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By Jack Ganssle

Summary: Four bits is still around, though seems to be as stealthy as an F-117.

Robert Cravotta and I had an interesting talk recently about four-bit processors. He has assembled the Embedded Processing Directory ( which lists a wealth of CPUs, but tiny 4 bitters aren't included. Does that mean they're dead?

Apparently not. In a recent post about the subject, Robert ( identifies several areas where these microcontrollers are still steadily cranking away. One example is the Gillette Fusion ProGlide razor (, a "manual" device that incorporates a four-bitter. "Manual" is in quotes since it is not electric, other than the circuitry included for running the CPU and powering whatever it is the software controls. A gift pack that includes the razor and a number of other manly-grooming accessories costs a mere $8.50 at Walmart ( The razor itself has got to run just a buck or two in manufacturing costs. Even an 8 bit CPU that costs a couple of dimes would eat too much of the BOM. I wonder what the microcontroller costs in the millions of units Gillette buys? It must be pennies.

Breathtaking volumes of low-end products scream for the cost advantages of a truly tiny processor.

It seems, though, that 4 bits has fallen off the radar. The processors are never covered in the press. It's hard to even find datasheets. Here's one ( for the EM6580 from EM Microelectronic. I'm unable to find a programming guide, but the datasheet claims this is a four bit controller with 72 instructions, suggesting it's a long cry from a RISC device.

Small isn't new. Motorola had a single-bit CPU in the 70s that assembled the bit stream into (internal) four bit instructions. Way cool, but hardly new even then. Data General's Nova 1200 minicomputer was a 16 bit machine with a four bit ALU. Logic sequenced operations through the ALU a nibble at a time. The result: Novas were cheap, at least for computers in the pre-microprocessor age. They were "embedded" in all sorts of applications. We put them into instrumentation. Though we yearned for the much nicer architecture of DEC's PDP-11 units, the price difference made that impossible. Cost has always been an important engineering consideration, which surely is the motivation behind today's continued use of nibble-wide CPUs.

Do you use really low-end processors? Which ones and why?

Published December 13, 2010