a full, stand-alone computer for each user, CP/CMS could run any S/360 software in a time-sharing environment, not just applications specifically designed for time-sharing. By using lightweight CMS as the primary user interface, CP/CMS achieved unprecedented time-sharing performance. In addition, the simplicity of CMS made it easier to implement user interface enhancements than in traditional OS. IBM reimplemented CP/CMS as its VM/370 product line, released in 1972 when virtual memory was added to the S/370 series. VM/370's successors (such as z/VM) remain in wide use today. (It is important to note that IBM reimplemented CP-67, as it had CP-40, and did not simply rename and repackage it. VM coexisted with CP/CMS and its successors for many years. It is thus appropriate to view CP/CMS as an independent OS, distinct from the VM family.) 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. IBM provided optional source code for, e.g., OS/360, DOS/360, several later mainstream IBM operating systems. With all these systems, some awareness of system source code was also involved in the 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 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 (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 contributed by IBM personnel (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