VSEn (Virtual Storage Extended) is an operating system for IBM mainframe computers, the latest one in the DOS/360 lineage, which originated in 1965. DOS/VSE was introduced in 1979 as a successor to DOS/VS; in turn, DOS/VSE was succeeded by VSE/SP version 1 in 1983, and VSE/SP version 2 in 1985. In February 2005, IBM announced z/VSE as successor to VSE/ESA 2.7, which was named to reflect the new System z branding for IBM's mainframe product line. In June 2021, 21st Century Software Inc announced that it had licensed the z/VSE source code from IBM with the intention of developing new versions of the operating system. As part of this transfer, z/VSE was renamed to VSEn. It is less common than z/OS and is mostly used on smaller machines. In the late 1980s, there was a widespread perception among VSE customers that IBM was planning to discontinue VSE and migrate its customers to MVS instead, although IBM relented and agreed to continue to produce new versions of VSE. Overview DOS/360 originally used 24-bit addressing. As the underlying hardware evolved, VSE/ESA acquired 31-bit addressing capability. IBM released z/VSE Version 4, which requires 64-bit z/Architecture hardware and can use 64-bit real mode addressing, in 2007. With z/VSE 5.1 (available since 2011) z/VSE introduced 64-bit virtual addressing and memory objects (chunks of virtual storage), that are allocated above 2 GB. User interfaces Job Control Language (JCL) A Job Control Language (JCL) that continues in the positional-parameter orientation of earlier DOS systems is VSEn's batch processing primary user interface. There is also another, special interface for system console operators. Beyond batch VSEn, like z/OS systems, had traditionally provided 3270 terminal user interfaces. However, most VSEn installations have at least begun to add Web browser access to VSEn applications. VSEn's TCP/IP is a separately priced option for historic reasons, and is available in two different versions from two vendors. Both vendors provide a full function TCP/IP stack with applications, such as telnet and FTP. One TCP/IP stack provides IPv4 communication only, the other IPv4 and IPv6 communication. In addition to the commercially available TCP/IP stacks for VSEn, IBM also provides the Linux Fastpath method which uses IUCV socket or Hipersockets connections to communicate with a Linux guest, also running on the mainframe. Using this method the VSEn system is able to fully exploit the native Linux TCP/IP stack. IBM recommends that z/VSE customers run Linux on