This page is provided as support for my talk about emulation of old computer systems. This talk was given on the 12th June 2025 under the auspices of the BCS Kent Branch..

It is provided primarily so that references can be provided in a useful and accessible form. Suggestions for additions or improvements are welcomed.

The Lecture

The following direct material is available:

• A link to the full video recording will appear here, once BCS have edited and redacted it as they see fit.
• Just the slides for the lecture.
• These are the slides for a previous talk on emulation, given back in 2019. They concentrate much more on the PDP-8, and the hardware used to produce replicas. There is also a recording here (it's on YouTube as well, but this avoids the adverts). Be warned that the audio level is quite low on this one, so you may have to wind up the volume quite a bit!

Closed source emulators

These are emulators which are commercial, or just closed source. Details of others are welcomed, if only for completeness.

• A company named Stromasys, who provide emulators for a number of platforms including PDP-11, VAX and Sparc.
• WinEight, which is a rather old, closed source (but freely available) emulator for the PDP-8.

Open source emulators

These emulators are open source, but the restrictions on copying and redistribution do vary.

• DOSBox is a very complete emulator for DOS on the PC, and can run a large number of DOS based games.
• PCjs is a browser based emulator for several machines.
• Hercules is a full featured emulator for IBM mainframe systems, including System/370, ESA/390, and z/Architecture.

Worked examples

These pages provide step by step instructions for installing a few systems on selected emulated hardware.

• Installation of Mini-UNIX (not Minix) on an emulated PDP-11.
• Installation of Quasijarus (a fork of 4.3 BSD UNIX) on an emulated VAX.
• Building an ITS system on an emulated PDP-10.
• Installing NetBSD on an emulated VAX.

More about SIMH

SIMH (SIMulator for Hobbyists) was started by Bob Supnik (formerly of DEC, as engineer and vice president) over 20 years ago. It was targeted at DEC machines, of which he has extensive experience. He designed SIMH as a framework which makes it relatively easy to write a new simulator.

A few years ago, a group of enthusiasts forked the project, and that version was put on GitHub. Many people contributed, but there was little central management which resulted in a lack of formal releases. Bob Supnik continued to develop "Classic" SIMH (as it became known), as version 3.x. The version on GitHub was version 4.x.

In May 2022, the MIT License of SIMH version 4 on GitHub was unilaterally modified by a contributor to make it no longer free software, by adding a clause that revokes the right to use any subsequent revisions of the software containing their contributions if modifications that "influence the behaviour of the disk access activities" were made. As of 27 May 2022, Supnik no longer endorsed version 4 on his official website for SIMH due to these changes, only recognising the "Classic" version 3.x releases.

On 3 June 2022, the last revision of SIMH not subject to this clause (licensed under BSD licenses and the MIT License) was forked by the group Open SIMH, with a new governance model and steering group that includes Supnik and others. The Open SIMH group cited that a "situation" had arisen in the project that compromised its principles.

Bob (Eager) has stayed with the "Classic" SIMH project for his own development, as it is not such a fast changing target (and is in fact very stable). This approach has also been adopted by the developer of the HP simulators.

Here are some useful links.

• The official SIMH page, maintained by BOB Supnik.
• The Open SIMH project.
• User's Guide for "Classic" SIMH.
• "Classic" SIMH FAQ.
• Papers about simulation and SIMH.
• Documentation on developing a new simulator under "Classic" SIMH. Bob Eager has produced an updated version of this here.

The Pironman 5

The Pironman 5 is a case kit for the Raspberry Pi 5. It makes an ideal platform for a 'pocket emulator'.

• The primary supplier is Sunfounder, where more details can be found.
• UK/EU suppliers include Amazon, Elektor, RobotShop and DigiKey.

Other useful links

These are miscellaneous links to places of interest. Additions are welcomed.

• These are software kits suitable for running on various machines emulated by SIMH.
• An interesting paper on why the VAX ended up with an emulated POLY instruction.
• Some more details on some of Bob Eager's PiDP systems can be found on his his projects page.
• The Computer Conservation Society have a collection of emulators for old machines. Availability varies, but the starting page can be found here.
• The Paper Computer is the ultimate in low-tech emulation. This is cited as circa 1983, but Bob saw something similar (using boxes and index cards) about 10 years earlier!
• The Obsolescence Guaranteed website (run by Oscar Vermeulen) has information, inter alia, on emulated hardware front panels.
• This is a fairly comprehensive PDP-8 information page. It also shows its relation to the other PDP systems.
• A good starting point for information on the PDP-11.
• This has some good links for information on the PDP-10. You can see the console (as on the PiDP-10) if you look carefully at one of the pictures (the wide view); it's in the background. The panel with some switches and the inscription 'KL10' is actually the diagnostic and bootstrap pricessor, and is a PDP-11 with a different colour scheme.
• This is a presentation to a BCS Branch, covering the history of VME/OpenVME. It is interesting because it shows how emulation was used to provide a high degree of backward compatibility. This link jumps in at a relevant point, or click here for the whole thing.