# Time-sharing system evolution

This article covers the evolution of time-sharing systems, providing links to major early time-sharing operating systems, showing their subsequent evolution.
The meaning of the term time-sharing has shifted from its original usage. From 1949 to 1960, time-sharing was used to refer to multiprogramming; it evolved to mean multi-user interactive computing.

## Time-sharing

Time-sharing was first proposed in the mid- to late-1950s and first implemented in the early 1960s. The concept was born out of the realization that a single expensive computer could be efficiently utilized by enabling multiprogramming, and, later, by allowing multiple users simultaneous interactive access. In 1984, Christopher Strachey wrote he considered the change in the meaning of the term time-sharing to be a source of confusion and not what he meant when he wrote his original paper in 1959.
Without time-sharing, an individual user would enter bursts of information followed by long pauses; but with a group of users working at the same time, the pauses of one user would be filled by the activity of the others. Similarly, small slices of time spent waiting for disk, tape, or network input could be granted to other users. Given an optimal group size, the overall process could be very efficient.
Each user would use their own computer terminal, initially electromechanical teleprinters such as the Teletype Model 33 ASR or the Friden Flexowriter; from about 1970 these were progressively superseded by CRT-based units such as the DEC VT05, Datapoint 2200 and Lear Siegler ADM-3A.
Terminals were initially linked to a nearby computer via current loop or serial cables, by conventional telegraph circuits provided by PTTs and over specialist digital leased lines such T1. Modems such as the Bell 103 and successors, allowed remote and higher-speed use over the analogue voice telephone network.

## Family tree of major systems

See details and additional systems in the table below. Relationships shown here are for the purpose of grouping entries and do not reflect all influences. The Cambridge Multiple-Access System was the first time-sharing system developed outside the United States.
Family tree of major time-sharing operating system families Influences:     ⇶ derivation     ⇉ strong influence     → some influence/precedence :
| CTSS → |
| CTSS → | •IBM mainframes: |
| CTSS → | CP-40/CMS ⇶ CP[-67]/CMS ⇶ | VM/370 ⇶ VM/SE versions ⇶ VM/SP versions ⇶ VM/XA versions ⇶ VM/ESA ⇶ z/VM |
| CTSS → | CP-40/CMS ⇶ CP[-67]/CMS ⇶ | VP/CSS |
| CTSS → | TSS/360 |
| CTSS → | OS/360 MVT-TSO ⇶ OS/VS2 SVS-TSO ⇶ MVS-TSO ⇶ OS/390-TSO ⇶ z/OS-TSO |
| CTSS → | Transactional systems: CICS, TPF ⇶ z/TPF |
| CTSS → | •IBM mainframes with non-IBM operating systems: |
| CTSS → | Michigan Terminal System (MTS) |
| CTSS → | MUSIC/SP |
| CTSS → | ORVYL |
| CTSS → | •DTSS/Multics family: |
| CTSS → | Dartmouth Time-Sharing System (DTSS) |
| CTSS → | MULTICS → UNIX family — see also UNIX MULTICS → PRIMOS → Domain/OS MULTICS ⇉ Stratus VOS |
| CTSS → | •DEC systems: |
| CTSS → | BBN Time-Sharing System |
| CTSS → | TOPS-10 → TENEX ⇉ TOPS-20 |
| CTSS → | RSTS/E |
| CTSS → | RSX-11M ⇉ VMS ⇶ MICA |
| CTSS → | Incompatible Timesharing System (ITS) |
| CTSS → | •UNIX: |
| CTSS → | UNIX family ⇉ Linux |

