ITS, the Incompatible Timesharing System, is an operating system for the PDP-10 computer family. It was written by hackers at MIT in the 1960s. The MIT site was shut down in 1990, but enthusiasts continue to operate ITS systems to this day.

Documentation

See the doc subdirectory for documentation.

Building it from scratch

This repository contains source code, tools, and scripts to build ITS from scratch.

First, magnetic tape images are created from files in src and bin. There are two bootable tapes, and two tapes with files in DUMP backup format.
Then the tapes are used to create a file system on an RP06 disk and populate it with a minimal system.
Next, the system is booted from the disk. Source code is loaded from tape, and the following programs are rebuilt:
- MIDAS, the assembler.
- DDT, debugger and HACTRN user login shell.
- The ITS monitor (kernel).
- Exec DDT, standalone debugger.
- NSALV, standalone file system tool.
The system is restarted, the rebuilt monitor and salvager are combined into a new ITS binary, which is then started.
Remaining programs are rebuilt:
- TECO, editor.
- DUMP, tape backup and restore.
- PDSET, set time and date.
- KSFEDR, manipulate front-end file system.
- LOCK, shut down system.
- ATSIGN DEVICE, load device drivers.
- TCTYP and CRTSTY, terminal handling.
- PEEK, system monitoring.
- ARCDEV, transparent file system access to archive files.
Finally, the whole file system is dumped to tape.