HP35: A Bit-Serial Wonder – 0. Prelude

Part0. Prelude

This part is the prelude to a series of articles on the internal design of the first pocket scientific calculator: HP35.

(The top image is a part of the cover of HP Journal 1972 issue 6)

Calculators, just like any other office stationeries, are taken for granted these days. Beneath the black blobs of those cheap and plentiful scientific calculators, modified 6502s or some other ultra-high efficiency, low sleeping power consumption CPUs make them tick. Given more power, anyone with 5-minute of C experience can build a calculator from a single microcontroller with no problem. So it’s nothing interesting huh?

But if we look backward, toward an age where microcontroller doesn’t even exist, toward an age where any single little-bandwidth transistor used in the chip design counts to the final price. It’s no longer trivial to see a pocket scientific come into being.

The world’s first scientific calculator HP35 holds its significance in the history of microelectronic technology. At the time when pocket calculation is all about adding, subtracting, multiplying and dividing, HP hit the market with an astonishing pocket calculator that can do logarithmic, trigonometric, exponential functions and more, and the device cost a reasonable  395USD (1972 money, which is equivalent to about 2300USD today), an absolute overkill. Any command takes less than 1sec to execute. Breathtakingly fast in 1972 (Said by Tony, the host of teenix.org, who provided a very intuitive microcode simulator: CCE33, more on that later). And the resolution of the result was so high that the engineers at HP even have a problem verifying the result using an IBM mainframe! There’s no doubt that they have combined the darkest magic in both hardware design and software design to achieve it.

After a month of investigation into the design of an HP45 (In case you don’t know, HP45 is the enhanced version of HP35), I believe I’m now confident to write an abstracted description of the actual hardware (RTL) implementation of the original ARC and CTC calculator chipset used in countless vintage HP calculator models. (The later ACT and more advanced NUT and Saturn and whatever chipsets are actually improved variants of the chips in the original HP35). In this long writing, I’ll give my understanding of the design, and give the HDL of my understanding. Stay tuned!

Disclaimer: This article contains my own understanding of the HP35 chipset design, the origin of my idea comes from the HP Patent Document of the HP45 (US Patent 4001569),  the detailed software investigation done by Jacques Laporte, and the beautiful HDL implementation of NUT done by Monte Dalrymple @ systemyde.com. Please accept my highest respect. As an immature undergraduate, I can’t guarantee anything I say here. If you have any issue with the content, please make comments.

Until next time, thanks for reading!

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