Although many time-sharing services simply closed, Rapidata held on, and became part of National Data Corporation. It was still of sufficient interest in 1982 to be the focus of "A User's Guide to Statistics Programs: The Rapidata Timesharing System". Even as revenue fell by 66% and National Data subsequently developed its own problems, attempts were made to keep this timesharing business going. UK Time Sharing Limited (TSL, 1969-1974) - launched using DEC systems. PERT was one of its popular offerings. TSL was acquired by ADP in 1974. OLS Computer Services (UK) Limited (1975-1980) - using HP & DEC systems. The computer utility Beginning in 1964, the Multics operating system was designed as a computing utility, modeled on the electrical or telephone utilities. In the 1970s, Ted Nelson's original "Xanadu" hypertext repository was envisioned as such a service. Security Time-sharing was the first time that multiple processes, owned by different users, were running on a single machine, and these processes could interfere with one another. For example, one process might alter shared resources which another process relied on, such as a variable stored in memory. When only one user was using the system, this would result in possibly wrong output - but with multiple users, this might mean that other users got to see information they were not meant to see. To prevent this from happening, an operating system needed to enforce a set of policies that determined which privileges each process had. For example, the operating system might deny access to a certain variable by a certain process. The first international conference on computer security in London in 1971 was primarily driven by the time-sharing industry and its customers. Time-sharing in the form of shell accounts has been considered a risk. Notable time-sharing systems Significant early timesharing systems: Allen-Babcock RUSH (Remote Users of Shared Hardware) Time-sharing System on IBM S/360 hardware (1966) → Tymshare AT&T Bell Labs Unix (1971) → UC Berkeley BSD Unix (1977) BBN PDP-1 Time-sharing System → Massachusetts General Hospital PDP-1D → MUMPS BBN TENEX → DEC TOPS-20, Foonly FOONEX, MAXC OS at PARC, Stanford Low Overhead TimeSharing (LOTS), which ran TOPS-20 Berkeley Timesharing System at UC Berkeley Project Genie → Scientific Data Systems SDS 940 (Tymshare, BBN, SRI, Community Memory) → BCC 500 → MAXC at PARC Burroughs Time-sharing MCP → HP 3000 MPE Cambridge Multiple Access System was developed for the Titan, the prototype Atlas