M.I.T. Computation Center

“ In addition to allowing the development of usable and sophisticated debugging techniques, an efficient time-sharing system should make feasible a number of relatively new computer applications which can be implemented only at great cost in a conventional system. Any problem requiring a high degree of intermixture of computation and communication on a real-time basis should readily lend itself to time-sharing techniques. ”

“ A user has written a subprogram in a compiler language and wishes to incorporate it into his set of programs already developed and kept in the central disk file. After sitting down at a console, he first gives a login command to identify himself. His next command, input, turns his console into a pseudo-keypunch. Using simple conventions, he types in his subprogram for storage with other subprograms on the disk. If he should note any typing or logic errors in his new file, now or at any later time, he can correct the file using an edit command. ”

“ Because CTSS is basically a system which will allow an evolutionary development of time-sharing while continuing to allow more conventional background systems to operate, it is expected that the present manual will of necessity be revised many times before it reaches a final form. A good deal of the difficulty arises from, on the one hand, the rather drastic change in user operating techniques which time-sharing permits, and on the other hand the immense amount of programming required to fully implement the system. ”

“ In the current implementation of the algorithm a user-computer interaction consists of the period from an input message to an output message. Although the response time is only a few milliseconds if the user's program is already in core, whenever there is more than one user the usual minimum time for a response, which is the minimum computer usage time per response, is (in the current drumless system) approximately seconds plus the response computation time, where is the number of program words in thousands. ”

“ The devices (e.g., a PDP-1 computer or a 1620 computer) will appear as input-output units, and a user, from his console, may request to have them logically attached to his program. For convenience each device can be considered by the programmer to consist of several logical subunits; in particular, for control purposes one of these subunits (e.g., the typewriter keyboard) can represent a user console, so that an auxiliary console is not needed. ”

“ When the supervisor determines that the background system is to be saved while a foreground program receives attention, the background system's machine conditions must be saved. In order to save the status of an I/O channel, the supervisor must wait for current I/O activity to stop. If the background system reads or writes long records on tape, the time spent waiting in the supervisor can be detrimental to the foreground response time. ”

“ All users and systems use a single standard supervisor subroutine for reading and writing the disk. Because of the input-output trapping and memory protection features, mishaps will not occur if the user executes wild transfers, tries to store spurious information in the subroutine, or tries to execute explicit instructions for writing the disk. ”

“ Core memory is considered to be divided into 256-word blocks. There are two 7-bit protection registers which, when the computer is in the normal mode, can be set by program to any block numbers. Whenever a user program is run, the supervisor, as a final step just before transferring to the user program, switches the computer to a special mode such that if execution of any memory address outside the range of the protection register block numbers is attempted, the normal mode is restored and a trap occurs to the supervisor. ”

“ Assignment of Input-Output UnitsAssignment of Input-Output Units There are numerous devices such as tape units, scopes, plotters, satellite computers, etc., which are of such a nature that it is desirable to assign them to a single user for his session at his console. This assignment is done by means of an attach command, where the user may receive a busy signal whenever requested units are actively engaged. Addressing conventions of input-output units will always be by logical unit numbers except when location is relevant. ”

“ In this mode, every input message (i.e., up to the break character) will immediately be copied back as an output message. If a user detects a garble he should issue a quit signal or take whatever action he has anticipated in his program.
When a person is working closely with his console at the command level, it will be possible to enter a brief mode. All programs which have been properly prepared will be able to interrogate the brief mode status indicator and to adjust the verbosity of their output accordingly. Both the brief and confirmation modes are set by the use of a command. ”

“ Each active user ceases to run, a cutoff message is sent to his console, all files are closed, all attached input-output units are disconnected, the user's program is created as a file of name cutoff with class saved, the user is given a logout command, and a message is placed in the user's console message file on the disk giving a summary of the cutoff process including the unread characters in the console input buffer. ”

“ These messages are placed in an input message pool for the receiving user along with the user number of the sender. The receiving user program can read its message pool at any time by a supervisor call similar to that for reading its own input console; an input-wait status can occur if no messages are present. If a receiver fails to read or acknowledge a message this is assumed to be intentional. The user number of another console must be determined by supervisor subroutine calls which give the desired user number on the basis of the console location, problem number and/or programmer number. ”

