accessing the centrally managed file *MESSAGES that was used by the MTS $Messagesystem to hold mailboxes and messages for individual users. The addition of file locking allowed removal of the restriction that a single userID could only be signed on once. Instead, the number of simultaneous sign ons was controlled by a maximum that could be set in the user's accounting record by the project manager or a site's business office. File save and restore Files are regularly backed up to tape unless they have been marked as NOSAVE. The file save process includes full and partial backups. Full saves are typically done once a week with no users signed on to the system. Partial saves save just the files that have changed since the last full or partial save and are typically done once each day in the late evening or early morning during normal operation with users signed on to the system. At the University of Michigan two copies of the full save tapes were made and one copy was stored "off-site". Save tapes were kept for six weeks and then reused. The tapes from every sixth full save were kept "forever". Files are saved to allow recovery from "disk disasters" in which the file system becomes damaged or corrupt, usually due to a hardware failure. But users could restore individual files as well using the program *RESTORE. Terminal support At its peak, MTS at the University of Michigan simultaneously supported more than 600 terminal sessions as well as several batch jobs. Terminals are attached to MTS over dial-in modems, leased or dedicated data circuits, and network connections. The Michigan Communications Protocol (MCP), a simple framing protocol for use with asynchronous connections that provides error detection and retransmission, was developed to improve the reliability of terminal to MTS and computer to MTS connections. A very wide range of terminals are supported including the 10 character per second (cps) Teletype Model 33, the 30 cps LA-36 and 120 cps LA-120 DECWriter, the 14 cps IBM 2741, and at ever increasing speeds up to 56,000 bits per second, the VT100 display, the Visual 550 display, the Ontel OP-1 and OP-1/R displays, Tektronix 4000 series of graphic displays, and personal computers from Apple (AMIE for the Apple ][), IBM (PCTie for DOS), and others running terminal emulation programs, including some specifically developed for use with MTS. Most terminals that are compatible