Magazine, July 16, 1994, article: "Gary Kildall's Work Lives On";

"Gates" by Stephen Manes and Paul Andrews, Doubleday, 1993;

"Portraits in Silicon," by Robert Slater, MIT Press, 1987.


CP/M - A Further Look

(This article is courtesy of Kirk L. Thompson, reprinted with permission Copyright (C) 1995, Kirk L. Thompson)

CP/M is divided into three functional modules:

(1) Command Console Processor (CCP), which intercepts and interprets activity from the keyboard while at the command prompt;


(2) Basic Disk Operating System (BDOS), the fixed core of the system;

(3) Basic Input/Output System (BIOS), which handled communication between the keyboard, terminal, screen, system core, and application software; use of memory and the controlling of peripheral devices, such as disk drives, printers, and mouse devices.

In the early days of microcomputers, there was little in the way of hardware standards. To overcome this problem, the BIOS had to be customized for each different make or model it ran on. On some of the early systems, such as those from Heath Company, the basic system came ready to use, the manufacturer supplied assembler source code and special utilities to allow the user to customize the system further, to make it compatible with the user's hardware add-ons.

Various aspects of CP/M were also "cloned" by some developers to provide greater speed and flexibility over the original system. ZCPR3 was prepared by Richard Conn in the early 1980's as a replacement for the CCP. In the mid 1980's, Dennis Wright, Joe Wright, and Echelon, Inc., used ZCPR3 as a base to develop Z-System. By this time, all CP/M systems were using Zilog's Z-80 8-bit microprocessor as the CPU, rather than Intel's 8080 processor that CP/M was originally written for. The Wrights and Echelon wrote their system for the more powerful Z-80 chip and added many MS DOS-like commands.

With the increasing dominance of the IBM PC and its clones in the mid and late 1980's, many CP/M users wanted to run their applications on the new platform or at least be able to move their old data files to it. Consequently, a number of software "emulators" were developed, which allowed PCs to run applications originally written for CP/M.

Due to the fact that there was no standard CP/M diskette format in the early years, there were several diskette-format emulation utilities written. These utilities enabled a PC to directly read and write the several dozen most popular CP/M 5-1/4 inch diskettes.

Although the continued use of CP/M is now primarily found with devotees of older equipment, it is also licensed to the U.S. Department of Defense where it is used as a development environment for missile guidance software.


ZRDOS 1.0: Echelon Z-System Disk Operating System, Programmer's Guide, by Dennis L. Wright, 1985.

ZCPR3: The Manual, by Richard Conn, Zeotrope, 1985.

CP/M Version 2.2 for the Heath/Zenith 8-bit Computer Systems, Zenith Data Systems, 1981.

CP/M Today

Gary Kildall s Digital Research Incorporated ("DRI") was bought out by Novell. On Sept 10, 1996 Caldera bought all of the Digital Research assets from Novell. They have released all of the source code for DR products. Their web site is http://www.caldera.com. Caldera also provides data on Linux and DRDOS.

The last known source for new, legal copies of CP/M (with documentation,
$9, plus $6 shipping), is:

California Digital, Inc.
17700 Figueroa Street
Gardena CA 90248
310-217-1951 Fax


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