## System descriptions and relationships

Important time-sharing systems, 1960–1990 (and successors); listed alphabetically :
| Influences: ⇶ derivation ⇉ strong influence → some influence/precedence ||||||
| System | Platform | Dates in use | Developer | Description | Influences: from→ →to |
| ACP | S/360 and S/370 | 1965–1979 | IBM | High-performance mainframe transaction platform used in SABRE and PARS | ⇶ TPF ⇶ z/TPF |
| APL ("A Programming Language," also "Iverson's Language") | System/360, others later | 1964–present | Kenneth Iverson | Mathematically oriented language and interactive environment, noted for incredible terseness and powerful set processing operators | |
| Berkeley Timesharing System | SDS 940 | 1964–1972 | Project Genie | Early general-purpose | ⇉ TENEX |
| Cambridge Multiple-Access System | Titan, the prototype Atlas 2 | 1967–1973 | University of Cambridge and Ferranti | Multiple Access System | Project MAC⇶ ⇶UNIX |
| CANDE | Burroughs Large Systems, Burroughs Medium Systems | 1965?–present | Burroughs | first IDE | (separate evolution) |
| CICS | S/3x0 | 1969–present | IBM | Ubiquitous mainframe transaction platform; often used with IBM 3270 terminals and COBOL | |
| CP-40/CMS | customized S/360-40 | 1967–1972? | IBM's Cambridge Scientific Center | First implementation of full virtualization | CTSS→ ⇶ CP-67 |
| CP-67/CMS | IBM System/360-67 | 1967–1975? | IBM's Cambridge Scientific Center | Influential precursor to IBM's VM series, widely distributed as open source | CP-40 ⇶ ⇶ VP/CSS ⇶ VM/370 ⇶ z/VM |
| CTSS ("Compatible Time Sharing System") | modified IBM 7090 and modified IBM 7094 | 1961–1973 | MIT Computation Center | First-generation "grandfather" of time-sharing systems | FMS→ ⇉ CP-40 ⇉ Multics ⇉ ITS → [numerous other systems] |
| DTSS ("Dartmouth Time Sharing System") | GE 200, GE 635, Honeywell 6000 series | 1963–1999 | Dartmouth College | Early time-sharing system running Dartmouth BASIC and other tools; became the first commercial time-sharing system through General Electric Information Services Division | FMS→ ⇉ CP-40 ⇉ Multics ⇉ ITS → [numerous other systems] |
| ITS ("Incompatible Timesharing System") | PDP-6, PDP-10 | 1967–1990 | MIT Artificial Intelligence Laboratory | "Subversive" operating system developed to counter direction of CTSS. The original platform for Macsyma, EMACS and other important applications. | CTSS→ → [numerous later systems] |
| JOSS ("JOHNNIAC Open Shop System") | JOHNNIAC, PDP-6 | 1963–1971? | RAND Corporation | Lightweight, interactive computing language for non-specialists; did not distinguish operating system from language | Highly regarded, but no obvious successors |
| Linux | ubiquitous | 1991–present | Linus Torvalds, GNU project, open source | Operating system dominating current open source activities | UNIX ⇉ minix ⇉ → [numerous other systems] |
| Microsoft Windows (Remote Desktop Services) | x86, IA-64, others | 1985–present | Microsoft | Ubiquitous GUI operating system | MS-DOS ⇉ OS/2 ⇉ VMS ⇉ MICA ⇉ Smalltalk ⇉ |
| MTS (Michigan Terminal System) | IBM S/360-67, S/370 | 1967–1999 | University of Michigan and 7 other universities | First (Nov. 1967) OS to use the virtual memory features of the S/360-67. Early (Sept. 1968) S/360-67 multiprocessor support. | CTSS→ DTSS→ → UNIX (BSD) |
| Multics | GE 645, Honeywell 6180 | 1969–2000 | Project MAC | Rich, important system | CTSS ⇉ ⇉ UNIX ⇉ [many other systems] |
