Is 4-bits dead?

December 13, 2010

Jack Ganssle-December 13, 2010

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?

Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges. Contact him at His website is

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