Compatible Time-Sharing System

By July, 1961[10] 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.[11] 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.

The first version of the DOTSYS braille translation software ran on CTSS and could output to a BRAILLEMBOSS braille page printer.[12][13] DOTSYS on CTSS was first demonstrated on August 18, 1966 as part of a feasibility study where teletypesetter tape, in the form of news, was converted to Grade 2 Braille. The following month the feasibility of converting textbook information on teletypesetter tape to error-free Grade 2 Braille was successfully demonstrated.[14] As MIT CTSS was an academic system, a research vehicle and not a system for commercial computing, two years later a version of DOTSYS stripped of CTSS dependencies for software portability was used on an IBM 709 at the American Printing House for the Blind to print the first braille edition of a book produced from teletypesetter input, only a few weeks after the ink-print version.[15] The following year, on CTSS, a demonstration of printing mathematical tables in braille was shown. A short FORTRAN II program was written to produce a conversion table from inches to millimeters in braille via the BRAILLEMBOSS braille page printer.[16]

Diagram of the Intrex running on CTSS

The Intrex Retrieval System ran on CTSS. Intrex was an experimental, pilot-model machine-oriented bibliographic storage and retrieval system with a database that stored a catalog of roughly 15,000 journal articles. It was used to develop and test concepts for library automation.[17][18][19] A deployment of three BRISC CRT consoles for testing at the MIT Engineering Library showed that it was preferred over two other systems, ARDS and DATEL.[20]

CTSS used a modified IBM 7090 mainframe computer that had two 32,768 (32K) 36-bit-word banks of core memory instead of the default configuration which provides only one.[27] 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.[8] Processor allocation scheduling with a quantum time unit 200 ms, was controlled by a multilevel feedback queue.[27] It also had some special memory-management hardware, a clock interrupt and the ability to trap certain instructions.

• 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

CTSS at first had only an assembler, FAP, and a compiler, MAD. Also, Fortran II code could be translated into MAD code by using MADTRN. Later half of the system was written in MAD. Later there were other programming languages including COMIT II, LISP 1.5 and a version of ALGOL.

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

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[28]
• 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.