| MVS/TSO | System/370 and successors | 1971–present | IBM | Probably the most widely used version of TSO, extended version TSO/E, current version zOS-TSO | CTSS→ TSS/360→ ⇶ z/OS-TSO |
| NOS | CDC 60-bit platforms | 1976-?? | Control Data Corporation | System used on most CDC machines | MACE→⇶ Kronos ⇉ |
| NOS/BE | CDC 60-bit platforms | 1976-?? | Control Data Corporation | System used on most CDC machines | COS ⇶ SCOPE →⇶ |
| ORVYL | IBM System/360 Model 67, IBM System/370, and successors | 1967-?? | Stanford University | Early time-sharing system; source of the WYLBUR editor later used on System/370 platforms | |
| OS/2 (as Citrix Multiuser) | x86 | 1987–present | IBM/Microsoft | Joint OS effort, now moribund. Still available as eComStation and ArcaOS. | DOS ⇶ Microsoft Windows ⇉ OS/2 ⇶ eComStation ⇶ ArcaOS 5.0 |
| ROSCOE | System/360 and successors | 1969–present | Applied Data Research (ADR) | Early time-sharing editor environment, often used as an alternative to TSO | ⇶ WRAP |
| RSTS/E | PDP-11 | 1972–1992+ | DEC | General-purpose time-sharing for the PDP-11 | |
| RSX-11 | PDP-11 | 1972-?? | DEC | Real-time operating system for the PDP-11 | ⇶ IAS ⇉ VMS |
| Smalltalk | Xerox Alto, later made portable | 1972–present | Xerox PARC, successors | Seminal system for experimental programming, responsible for many modern user interface concepts | ⇉ Apple Lisa ⇉ Apple Macintosh ⇉ Microsoft Windows ⇉ [all GUI platforms] |
| Stratus VOS | i860, x86, PA-RISC, 68k | 1980?–present | Stratus Technologies | High-availability fault-tolerant transaction processing | MULTICS ⇉ |
| TENEX | PDP-10 | 1970?-?? | Bolt Beranek and Newman | Influential system widely used at research and government sites | ⇉ TOPS-20 ⇉ VMS |
| TOPS-10 | PDP-10 | 1970–1988? (as TOPS-10) 1964–1970 (as PDP-6 Monitor) | DEC | Widely used at research and academic sites | PDP-6 Monitor ⇶ → TENEX ⇉ CP/M |
| TOPS-20 | DECsystem 20 | 1976-?? | DEC | Successor to TOPS-10 but more like TENEX | TENEX→ TOPS-10→ |
| TPF | S/3x0 | 1979–present (TPF) 2005–present(z/TPF) | IBM | High-performance mainframe transaction platform, successor to ACP, still available as z/TPF | ACP ⇶ ⇶ z/TPF |
| TSOS | RCA Spectra 70 and successors | 1968-today | RCA | Early general purpose mainframe OS | |
| TSS-8 | PDP-8 | 1967–?? | DEC | Simple minicomputer OS | → RSTS/E |
| TSS/360 TSS/370 | IBM System/360-67 and successors | 1967–1971? | IBM | IBM's original "official" time-sharing system; not a success | CTSS→ ⇶ TSS/370 |
| Unisys/UNIVAC EXEC 8 | UNIVAC 1108 and successors | 1964–present | Sperry-Rand et al. | Many universities and government agencies were early users | EXEC 8 ⇶ OS 1100 ⇶ OS 2200 |
| UNIX and derivative systems | ubiquitous | 1969–present | Bell Laboratories and successors | Ultimately dominated operating system thought, in both proprietary and open-source descendants | Multics ⇉ ⇉ Linux |
| VM/370 VM/SE VM/SP | System/370 and successors | 1972–1988 2000–present (z/VM) | IBM | Proprietary reimplementation of CP/CMS, still available as z/VM | CP-40 ⇶ CP-67 ⇶ ⇶ VM/ESA ⇶ z/VM |
| OpenVMS | VAX, IA-64, DEC Alpha, x86-64 | 1977–present | DEC | Popular DEC operating system | TENEX→ RSX-11M ⇉ ⇉ MICA ⇉ Windows NT |
| VP/CSS | IBM System/360-67, System/370 and successors | 1968–1986? | National CSS | Proprietary fork of CP/CMS developed by a time-sharing vendor | CP/CMS ⇶ |
| WYLBUR | System/370 and successors | 1967–2009? | Stanford University | Popular editor system originally from ORVYL, used under OS/VS as an alternative to TSO | ⇶ SuperWylbur |