Main Content

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 masks were carefully cleaned and scanned by The CPU Shack, and stitched by Antoine Bercovici. 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.

The Nanoprocessor was designed in 1974, the same year that the classic Intel 8080 and Motorola 6800 microprocessors were announced. However, the Nanoprocessor’s silicon fabrication technology was a few years behind, using metal-gate transistors rather than silicon-gate transistors that were developed in the late 1960s. This may seem like an obscure difference, but silicon gate technology was much better in several ways. First, silicon-gate transistors were smaller, faster, and more reliable. Second, silicon-gate chips had a layer of polysilicon wiring in addition to the metal wiring; this made chip layouts about twice as dense.8 Third, metal-gate circuitry required an additional +12 V power supply. The Intel 4004 processor used silicon gates in 1971, so I’m surprised that HP was still using metal gates in 1974.

A bizarre characteristic of the Nanoprocessor is its variable substrate bias voltage. For performance reasons, many 1970s microprocessors applied a negative voltage to the silicon substrate, with -5V provided through a bias pin. The Nanoprocessor has a bias pin, but strangely the bias voltage varied from chip to chip, from -2 volts to -5 volts. During manufacturing, the required voltage was hand-written on the chip (below). Each Nanoprocessor had to be installed with a matching resistor to provide the right voltage. If a Nanoprocessor was replaced on a board, the resistor had to be replaced as well. The variable bias voltage seems like a flaw in the manufacturing process; I can’t imagine Intel making a processor like that.”

Link to article