Major Differences Between the Distributions

This software distribution began as a series of improvements Warren Young made to Oscar Vermeulen's software originally made available for the 2015 PiDP-8/I kit. Development has continued in the Tangentsoft Fossil repository ever since, with occasional snapshots made available as pre-built SD card images or tagged releases for convenience. Users coming from any earlier starting point — whether Oscar's original 2015 release, the 2021.02.14 tagged release, or one of Oscar's pre-built SD card images — will find the significant differences described below.

The 2026-04-13 release is the current stable snapshot of this ongoing development. It reflects a great deal of work since 2021, including the introduction of Cycle Accurate front panel behavior and a modernized binary image build process.

For the complete user-focused list of changes, see the ChangeLog. For the low-level blow-by-blow, see the project timeline.

Front Panel Switch Combos

The introduction of Cycle Accurate (also called Cycle Realistic) operation in this release changed the behavior of the Sing_Step and Sing_Inst switches fundamentally: they now behave as they did on a real PDP-8/I. Because of this, the old method of holding several switches down simultaneously no longer works for the special host-control functions. The switch sequences are now ordered: the switches must be operated in a specific sequence rather than all at once.

Booting a Different OS or Mounting USB Media

Set the DF and IF switches as before to select your target, then:

Turn Sing_Inst on
Hold Cont
Toggle Sing_Step

After mounting USB media, turn Sing_Step and Sing_Inst back off, then press Cont to resume. When booting a different OS, pressing Cont may not be necessary — but will be needed if you were too slow releasing Sing_Step and Sing_Inst.

Rebooting the Host Pi

Turn Sing_Inst on
Hold Start
Toggle Sing_Step

Shutting Down the Host Pi

Turn Sing_Inst on
Hold Stop
Toggle Sing_Step

General Note on Switch Combos

In all cases, the order of operations matters. The action triggers on the toggle of Sing_Step, while the other switch(es) are held. This is a deliberate consequence of Cycle Accurate mode restoring authentic PDP-8/I front panel behavior.

Incandescent Lamp Simulator

One of the early improvements over the Vermeulen releases was the Incandescent Lamp Simulator, originally by Ian Schofield, then improved by Warren Young.

As of the 2026-04-07 release, this has been improved yet again. First, by an algorithm improvement by Heinz Berndt Eggenstein that refined the brightness simulation. Next came integration with the Cycle Accurate front panel work by Steve Tockey. Finally, there was a bunch of work to support the Pi 4 and Pi 5 through the picntrl revision. The net result is that the ILS now runs well on all currently supported hardware.

Hardware Support

As of the 2026-04-07 release, we have dropped direct support for 32-bit-only and single-core Pi boards. This is purely out of wish not to put scant hobby time into backwards compatibility for obsolete hardware. The old software that ran on them remains available.

We now require something at least as capable as a Raspberry Pi 2. This includes the Zero 2 W.

Differences from the Software Originally Made Available for the 2015 Kit

For users migrating from Oscar's original 2015 release, the major categories of change introduced over the years since then are summarized here. Each of these has long since become the baseline that all users can expect.

Build and installation — A standard ./configure && make && sudo make install build system replaced the original manual build process.
Command and path names — Commands were renamed to match the "8i" platform name (e.g. pidp8i, pidp8i-sim, pidp8i-test), and file locations were reorganized to separate build and execution concerns, conforming to the Linux Filesystem Hierarchy Standard.
Service management — The background simulator service moved to systemd, replacing the original System V init script.
Multiple installations — The software can be unpacked, built, and installed into separate directories, and re-installation no longer overwrites local media images.
OS compatibility — The software now builds and runs on a much wider range of OS versions than the original release, which was essentially tied to Raspbian and close relatives.

PiDP-8/I Software