It has been almost a year since I got a different implementation of my PDP-8/e replica project for my birthday.

Yes, it was a cake, and I have neglected to share it with the world so far. To be fair, there was a time this year where everything was a cake, at least on Twitter, and adding another one would have just been pouring gasoline on the fire.

But here it is, first a side by side comparison of the implementations:

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A few weeks back, I was showing my PDP-8/e project at the Vintage Computer Festival in Zurich. While I was doing my project, I haven’t really checked if there were other projects like this. At least for the PDP-8 I knew there wasn’t, the only FPGA core I could find was a new implementation of the architecture that is binary compatible but doesn’t attempt to replicate the structure and instruction cycles of any specific PDP-8.

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As I have noted, I have been working on packaging the VHDL simulator GHDL for Debian after it has dropped out of the archive for a few years. This work has been on slow burner for a while and last week I used some time at DebConf 18 to finally push this to completion and upload it. ftpmasters were also working fast, so yesterday the package got accepted and is now available from Debian unstable.

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This is, at long last, part 3 of the overview of my PDP-8/e replica project and offers some details of the low-level implementation.

I have mentioned that I build my PDP-8/e replica from the original schematics. The great thing about the PDP-8/e is that it is still built in discrete logic rather than around a microprocessor, meaning that schematics of the actual CPU logic are available instead of just programmer’s documentation. After all, with so many chips on multiple boards something is bound to break down sooner or later and technicians need schematics to diagnose and fix that¹. In addition, there’s a maintenance manual that very helpfully describes the workings of every little part of the CPU with excerpts of the full schematics, but it has some inaccuracies and occasionally outright errors in the excerpts so the schematics are still indispensable.

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Over the holidays my old Nintendo Game Boy (the original DMG-01 model) has resurfaced. It works, but the display had a bunch of vertical lines near the left and right border that stay blank. Apparently a common problem with these older Game Boys and the solution is to apply heat to the connector foil upper side to resolder the contacts hidden underneath. There’s lots of tutorials and videos on the subject so I won’t go into much detail here.

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It has been a few years since Debian last had a working VHDL simulator in the archive. Its competitor Verilog has been covered by the iverilog and verilator simulator packages, but GHDL was the only option for VHDL in Debian and that has become broken, orphaned and was eventually removed. I have just submitted an ITP to make my work on it official.

A lot has changed since the last Debian upload of GHDL. Upstream development is quite active and it has gained free reimplementations of the standard library definitions (the lack of which frustrated at least two attempts at adoption of the Debian package). It has gained additional backends, in addition to GCC it can now also use LLVM and its own custom mcode (x86 only) code generator. The mcode backend should provide faster compilation at the expense of lacking sophisticated optimization, hence it might be preferable over the other two for small projects.

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This is an overview of the hardware and internals of the PDP-8/e replica I’m building.

The front panel board

If you know the original or remember the picture from the first post it is clear that this is a functional replica not aiming to be as pretty as those of the other projects I mentioned. I have reordered the switches into two rows to make the board more compact (which also means cheaper) without sacrificing usability.

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I am creating a replica of the DEC PDP-8/e architecture in an FPGA from schematics of the original hardware. So how did I end up with a project like this?

The story begins with me wanting to have a computer with one of those front panels that have many, many lights where you can really see, in real time, what the computer is doing while it is executing code. Not because I am nostalgic for a prior experience with any of those — I was born a bit too late for that and my first computer as a kid was a Commodore 64.

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So I finally got myself a blog to write about my software and hardware projects, my work in Debian and, I guess, stuff. Readers of planet.debian.org, hi! If you can see this I got the configuration right.

For the curious, I’m using a static site generator for this blog — Hugo to be specific — like all the cool kids do these days.