Editing CP/CMS (section)

==CP/CMS as free software==
CP/CMS was distributed in [[source code]] form, and many CP/CMS users were actively involved in studying and modifying that source code. Such direct user involvement with a vendor-supplied operating system was unusual.

In the CP/CMS era, many vendors distributed operating systems in machine-readable source code{{clarify|date=April 2016}}. IBM provided optional source code for, e.g., [[OS/360]], [[DOS/360]], and several later mainstream IBM operating systems. With all these systems, some awareness of system source code was also involved in the [[System generation|SYSGEN]] process, comparable to a [[kernel build]] in modern systems also in installing a [[Starter Set]]. (Forty years later, the [[Hercules emulator]] can be used to run fossilized versions of these systems, based on source code that is now treated as part of the [[public domain]].)

The importance of operating system source code has changed over time. Before [[History of IBM#1960–1969: The System/360 era, Unbundling software and services|IBM unbundled software from hardware]] in 1969, the OS (and most other software) was included in the cost of the hardware. Each vendor had complete responsibility for the entire system, hardware and software. This made the distribution medium relatively unimportant. After IBM's unbundling, OS software was delivered as [[IBM System Control Program software IBM System Control Program (SCP) software]], eventually in object code only (OCO) form.

For complicated reasons, CP/CMS was not released in the normal way. It was not supported by IBM, but was made part of the unsupported [[IBM Type-III Library]], a collection of software contributions from IBM staff members (similarly software contributed by customers formed the Type-IV Library). IBM distributed this library to its customers for use 'as is'. The lack of direct IBM support for such products forced active users to support themselves and encouraged modifications and mutual support. CP/CMS and other Type-III products were early forms of [[free software]].

Source code distribution of other IBM operating systems may have continued for some time (e.g. [[OS/360]], [[DOS/360]], [[DOS/VSE]], [[MVS]], and even [[TSS/370]], which all today are generally considered to be in the [[public domain]]) since they were arguably published without a copyright notice before 1978.<ref>cf. [[Hercules emulator]], which is apparently used to run these historic systems without raising complaints from IBM; the systems were published by IBM as machine readable source code and as microfiche</ref><ref>public domain: "Until the Berne Convention Implementation Act of 1988, the lack of a proper copyright notice would force an otherwise copyrightable work into the public domain...."</ref>{{Quote without source|date=June 2025}}} However, the unsupported status of CP/CMS placed different pressures on its user community and created the need for source code distribution.

CP/CMS was contributed to the Type-III Library by MIT's [[Lincoln Laboratory]] and ''not'' by IBM, despite the fact that the system was built by IBM's [[Cambridge Scientific Center]]. This decision has been described as a form of collusion to outmaneuver the IBM political forces opposed to time-sharing{{Citation needed|date=May 2010}}. It is thought that it may also reflect the amount of formal and informal input from MIT and [[Union Carbide]] that contributed to the design and implementation of CP-40, the S/360-67, CP-67, and CMS. See [[History of CP/CMS#Historical notes|History of CP/CMS (historical notes)]] for further insights and references on this topic.

Many CP/CMS users made extensive modifications to their own copies of the source code. Much of this work was shared among sites, and important changes found their way back into the core system. Other users, such as [[National CSS]] and some academic sites, continued independent development of CP/CMS, rather than switching to VM/370 when it became available. These efforts diverged from the community, in what today would be termed a [[fork (software development)|software fork]].

After IBM released VM/370, source code distribution of VM continued for several releases. (The VM project did not adopt the use of [[IBM PL/S|PL/S]], an internal systems programming language mandated for use within IBM on many comparable projects. The use of PL/S would have made source code distribution impossible. IBM attempted to turn away from assembly language to higher level languages as early as 1965, and was making substantial use of [[IBM PL/S|PL/S]] by 1969, e.g. in MVS. PL/S was considered a trade secret at the time and was not available to customers. IBM apparently made exceptions to this policy much later.<ref>Pugh ''et al., p.'' 737, note 166 &ndash; citing the C. H. Reynolds August 1965 ''PL/I policy statement'' mandating use of PL/I</ref><ref>W. R. Brittenham, ''The development of PL/S'', IBM Technical Report (1974) &ndash; cited in Pugh</ref>) The VM user community continued to make important contributions to the software, as it had during the CP/CMS Type-III period. Few OS or DOS sites exhibited active user involvement in deep operating system internals, but this was found at many VM sites. This reverse support helped CP/CMS concepts survive and evolve, despite VM's second class citizen status at IBM.