Inside the HP Nanoprocessor: a high-speed processor that can’t even add

The Nanoprocessor is a mostly-forgotten processor developed by Hewlett-Packard in 1974 as a microcontroller for their products. Strangely, this processor couldn't even add or subtract, probably why it was called a nanoprocessor and not a microprocessor. Despite this limitation, the Nanoprocessor powered numerous Hewlett-Packard devices ranging from interface boards and voltmeters to spectrum analyzers and data capture terminals. The Nanoprocessor's key feature was its low cost and high speed: Compared against the contemporary Motorola 6800, the Nanoprocessor cost $15 instead of $360 and was an order of magnitude faster for control tasks.

Recently, the six masks used to manufacture the Nanoprocessor were released by Larry Bower, the chip's designer, revealing details about its design. The composite mask image below shows the internal circuitry of the integrated circuit. The blue layer shows the metal on top of the chip, while the green shows the silicon underneath. The black squares around the outside are the 40 pads for connection to the IC's external pins. I used these masks to reverse-engineer the circuitry of the processor and understand its simple but clever RISC-like design.

This is a very detailed and in-depth article, so definitely not for the faint of heart. Definitely a little over my head, but I know for a fact there's quite a few among you that love and understand this sort of stuff deeply.