Intel 4004 — 50th Anniversary Project

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To celebrate the 52nd Anniversary of Intel’s November 15, 1971 product launch of the now-historic 4004 microprocessor, Swiss physicist, Klaus Scheffler, in a world’s first, built a complete 4-bit computer based on Intel’s original 4004 schematics using nearly 4,000 discrete SMD transistors on three giant printed circuit boards. He proved his computer works stand-alone by running an algorithm that computes the first 16 digits of Pi. This achievement was an international collaboration with Hungarian mathematician, Lajos Kintli, who worked with Scheffler from afar to debug the printed circuit “microprocessor,” by writing assembly-language algorithms for the 4004, “board bring-up” test software running on a Teensy, and Kintli’s indispensable circuit analyzer and verification software that made it possible for Sheffler to build and debug his fledgeling, printed circuit 4004 incrementally, comparing it against “oscilloscope traces” from Kintli’s known-working simulation of the entire 4004-family chip-set. But Scheffler did not stop there. After he got his transistor 4004 working, he went on to build a transistor 4002 RAM and a hybrid ROM+I/O board to create a complete, stand-alone, 4-bit computer. This project is a testament that a “big hack” (the good kind) can be accomplished by a small team of dedicated hobbyists or even solo, working just a few hours a week, yet persisting for months despite the inevitable technical challenges.

Check out Chen Steenvoorden's Busicom 141-PF Printing Calculator Simulator implemented in WebAssembly.

Note best: The the 141-PF was a vintage adding machine, so you'll see the digits you typed on "paper" only after you press an operator button like "plus." For example: To add 18 and 42, type 1 8 + 4 2 + = (The 2nd + is very important!) You can read the original (Busicom clone) Operating Instructions at the Internet Archive.

Federico Faggin published his memoirs in Italian and English (February 2021)

The autobiography of this award-winning pioneer of semiconductor technology, designer of the 4004, 8008, 8080, Z80, etc., first published in Italian (as Silicio), now out in English Silicon: From The Invention of the Microprocessor to the New Science of Consciousness

Last year's shout-out went to Erturk Kocalar (November 2020)

...for his line of educational (and open-source) Arduino-compatible "Retroshield" kits that let you play around with real, vintage microprocessors from the comfort of your laptop. He recently released his most ambitious shield yet—you guessed it—the ZIF socketed, preassembled 4004 Retroshield, complete with voltage level-shifters so the -15 volt PMOS 4004 can communicate with the 5 volt CMOS Arduino. I particularly like Erturk's Busicom calculator technical summary, which includes a nice explanation of how the Shinshu Seiki drum printer works.

Want to meet a community of vintage semiconductor engineers we inspired? Visit the Visual 6502 Project and Evil Mad Scientist Laboratories

Want to know more about the "F.F." signature in the corner of the chip? Read the perspective of Federico Faggin, the 4004's silicon designer.

The Exhibit Story

Genesis

Knowing that robotic "pick-and-place" machines were now the standard way of assembling electronics, and made the whole idea practical to produce, I started thinking. What could I build out of this "retro," one-gate-per-chip technology? Some sort of microprocessor seemed like a natural. But modern microprocessors have millions of transistors, so I knew I'd have to keep the project very modest, like restricting the data-path to 4-bits. This meant I'd have to design my own, or... go back in history and build a replica of a commercial micrprocessor. Besides, reproducing a commercial microprocessor, rather than re-inventing my own, would allow me to use existing software and existing manuals, data sheets, wisdom of the ages, etc.

The first microprocessor that came to mind was the Intel 4004. It was not nearly as famous as the 8080, and its 8-bit progeny, but my late step-father, Joe Robb, worked for Intel in the early 1970's, and I still remember when brought home some MCS-4 data sheets. MCS-4 (Microcomputer Set 4) was the name Intel gave to the 4004 chip family for its commercial introduction in November 1971.

In search of the circuit

During this research spree I also learned that the Intel Archives and the Intel Museum were somehow connected. This helped me turn a notion into a plan: I wrote to Tracey Mazur, the curator of the Intel Museum, and Rachel Stewart, Intel's corporate archivist, with a proposition: if you give me access to internal 4004 engineering documents, I will build you a giant, functioning replica of the 4004 suitable for a museum exhibit.

Around the same time I started looking for vintage software to run on the giant 4004 replica. I wrote to Federico Faggin and to Marcian "Ted" Hoff, two of the four co-inventors of the 4004 (I was never able to contact the other inventors, Busicom's Masatoshi Shima or Intel's Stan Mazor). Both Faggin and Hoff wrote back, Faggin with a general offer of advice, and Hoff with software for a 4004-based video game design that Intel pitched to a potential customer.

At this time, I was not optimistic about gaining access to schematics or the mask artwork that was used to create the original 4004 wafers, and so I began devising a "Plan B." I was even starting to wonder if there were any schematics to be found, having read interviews that suggested that no complete logic diagram was ever drawn of the 4004. In consultation with Federico Faggin and MIT's Tom Knight, I mentally prepared myself for reverse-engineering the 4004 "the old-fashioned way," by photographing the metal layer through a microscope, and then etching that top layer away to reveal the polysilicon layer, photographing that, and using enlargements of these photomicrographs to meticulously transcribe every transistor and wire.

A few weeks later I got word from Tracey Mazur that Intel's legal department would allow us access to the 4004 engineering documents, and Rachel Stewart was offering encouraging news about her research.

In late July of 2005 I flew to California to meet with Federico Faggin and Rachel Stewart at the Intel archives. The visit was amazingly brief and nearly a complete success. Faggin picked up three large engineering drawings on vullum, a translucent plastic sheeting, unrolled them one-by-one, and declared within 30 seconds, "Yes. This is all of them." Rachel and I looked at each other approvingly, as Faggin adds, "See here? The drawings are drawn just like the chip layout."

The Calculator connection

Reverse Engineering the schematics

Meanwhile I continued to develop the ROM reader, periodically sending Brian and Barry my results to see if the binary data I was extracting yielded valid 4004 code. After some struggles with the unusual voltage levels used by the 4001 ROMs, we succeeded in extracting a significant fraction of the Busicom software from the ROMs Faggin had found in his basement. The bad news was that there were large strings of zeros in three out of four of the calculator ROMs. We would have to find another source.

Digital Archeology Expedition

After Fred returned to Boston, I immediately sent the ROM contents to Brian and Barry, who used Fred's wiring diagram along with printed circuit board photographs I received from Christian Bassow, a German chip collector. Within a month, they had crafted an instruction-level simulator of the 4004 as well as simulated Busicom hardware, and by January 2006, they had a completely working simulation of the Busicom 141-PF printing calculator.

Verilog simulation of the 4004

Funding and Exhibit design

Letting visitors "look under the hood"

Exhibit Detail and Construction

Out of the vaults and into the world