First mover: Intel’s 4-bit 4004 CPU runs 50

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Thomas Nguyen, CC BY-SA 4.0

50 years ago today, on November 15, 1971, Intel launched the 4004 and, by extension, the modern computer age. The 4004 was the first commercially produced microprocessor or CPU. It was a huge technical achievement for Intel and the design team, led by Federico Faggin, broke new ground in several ways.

Intel hired Faggin to solve a problem: it had negotiated a deal with the Japanese calculator company called Busicom, in which Intel was to produce a four-chip calculator design for upcoming Busicom products. The problem was that no one at Intel could actually build the CPU. At the time, Intel was primarily a memory company. Faggin was briefed on the Busicom project late in its first day. In his account of the development of the 4004, published in 2009, Faggin he wrote:

Stan also told me that Shima would be arriving in a few days to check the progress, expecting to find the CPU logic design completed and the other chips in an advanced state of design. The problem was that since late 1969 no work had been done on the project and Busicom had not been informed.

When I saw the project plans that had been promised to Busicom, I was amazed: I had less than six months to design four chips, one of which, the CPU, was at the limit of the possible; a chip of that complexity had never been made before. I didn’t have anyone working for me to share the workload; Intel had never made custom random logic chips before, and unlike other companies in that industry, it had neither methodology nor design tools for quick, error-free design.

Faggin goes on to note that Intel people like Andy Grove, “… saw my project as a diversion imagined by the marketing guys to make money while waiting for the memory business – Intel’s true mission – to mature.”

The 4004 block diagram. Picture of Appaloosa, CC BY-SA 3.0

Despite these difficulties, Faggin delivered the CPU in December 1970. The 4004 was built using new silicon metal oxide semiconductor (MOS) gate technology. It contained 2,300 random logic transistors, with a basic instruction cycle time of 10.7us, or approximately 92,000 instructions per second. While this level of performance doesn’t exactly turn heads today, the 4004’s ability to deliver that kind of performance in a four-chip design was a breakthrough in 1971.

The chip naming convention was also new. Faggin started a new naming method, sequentially grouping 4001 (ROM), 4002 (RAM), 4003 (Shift Register), and 4004 (CPU), to show they were part of the same product family. The Intel 4004 would be replaced by the 8008, 8080, 8085 and finally the 8086. Faggin would go on to found Zilog, one of the earliest competitors (to who?) whose Z80 CPU was popular in the late 70’s and early 80’s. Zilog focused on the microcontroller market rather than chasing Intel in the high-performance microprocessor industry, but the company is still in business today.

As for the 4004, it’s not directly related to the x86 CPUs we use today, but you might think of it as an australopithecus to ours. homo sapiens. While Intel’s “8008” moniker was chosen to build directly on the success of the 4004, the 4004 and 8008 designs are unrelated and it was the latter that led to the x86.

Chips like the 4004 and 8008 (as well as various CPUs from Intel’s competitors like the 6502) a little later showed that there was a thriving market for small microprocessors that were nowhere near as powerful – or expensive – as the mainframe-class hardware available at the time from companies such as IBM. Intel would eventually move away from memory and invest more resources into CPU development.

Looking back at 4004, we underline how far we have come. The 4004 ran at 750KHz. It was shipped at a time when IPC (Instructions Per Cycle) was more generally discussed as CPI – Cycles Per Instruction and required a minimum of ~ 8 clock cycles to process a single instruction. It can be hard to remember that there was a time when a calculator required more chips (and more silicon area in total) than is now needed to power a modern iPhone or Android device. In less than a single human life, we have increased the clock speed by ~ 6000x, not to mention improvements in IPC, power consumption and area.

Happy 50th birthday to 4004 and the microprocessor revolution.

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