MCM/70 Reproduction

MCM/70 has been mentioned here before. Here’s a project to make a reproduction.

The MCM/70 computer was conceived, developed, and built in Canada in 1974. It was arguably one of the very first “personal” computers because it was portable, had a built-in keyboard, display, and support for the APL programming language, plus cassette storage. On the other hand, at a cost of $10,000, it was only really accessible to large corporations, the military, governments, and educational institutions.

As soon as I became aware of the MCM/70 I knew that I had to have one, and given the rarity of these machines that means making one. This project will chronicle my MCM/70 Reproduction journey.


It would be fun to make one with modern components, but the original had all sorts of awfulness. Massive lead-acid rechargeable batteries and a very, uh, lively voltage required by the display, to name but two. I was lucky enough to see a (possible the) MCM/70 on my friend’s repair bench when he was working on it for YUCoM. It did not look like a fun machine to work on


As an idea, it has to be one of my favourite machines. I’m so glad the firmware has been preserved. A recreation is a very nice idea, I think, even though it might be a challenge to enjoy using it.

I’m curious why you think it would be a challenge to enjoy using it? I’ve been working through the MCM/70 User’s Guide and it’s been a great experience learning APL on this machine.

Ah, I suppose it’s only a comment on my ability to learn new things - it’s a personal reaction. It’s great to hear that you’re learning APL.

Ah cool. Truth be told learning is probably too strong a term. I guess by working through the exercises I am trying to get a feel for the language and its capabilities. 100% chance I will never write an APL program :slight_smile:


I don’t get why you had to 3D-print another side panel just to have a relief slot. I’d have just pulled out my Dremel tool and cut a notch out of the one I already had. Of course, I’m also the sort of person who sees a flat & square-ish part as something to be cut out of sheet plastic rather than run through a 3D printer.

The lead-acid batteries was a built in UPS. I run mine with them disconnected but I still left them there because it’s another mainframe feature on your desktop with the MCM/70.

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Wow. You have an MCM/70 in working order? That’s very cool.

Yes. You can see me typing in the “Horse” game. If you skip to the end, you can see it run. Forgive the lack of talking as this was just a quick test to help confirm the timing for the MCM/70 emulator. This is take number 14 or so as I kept typing it in wrong and I was demonstrating how I was typing it in.

Thanks, that was great to see!

It made me realize that in APL we normally think from right to left, which makes typing in from left to right a bit awkward while developing. But that would be the case for a paper TTY as well, of course. Typing in an existing listing would be no problem, on the other hand.

One thing that would help is if you were able to move the cursor to the middle of the line and insert new characters there. I noticed that you didn’t do that in line 11 when you needed to insert a “P” before the “]”.

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I don’t believe there is cursor control. You can see the MCM/APL Quick Reference guide here: that I used as a reference.

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There’s a 280 page User Guide here. It looks like you can edit a line you’re entering, before you hit RETURN, by using BACKSPACE to move cursor left, then overtyping, or using CTRL-SPACE to insert a space (which you can then overtype.)

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Thanks for the link. It will be an interesting read. I will give what you suggest a shot. It might have saved me 13 retakes knowing that :wink:

Hee hee! I see function editing is on page 83. So, if you were putting together a complex expression, you might well put it in a function so you can revisit it.

Also CTRL-BACKSPACE will delete the current character under the cursor.

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