used to run any S/360 or S/370 system, but in practice the virtual machines were only used to debug MTS and so there may be subtle features that are not used by MTS that are not completely or correctly implemented. The MTS virtual machine was never updated to support the S/370-XA architecture (instead other tools such as SWAT and PEEK were used to debug MTS and IBM's VM/XA or VM/ESA were used to debug UMMPS). In the early 1970s work was done at Wayne State University to run a version of OS/MVT in a modified virtual machine (VOS) under MTS as a production service. "Student" virtual machines in MTS have also been created as teaching tools. Here the OS running in the virtual machine (written by the student) uses simulated devices and has no connection to the "real" outside world at all (except possibly a console). In addition to virtual machines, MTS provides two programs that implement virtual operating system environments. *FAKEOS, developed at the University of Michigan, allows programs from OS/360 to run as user programs in MTS. *VSS, developed at the University of British Columbia, allows programs from OS/VS1 and MVS/370 to run as user programs in MTS. Neither program actually runs the IBM operating system, instead they simulate enough of the operating environment to allow individual programs developed for those operating systems to run. Both programs can be run directly, but often they are run from driver files that give an end user the impression that they are running a regular MTS user program. Electronic mail At least three different implementations of e-mail were available under MTS at different times: *MAIL from NUMAC, but not available at all MTS sites; CONFER, the computer conferencing system written by Robert Parnes at UM; and $MESSAGESYSTEM from the University of Michigan Computing Center.MTS Volume 23: Messaging and Conferencing in MTS, University of Michigan Computing Center, Ann Arbor, Michigan CONFER and *MAIL only sent and received mail to and from "local" users. Available to users in July 1981, $MESSAGESYSTEM is the last of the three systems to be implemented and became the most widely used. Between 1981 and 1993 it was used to send and receive more than 18 million messages at the University of Michigan. It can send: local and network e-mail messages, dispatches (immediate messages displayed at another user's terminal unless dispatches were blocked by the other user), bulletins