Running the Busicom software in a Spartan-6

Eight years ago I decided I wanted to learn about programmable logic: PLAs, CPLDs, and FPGAs.

About the same time, I became aware that a group of engineers and computer history buffs had gotten Intel to release the schematics for first commercially-available microprocessor: the Intel 4004 CPU. They'd also retrieved the software that drove the first commercial product that used the i4004: the Busicom 141-PF.

I decided to re-create the i4004 CPU in an FPGA.

It turns out that's a lot like learning to swim by attempting to cross the English channel. But I've never been known to shy away from a challenge.


The project has taken some odd detours, and there are still many incomplete pieces. But last weekend I was able to load a bit file into the Spartan-6 LX9 FPGA in my rebuilt Canon P170-DH calculator and let the 49 year old Busicom software run once again.

Here's a video of the calculator in action:

Yes, that's the unmodified Busicom software scanning the keyboard, calculating an addition, and printing the results. All running on a CPU that is a faithful re-coding of the Intel schematics in Verilog HDL. It's as close to the original as I could make it, given the differences in technologies.

I still have a lot to do on this project. I want to rearrange the keyboard so it more closely matches that of the Busicom calculator. The printer emulation needs some work, as does the vacuum fluorescent display I added. And I'm still planning to finish implementing the i4004 CPU using discrete components, which will use this as its I/O interface.

But first I want to savor the results of eight years of study and experimentation.