CP/M,[3] originally standing for Control Program/Monitor[4] and later Control Program for Microcomputers,[5][6][7] is a mass-market operating system created in 1974 for Intel 8080/85-based microcomputers by Gary Kildall of Digital Research, Inc. CP/M is a disk operating system[8] and its purpose is to organize files on a magnetic storage medium, and to load and run programs stored on a disk.

[10] CP/M increased the market size for both hardware and software by greatly reducing the amount of programming required to port an application to a new manufacturer's computer.

[11][12] An important driver of software innovation was the advent of (comparatively) low-cost microcomputers running CP/M, as independent programmers and hackers bought them and shared their creations in user groups.

Kathryn Strutynski, a friend of Kildall from Naval Postgraduate School (NPS), became the fourth employee of Digital Research Inc. in early 1979.

[24] (Xerox included a Howard W. Sams CP/M manual as compensation for Digital Research's documentation, which InfoWorld described as atrocious,[25] incomplete, incomprehensible, and poorly indexed.

[26]) By 1984, Columbia University used the same source code to build Kermit binaries for more than a dozen different CP/M systems, plus two generic versions.

[30] Programs written for CP/M were typically portable among different machines, usually requiring only the specification of the escape sequences for control of the screen and printer.

[31] Many different brands of machines ran the operating system, some notable examples being the Altair 8800, the IMSAI 8080, the Osborne 1 and Kaypro luggables, and MSX computers.

[35][36] A benefit of the Rainbow was that it could continue to run 8-bit CP/M software, preserving a user's possibly sizable investment as they moved into the 16-bit world of MS-DOS.

The translator would also optimize the output for code size and take care of calling conventions, so that CP/M-80 and MP/M-80 programs could be ported to the CP/M-86 and MP/M-86 platforms automatically.

Upon the failure to obtain a signed non-disclosure agreement, the talks failed, and IBM instead contracted with Microsoft to provide an operating system.

Features of ZCPR as of version 3 included shells, aliases, I/O redirection, flow control, named directories, search paths, custom menus, passwords, and online help.

[51] First featured in the Sharp MZ-800, a cassette-based system with optional disk drives,[52] Personal CP/M was described as having been "rewritten to take advantage of the enhanced Z-80 instruction set" as opposed to preserving portability with the 8080.

American Microsystems announced a Z80-compatible microprocessor, the S83, featuring 8 KB of in-package ROM for the operating system and BIOS, together with comprehensive logic for interfacing with 64-kilobit dynamic RAM devices.

[54] On most machines the bootstrap was a minimal bootloader in ROM combined with some means of minimal bank switching or a means of injecting code on the bus (since the 8080 needs to see boot code at Address 0 for start-up, while CP/M needs RAM there); for others, this bootstrap had to be entered into memory using front-panel controls each time the system was started.

A utility program (MOVCPM) is provided with system distribution that allows relocating the object code to different memory areas.

This newly patched version can then be saved on a new disk, allowing application programs to access the additional memory made available by moving the system components.

Otherwise it attempts to find an executable file on the currently logged disk drive and (in later versions) user area, loads it, and passes it any additional parameters from the command line.

[55] Transient commands in CP/M include ASM, DDT, DUMP, ED, LOAD, MOVCPM [pl], PIP, STAT, SUBMIT, and SYSGEN.

[57] Transient commands in CP/M 3 include COPYSYS, DATE, DEVICE, DUMP, ED, GET, HELP, HEXCOM, INITDIR, LINK, MAC, PIP, PUT, RMAC, SET, SETDEF, SHOW, SID, SUBMIT, and XREF.

All of these variations in the hardware are concealed from other modules of the system by use of the BIOS, which uses standard entry points for the services required to run CP/M such as character I/O or accessing a disk block.

Two early outliners, KAMAS (Knowledge and Mind Amplification System) and its cut-down successor Out-Think (without programming facilities and retooled for 8080/V20 compatibility) were also written for CP/M, though later rewritten for MS-DOS.

A host of compilers and interpreters for popular programming languages of the time (such as BASIC, Borland's Turbo Pascal, FORTRAN and even PL/I[58]) were available, among them several of the earliest Microsoft products.

Using this capability, programmers developed and sold auxiliary desk accessory programs, such as SmartKey, a keyboard utility to assign any string of bytes to any key.

[68][69][70] Although CP/M provided some hardware abstraction to standardize the interface to disk I/O or console I/O, application programs still typically required installation to make use of all the features of such equipment as printers and terminals.

Programs (MBASIC is a notable example) were able to create filenames containing lower-case letters, which then could not easily be referenced at the command line.

The user area feature arguably had little utility on small floppy disks, but it was useful for organizing files on machines with hard drives.

A number of CP/M-80 derivatives existed in the former Eastern Bloc under various names, including SCP (Single User Control Program [de]), SCP/M, CP/A,[82] CP/J, CP/KC, CP/KSOB, CP/L, CP/Z, MICRODOS, BCU880, ZOAZ, OS/M, TOS/M, ZSDOS, M/OS, COS-PSA, DOS-PSA, CSOC, CSOS, CZ-CPM, DAC, HC and others.

[89][90][91] After Olmstead's death on 12 September 2001,[92] the distribution license was refreshed and expanded by Lineo, who had meanwhile become the owner of those Digital Research assets, on 19 October 2001.

[96] As of 2018[update], there are a number of active vintage, hobby and retro-computer people and groups, and some small commercial businesses, still developing and supporting computer platforms that use CP/M (mostly 2.2) as the host operating system.