Compatible Time-Sharing System
The Compatible Time-Sharing System (CTSS) was the first general purpose time-sharing operating system. Compatible Time Sharing referred to time sharing which was compatible with batch processing; it could offer both time sharing and batch processing concurrently.

CTSS was developed at the MIT Computation Center ("Comp Center"). CTSS was first demonstrated on MIT's IBM 709 (the "blue machine") in November 1961. Routine service to MIT Comp Center users began in the summer of 1963 and was operated there until 1968.

A second deployment of CTSS on a separate IBM 7094 that was received in October 1963 (the "red machine") was used early on in Project MAC until 1969 when the red machine was moved to the Information Processing Center and operated until July 20, 1973. CTSS ran on only those two machines however there were remote CTSS users outside of MIT including ones at the University of Edinburgh and the University of Oxford.

History
John Backus said in the 1954 summer session at MIT that "By time sharing, a big computer could be used as several small ones; there would need to be a reading station for each user". Computers at that time, like IBM 704, were not powerful enough to implement such system, but at the end of 1958, MIT's Computation Center nevertheless added a typewriter input to its 704 with the intent that a programmer or operator could "obtain additional answers from the machine on a time-sharing basis with other programs using the machine simultaneously".

In June 1959, Christopher Strachey published a paper "Time Sharing in Large Fast Computers" at the UNESCO Information Processing Conference in Paris, where he envisaged a programmer debugging a program at a console (like a teletype) connected to the computer, while another program was running in the computer at the same time. Debugging programs was an important problem at that time, because with batch processing, it then often took a day from submitting a changed code, to getting the results. John McCarthy wrote a memo about that at MIT, after which a preliminary study committee and a working committee were established at MIT, to develop time sharing. The committees envisaged many users using the computer at the same time, decided the details of implementing such system at MIT, and started the development of the system.

Experimental Time Sharing System
By July, 1961 a few time sharing commands had become operational on the Computation Center's IBM 709, and in November 1961, Fernando J. Corbató demonstrated at MIT what was called the Experimental Time-Sharing System. On May 3, 1962, F. J. Corbató, M. M. Daggett and R. C. Daley published a paper about that system at the Spring Joint Computer Conference. Robert C. Daley, Peter R. Bos and at least 6 other programmers implemented the operating system, partly based on the Fortran Monitor System. 

The system used an IBM 7090, modified by Herbert M. Teager, with added 3 Flexowriters for user consoles, and maybe a timer. Each of the 3 users had two tape units, one for the user's file directory, and one for dumping the core (program in memory). There was also one tape unit for the system commands, there were no disk drives. The memory was 27 k words (36-bit words) for users, and 5 k words for the supervisor (operating system). The input from the consoles was written to the buffers in the supervisor, by interrupts, and when a return character was received, the control was given to the supervisor, which dumped the running code to the tape and decided what to run next. The console commands implemented at the time were login, logout, input, edit, fap, mad, madtrn, load, use, start, skippm, listf, printf, xdump and xundump.

This became the initial version of the Compatible Time-Sharing System. This was apparently the first ever public demonstration of time-sharing; there are other claims, but they refer to special-purpose systems, or with no known papers published. The "compatibility" of CTSS was with background jobs run on the same computer, which generally used more of the compute resources than the time-sharing functions.

Features
 The original ELIZA ran on CTSS.
 CTSS was the first computer system to implement password login.
 CTSS had one of the first computerized text formatting utilities, called RUNOFF (the successor of DITTO).
 CTSS had one of the first inter-user messaging implementations, possibly inventing email.
 CTSS had one of the first instant messaging systems similar to write.
 MIT Computation Center staff member Louis Pouzin created for CTSS a command called RUNCOM, which executed a list of commands contained in a file. (He later created a design for the Multics shell that was implemented by Glenda Schroeder which in turn inspired Unix shell scripts.) RUNCOM also allowed parameter substitution.
 CTSS had an implementation of the text editor QED, the predecessor of ed, vi, and vim, with regular expressions added by Ken Thompson.