“ The job first processes control card files left on the disk by the time-sharing supervisor, and then the input tape control cards, before producing the requested dump tapes. In addition, the current history tapes are extended by the over-age files and any files which the user has asked to be deleted; corresponding file directory entries are marked along with the date dumped and the history tape position. Finally, the dump program produces an output tape for the 1401 of output requested by the dump control cards, both those submitted directly and those generated from consoles. ”

“ Besides easing the programming transition problems of changing from batch processing to time-sharing techniques, this compatibility feature eliminates the necessity for immediately satisfying all of the Center's 400 research users (and a similar number of student users) with the initial version of the time-sharing system.
To achieve programming compatibility, the time-sharing system uses the same programming languages normally available in the major Center system, FMS: Fap, Mad, and Fortran in the form of the nearly equivalent Madtran (memo CC-188) . ”

“ Keypunching and Card Input to the DiskKeypunching and Card Input to the Disk Normally, it is expected that casual program writing, editing and data generation will be done directly from the user's console. Even for the case of long programs which are too tedious for the programmer to type, a trained typist can still type the program directly into the disk memory. However, for various reasons such as distance, convenience, or exchange with other computer centers, it is still sometimes necessary to input punched cards. ”

“ It must then determine whether the program or programs currently in core must be dumped (to disk or drum) , in part or entirely, to leave room in core for the next user. The next user must then be retrieved from secondary storage together with the proper machine conditions.
In addition to maintaining input and output buffers for each user console, the supervisor keeps a record of the status of each user. The status of a user may be: "working," where a program is ready to continue running whenever it is next brought in ”

“ The supervisor will maintain for each user an input-output assignment table in which the logical-physical unit number interconnection will be kept. When a user gives a logout command at his console at the end of his session, all assigned units will be disconnected.
Magnetic Tape UsageMagnetic Tape Usage Although for a large number of cases the disk memory, because of its quasi-random access, is more appropriate for program data and temporary storage requirements, there are still many problems such as payrolls, bubble chamber scanner output, etc., where magnetic tapes are more appropriate. ”

“ The SupervisorThe Supervisor The supervisor program remains in A-core at all times when CTSS is in operation. Its functions include: handling of all input and output for the consoles; scheduling of console-initiated (foreground) and offline-initiated (background) jobs; temporary storage and recovery of programs during the scheduled swapping; monitoring of all input and output from the disk, as well as input and output performed by the background system; and performing the general role of monitor for all foreground jobs. ”

“ Background System RestrictionsBackground System Restrictions The normal user of CTSS will not need to concern himself with the following section which is primarily of interest to those programmers dealing with older pre-time-sharing programs.
Certain programming conventions or restrictions must be followed in programs running as background jobs in CTSS. These restrictions are principally dictated by the need periodically to interrupt all programs operating in CTSS. Normally, well-programmed systems will already satisfy the requirements. ”

“ Disk Editing ProceduresDisk Editing Procedures Despite the rather large capacity of the IBM 1301 disk modules for secondary storage they are still capable of being filled by various means, from runaway programs to careless user file "housecleaning." To minimize the latter effect, each user is assigned an initial but usually adequate track quota, which can only be extended by a formal request to the Center. ”

“ When using the IBM 1014 Selectric typewriter consoles, the quit signal is generated by the sequence of: inquiry request, "#," inquiry release. To initiate each line of input, the inquiry request key must be depressed. ”

“ If a programmer desires to interact more frequently with input messages (including character-by-character reading) , any arbitrary break character may be added to and deleted from his break character list by means of subroutine calls to the supervisor. The programmer, however, should anticipate response time delays and the extra computer time usage for each interaction.
Quit Signals and Console Input LevelsQuit Signals and Console Input Levels When a user issues a command, two actions are initiated. First, his console input level is logically dropped from level 0 (command level) to level 1 ”

“ A diagnostic will occur if an LCHX instruction is used. A "Reset and Load Channel" instruction, if given, must immediately follow the select instruction; otherwise, the I/O check light will be turned on. An exception is made for three instructions; up to three SPR's, or SPU's, and/or NOP's may be inserted between the select and reset and load instructions. ”

“ This will automatically be done on assembly if the BCORE pseudo-op is used in the Center version of the Fap assembler. I/O instructions, if generated, must also contain this bit. A diagnostic will be given if this condition is not met. ”