SITS

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

Its design is vaguely reminiscent of ITS. Like ITS, there is a core-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.

SITS version 1146, and NSITS 111 and 112 have been preserved on a 1975 ITS backup tape from the AI Lab PDP-10.

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. 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.

Preservation status

Only a few snapshots of SITS exist:

• SITS version 1146 from October 1975, and NSITS versions 111, 112 from December 1975.
• A set of disk images from May 1978 with an unknown SITS version.

Hardware support

This is the list of supported, or actually required hardware for SITS version 1146:

• KB11-A CPU with MMU
• 128K core memory
• EAE extended arithmetic element
• FP11 floating point coprocessor
• KW11P programmable clock
• PC11 paper tape reader/punch
• DC11 terminal controller
• DH11 terminal controller
• RF11 fixed head disk
• RK11 moving head disk
• Tom Knight's NG vector display controller
• Ron Lebel's TV raster display controller
• Lebel keyboards