SITS

SITS means "Small Incompatible Timesharing System". It was a time-sharing operating system for a PDP-11/45, created in the mid-1970s at MIT for running Logo.

Overview

SITS' design is vaguely reminiscent of ITS, with large influences from MIT's PDP-1-X Time Sharing System. Like ITS, there is a main memory-resident kernel debugger; SALV for managing disks; TECO for text editing, and DDT for debugging and running user programs. Processes can be PCLSR'ed just like in ITS. From the PDP-1-X it inherited capability-based security and the concept of spheres. It has a hierarchical tree-structured file system inspired by Multics. SITS supported raster displays similar to those used with the MIT-AI Knight TV, a few vector displays, and General Turtle 2500 terminals. Programs can be stored in either absolute loader format, or pure dump format.

The kernel debugger was written by Radia Perlman and was called RUG from the phrase "snug as a bug in a rug". In addition to debugging, it can also read and write files in the SITS file system which is how SITS is booted. It can also read files from paper tapes, or from ITS through the Rubin 10-11 shared memory interface. The default file name is DAZZEL, perhaps a nod to the Dazzle Dart game that was hosted on the same machine.

AI memo 356 "Logo Progress Report 1973-1975" has this:

Originally Logo was implemented in assembly language on the PDP-10. In order to provide a computer system dedicated to educational use, it was adapted for the PDP-11. The first milestone in this direction was the completion in 1973-1974 of a dedicated timesharing system running 11LOGO.

The was not an entirely satisfactory solution because of the inability of the system to be self-maintaining or to run other languages or special purpose jobs (like a simulation environment or an educational real-time game). During 1974-1975, our programming staff, under the direction or R. Lebel, completed the design and implementation of a general purpose multi-language timesharing system for the PDP-11/45. The SITS timesharing system was developed to provide an environment suitable for running Logo and other PDP-11/45 programs. It incorporates a Multics-like tree structured file system including (potentially) full access control. It also provides unique capabilities for running programs as multiple process systems, rather than the more common single process approach, and the ability for each user to run many jobs simultaneously. The system include provisions for using both the older refreshed displays and our new raster displays.

When the SITS PDP-11/45 was decommissioned from the Logo group, it was donated to what became the Concourse Computer Center. It did not continue running SITS due to instability problems. Instead the computer ran UNIX V7, later 2BSD, and was placed on the Chaosnet.

Little SITS

To provide the educational market with an inexpensive computer capable of running Logo, it was ported to an LSI-11 based computer called the 3500. It was done using a system abstraction layer called the Little SITS emulator.

Preservation status

Several snapshots of SITS exist on ITS backup tapes from the AI Lab PDP-10.

• SITS 68 from 1973.
• SITS 455 and 456 from 1974.
• SITS version 1146 from 1975.
• NSITS 111 and 112 from 1975. Version number reset?
• SITS 103 from 1976.
• SITS 117, 118, and 119 from 1977.
• SITS 134 from 1978.
• SITS 136 from 1979.
• A set of disk images from May 1978 with an unknown SITS version.

Unsorted files from SITS, PDP-10 Logo, and PDP-11 Logo can be found here, courtesy of Ron Lebel: https://github.com/larsbrinkhoff/mit-logo-and-sits-raw-files

Hardware support

This is the list of supported hardware for SITS version 119:

Device	Description	Address	Vector
KB11-A CPU	CPU
KC11-C	MMU
Core memory	108Kword	000000-577777
FP11-B	Floating-Point Processor
KW11-P	Programmable clock	772540-772545	104
PC11	Paper tape reader/punch	777550-777557	70-74
DC11	Asynchronous serial line interface	774000-774007	300-304
DH11	Asynchronous serial line interface	760020-760037	340-344
RK11	Moving head disk	777400-777417	220

Optional hardware:

Device	Description	Address	Vector
RF11	Fixed-head disk	777460-777471	204
NG	Tom Knight's vector display controller	764040-764043	270
	Lebel keyboards	764100-764107
TV	Ron Lebel's raster display controller	764140-764163
	Frame buffer	600000-757777
MB11	MAR and history	770000-770017	364

See also

• Installing SITS on SIMH
• List of SITS system calls