Implementation

Kernel
CTSS used a modified IBM 7090 mainframe computer that had two 32,768 (32K) 36-bit-word banks of core memory instead of the normal one.  One bank was reserved for the time-sharing supervisory program, the other for user programs. CTSS had a protected-mode kernel, the supervisor's functions in the A-core (memory bank A) could be called only by software interrupts, like in the modern operating systems. Causing memory-protection interrupts were used for software interrupts. Processor allocation scheduling with a quantum time unit 200 ms, was controlled by a multilevel feedback queue. It also had some special memory-management hardware, a clock interrupt and the ability to trap certain instructions.

Supervisor subroutines
 RDFLXA – Read an input line from console
 WRFLX – Write an output line to console
 DEAD – Put the user into dead status, with no program in memory
 DORMNT – Put the user into dormant status, with program in memory
 GETMEM – Get the size of the memory allocation
 SETMEM – Set the size of the memory allocation
 TSSFIL – Get access to the CTSS system files on the disk
 USRFIL – Change back to user's own directory
 GETBRK – Get the instruction location counter at quit

Programming languages
CTSS at first had only an assembler FAP and a compiler MAD. Also, Fortran II code could be translated into a MAD code. Later half of the system was written in MAD. Later there were other programming languages like LISP and a version of ALGOL.

File system
Each user had their own directory, and there were also shared directories for groups of people with the same "problem number". Each file had two names, the second indicating its type as did the extension in later system. At first, each file could have one of four modes: temporary, permanent, read-only class 1, and read-only class 2. Read-only class 1 allowed the user to change the mode of the file. Files could also be symbolically linked between directories. A directory listing by listf:
 	10 FILES	20 TRACKS USED
 DATE		NAME		MODE	NO. TRACKS
 5/20/63		MAIN	MAD	P	15
 5/17/63 	DPFA	SYMTB	P	1
 5/17/63 	DPFA	BSS	P	1
 5/17/63 	DPFA	FAP	P	2

Peripherals
Input-output hardware was mostly standard IBM peripherals. These included six data channels connecting to:
 Printers, punched card readers and punches
 IBM 729 tape drives, an IBM 1301 disk storage, later upgraded to an IBM 1302, with 38 million word capacity
 An IBM 7320 drum memory with 186K words that could load a 32K-word memory bank in one second (later upgraded to 0.25 seconds)
 Two custom high-speed vector graphics displays
 An IBM 7750 transmission control unit capable of supporting up to 112 teleprinter terminals, including IBM 1050 Selectrics and Model 35s. Some of the terminals were located remotely, and the system could be accessed using the public Telex and TWX networks.

Influences
CTSS was described in a paper presented at the 1962 Spring Joint Computer Conference, and greatly influenced the design of other early time-sharing systems.

Maurice Wilkes witnessed CTSS and the design of the Titan Supervisor was inspired by that.

Dennis Ritchie wrote in 1977 that UNIX could be seen as a "modern implementation" of CTSS.  Multics, which was also developed by Project MAC, was started in the 1960s as a successor to CTSS – and in turn inspired the development of Unix in 1969. One of the technical terms inherited by these systems from CTSS is daemon.

Incompatible Timesharing System (ITS), another early, revolutionary, and influential MIT time-sharing system, was produced by people who disagreed with the direction taken by CTSS, and later, Multics; the name was a parody of "CTSS", as later the name "Unix" was a parody of "Multics".

See also
 Timeline of operating systems
 Time-sharing system evolution

References

Further reading

External links
 Oral history interview with John McCarthy, Charles Babbage Institute, University of Minnesota.  Discusses computer developments at MIT including time sharing.
 Oral history interview with Fernando J. Corbató, Charles Babbage Institute, University of Minnesota.  Discusses many computer developments at MIT including CTSS.
 Oral history interview with Robert M. Fano, Charles Babbage Institute, University of Minnesota.  Discusses computer developments at MIT including CTSS.
 The IBM 7094 and CTSS: personal memoir of Tom Van Vleck, a system programmer on CTSS
 CTSS Source version MIT8C0 in Paul Pierce's collection.
 Dave Pitts' IBM 7094 support – Includes a license-free simulator, cross assembler and linker that can be used to build and run CTSS.
 CTSS sources and binaries, a more complete kit which runs on SIMH.
 CIO: 40 years of Multics, 1969-2009: Interview with CTSS and Multics developer Fernando J. Corbato.
 Jerome Saltzer's CTSS bookshelf via CSAIL.

1960s software
1970s software
Discontinued operating systems
Massachusetts Institute of Technology software
Time-sharing operating systems