Recreating MIT's ITS: From PDP-10 Emulation to Physical Replica

When the Incompatible Timesharing System (ITS) at MIT was finally shut down, its files were made available on an FTP site, primarily for original users to retrieve their old data. However, someone scraped all these files, preserving them for posterity.

The Rise of PDP-10 Emulation

Following the shutdown, several PDP-10 emulators emerged, allowing enthusiasts to run PDP-10 software and ITS using the archived files. This is where Lars, one of the project's key figures, became involved. His initial goal was to create automated build scripts to install ITS from tape images, eliminating the need for manual input. This project quickly grew, snowballing into a comprehensive build script capable of building everything from source code.

Lars was not working alone; he was part of a small community dedicated to this effort. A significant contribution came from Rick Richard Cornwell, who developed a KA10 emulator, precisely replicating the processor used in the original MIT ITS machine. This emulator was a game-changer, enabling the community to add emulators for other MIT hardware, moving beyond generic PDP-10 emulation to recreate the entire MIT lab hardware environment.

From Emulation to Physical Replica

The journey from software emulation to a physical replica began when Oscar and Lars met at VCF Seattle in 2019. Oscar, initially skeptical that the project could fully replicate the complex hardware, including the PDP-10 and its peripherals like the ARPANET IMP, was challenged by Lars to download and compile the software from GitHub. It took Oscar four hours to compile the code in his hotel room, but it worked.

Oscar described the experience as falling into a "huge rabbit hole." He likened exploring the ITS operating system, with its hundreds of contributors and their unique code, to walking through Pompeii, but with fully standing buildings instead of ruins. Having previously worked with PDP-8 and PDP-11 systems, he recognized ITS as the "ultimate computer history capsule." This realization inspired him to create a physical replica.

The replica project faced significant challenges, primarily due to Oscar's desire for exact historical accuracy. An initial attempt with a wooden box was discarded because it didn't match the KA10 model. The idea of 3D printing was also deemed insufficient, leading to the ambitious decision to create an injection mold. This proved difficult due to the size and complexity, taking two years to secure funding and courage, but ultimately, the replica was completed.

The Technology Behind the Replica

The physical replica is powered by a Raspberry Pi, specifically a Pi 5 for optimal performance. The Pi 5 is powerful enough to run all the emulated components, including graphics terminals, a PDP-6, and a PDP-11. Older Raspberry Pi models can also be used, though they might require focusing on a subset of the emulated systems.

This setup highlights a remarkable technological contrast: a $55 microcontroller can now run a system that, in its original form, would have cost millions of dollars in its day. The original MIT AI lab, with all its components, represented an investment of several million dollars in 1970s money.

The Vision: Computer History Capsules and ARPANET Reconstruction

The creators view these replicas as "computer history capsules," with the ITS replica being one of the richest examples. While the physical replica offers a tangible experience, anyone can download the GitHub repository and install the software on a Raspberry Pi to experience ITS without the blinking lights of the front panel.

The project's ambitions extend beyond single machines. MIT had three PDP-10s, and a similar system from Stanford, called WAITS (frozen in time in December 1974), has also been reconstructed. The next logical step for the team is to reconstruct the ARPANET.

Lars discovered that the ITS operating system tapes already contained the software (NCP - Network Control Program) to connect to an ARPANET. With the help of Rich Greenblatt, hardware emulation was added to the PDP-10 simulator to utilize this software. The simulation of the IMP (Interface Message Processor) router, the crucial infrastructure of the ARPANET, had already been developed by others.

Now, with connected IMP routers talking to ITS on the PDP-10, the team is capable of reconstructing an ARPANET, marking another significant step in preserving and bringing to life early computer history.