IBM announced its System/370 series in 1970 without the virtual memory feature needed for virtualization, but added it in the August 1972 Advanced Function announcement. Virtualization has been featured in all successor systems (all modern-day IBM mainframes, such as the zSeries line, retain backward compatibility with the 1960s-era IBM S/360 line). The 1972 announcement also included VM/370, a reimplementation of CP/CMS for the S/370. Unlike CP/CMS, IBM provided support for this version (though it was still distributed in source code form for several releases). VM stands for "Virtual Machine", emphasizing that all, and not just some, of the hardware interfaces are virtualized. Both VM and CP/CMS enjoyed early acceptance and rapid development by universities, corporate users, and time-sharing vendors, as well as within IBM. Users played an active role in ongoing development, anticipating trends seen in modern open source projects. However, in a series of disputed and bitter battles, time-sharing lost out to batch processing through IBM political infighting, and VM remained IBM's "other" mainframe operating system for decades, losing to MVS. It enjoyed a resurgence of popularity and support from 2000 as the z/VM product, for example as the platform for Linux on IBM Z.