LGP-30 Replica: LittleGP-30

Imagine a computer built from just 113 electronic tubes and 1450 diodes.

LGP-30

This is an original photo of Stan Frankel's masterpiece, the LGP-30. Introduced in 1956, it paved the way to the idea of a personal computer. A Desk Computer, was the word. Which is not to be confused with a desktop computer, obviously. The difference is an order of magnitude in both weight and size.

For someone used to today's computers, the LGP-30 demands a rethink - which is why it is interesting. It works on a serial stream of bits. Not 16, 12, or 8 bits, but just 1 bit of data flows through its veins. The whole thing is a box of shift registers, with a tiny bit of logic added. Jürgen Müller's cycle-exact replica shows the (replica) control panel in more detail:

LittleGP-30 v2.0: a work in progress

We expect the LGP-30 replica to be ready in 2026. The electronics have been up and running for years - Jürgen made a small run of machines around 2018. And the 2025 update is already done, too. No, the remaining challenge is to produce an appropriate housing for the machine. This time though, this will be a desktop computer - not a desk computer as the original. But we want to make it out of proper metal, with the proper 1950s paint job. If you are interested in the project, please email us at oscar@ceds.dev!
LGP-30 replica 3D model
LGP-30 replica panel
LGP-30 replica 3D model
LGP-30 replica 3D model
LGP-30 replica panel
LGP-30 replica 3D model
LGP-30 replica 3D model
LGP-30 replica 3D model

More interesting hardware facts

The LGP-30 does not have RAM or core memory. It does not even have an accumulator in the normal sense of the word. Instead, a rotating drum stores 4096 words of 31 bits each. The first three tracks on the drum store the serial streams that amount to the data of the accumulator, the instruction register and the Control register (program counter, address of next instruction).

With its 113 tubes, the LGP-30 has 16 instructions. Which is enough for anyone, of course, and makes machine code programming relatively simple. Except it isn't, because of the drum storage.

If you want to get performance out of the LGP-30, you have to time the drum. Store a variable at the right distance from the instruction using it, and the serial stream flows uninterrupted. Store it somewhere else, and you have to wait for the drum to rotate up to where your data sits. Hence the nicely unintelligible code of Mel.

The LGP-30 does not have a classical Blinkenlights front panel. Instead, it uses a little built-in oscilloscope to show you what is in its three registers.

So. This is fun. Rethinking computer architecture, getting your mind around serial computers, being amazed about how little you need to make a very functional computer. Even a PDP-8 looks bloated compared to this. And still it works!

To prove that point, quite a lot of recovered software is built in to the replica. From the famous Black Jack game, through an assembler and the ACT-II compiler, down to what you could call the predecessor of an operating system - of sorts.



The Story of Mel

Ed Nather’s 1983 poem The Story of Mel is a cornerstone of hacker folklore, celebrating the ingenuity and ethos of “Real Programmers.” It tells of Mel Kaye, who wrote blackjack code for the LGP-30 in raw machine language, timing instructions to the drum memory’s rotation. When asked to cheat users, Mel subverted the request—ensuring the computer won instead. His code used self-modifying loops so precise they baffled future maintainers.

Rather than simplify or refactor, Nather walked away in awe. Mel’s story remains a tribute to low-level mastery and a reminder that elegance often hides deep within the machine. Immerse yourself in the 1950s coding world on the LGP-30 through this link: melsloop.com

Explaining the control panel

LGP-30 controls The LGP-30 has
  • storage/memory (same thing) in the form of a magnetic drum with 64 tracks of 64 words each. So 4096 memory locations.
  • Then, an extra 12 words on the drum are used as CPU registers. You'll use a Program Counter PC, an Instruction Register IR and an Accumulator A.
  • A Flexowriter typewriter provides a dual input device, called TTI, with Keyboard and Paper tape reader. And a dual output device, TTO, with typewriter and paper tape puncher.
  • You can also switch in the fast paper tape reader/punch. This was an expensive option at the time, but it does the same thing the Flexowriter does already (mostly, normally).

LGP-30 Controls: Controlling the computer is a bit confusing, as there are buttons on the LGP-30 front panel, and then there are switches on the Flexowriter (teletype) as well. And some Flexowriter controls have the same name as buttons on the LGP-30. So here is the overview:
  • Especially confusing, there's a MANUAL INPUT on the front panel and a MANUAL on the typewriter. Different things.
  • And a START on the LGP, versus a START COMP on the typewriter. Not different things.


The Replica

Designed by Jürgen Müller, this is a cycle-exact replica on FPGA. For now, as this page is under construction still, visit the project development page if you want more information.

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