the KENBAK-1 Computer



arguably the first personal computer

What is the Kenbak-1?

The Kenbak-1 has often been considered "the first personal computer." It was designed, built, and marketed by John Blankenbaker in 1971, and was made from Small Scale Integrated (SSI) circuits before microprocessors were available. In spite of it not being commercially successful (only about 50 were made and sold) it made a lasting impact. It was named the "first personal computer" by a group of computer industry experts at a contest by Boston's Computer Museum in 1986. It is still considered the first personal computer by the Computer History Museum in Mountain View, CA, the Computer Museum of America in Roswell Georgia, the Deutsches Museum, in Munich Germany, and most major computer museums.

For John V. Blankenbaker's biography, click here. For more information about this title of "the first personal computer and the "First PC Contest" click here.

John Blankenbaker with his computer in 1986 (photo courtesy of Jerry Rabinowitz)

Advertisement in Scientific American, 1971

What was it Used for?

From the beginning, the Kenbak-1 was marketed as a computer for learning computer concepts and machine code programming. It was limited to just 256 bytes of memory, 8 input switches and 8 output lights, but its rich instruction set and capabilities made it similar to the "big computers" of the time, and actually easier to program than some of the microprocessors which followed.

According to John Blankenbaker, some of his first sales were made to educational institutions. "The first two Kenbak-1 computers were sold to a private girl's school in Hawaii. Two were sold to a private boy's school. Four were sold to a high school in NY. Foreign sales included Mexico, Canada, France, Spain, and Italy. Several community/technical colleges bought some. Individuals bought some." This advertisement on the left from Scientific American was the most successful (and expensive) ad, but John did get advertisements and mentions in Amateur Computer Society Newsletter and Computer World and even some educator magazines like College Management and Nations Schools.

How Was it Used?

To the user/programmer, the Kenbak-1 appears as a memory bank of 256 8-bit numbers (bytes) with a unusually "rich" instruction set to perform calculations. Some of those 256 memory locations have "special functions" such as special math registers or the input/output functions. The programmer would manually load "machine code" instructions and data into the desired memory locations with the front panel buttons, then press the "start" button to execute that program.

The instruction set was unusually extensive for a simple machine with only 256 bytes. It included the usual 8-bit math instructions (add, subtract, complement) bit manipulation and shifting, conditional jumps, and indexed or indirect memory addressing. It was a very "complete" instruction set. Tom Crosley, an early user, said he felt the instruction set was better than the PDP-8 or the Intel 8080 which followed.

After storing the program in memory, the starting location would be stored in the "P Register" (memory location 3) the start button would be pressed, and the program would begin. The simple program at the right will just count from zero to 255 repeatedly on the front panel lights. See an online emulator running this below. For more details about programming, and some example programs, CLICK HERE.

How was it Electronically Designed?

At the time the Kenbak-1 was designed and made, microprocessors were not invented yet. So the Kenbak-1 was designed with small scale integration (SSI) logic chips. Individual logic gates, inverters, and latches were used, as well as a few transistors, resistors, capacitors, and diodes. A state-machine with 29 states controlled the internal workings. The Arithmetic Unit was implemented with a serial adder, where a single bit of the answer is processed on each clock tick. This reduced the chip count greatly, but at the expense of execution speed, so the Kenbak-1 could only execute about 750 instructions per second (for typical 2 byte instructions), in spite of a clock speed of 1 megahertz. About 131 different SSI chips were needed to implement this state machine, ALU, and extra logic.

Making 256 bytes of memory was not simple at the time. Random access memory chips were not available, and many computers and calculators of the time used "serial memory." This was sometimes implemented in period computers using waves in a pool of mercury, or torsion waves traveling down a metal wire. Luckily, Intel developed a 1024-bit shift register integrated circuit about this time, so two of these were utilized to give the 256 bytes. Every time an instruction or data was to be read or written to memory, a wait of 512 clock cycles, on average, would have to pass for the correct byte to circulate through the memory so it could be read. John figured the slower speed wouldn't be a problem, and might allow learners to see the progress of their program's execution. For detailed information about the circuitry, and electronic implementation, click here for details about the Circuit Design.

Just one page, of the 26 pages of logic schematics, including the shift register memory chips.

One of 3 pages of state diagrams. Left click to open in new window.

How Can I Get My Own Kenbak-1?

While original Kenbak-1 computers are rare, modern day emulators (or simulators) and reproductions readily available.

The best "free" way to play with a Kenbak-1 is through an online emulator, and a favorite is this one from neocomputer.org. It's running the "simple counting program" above. This emulator is remarkably similar to using the real thing at no cost at all.

An online emulator from neocomputer.org

Three different Kenbak-1 emulators

from adwaterandstir.com

An even more authentic experience comes from several of the physical Kenbak-1 emulators which are designed around modern single chip computers. While they don't implement the low level circuitry and state-machine logic, the experience is very accurate. My favorite is from adwaterandstir.com web site, which makes and sells several models of Kenbak-1 emulators. Chris Davis, who runs the website, even located the original "Bud Industries Grand Prix" case specifications, so he could authentically make up a metal cases for these reproductions.

Is It Possible To Make a Kenbak-1 From the Original Components and PC Boards?

Yes, that's exactly what a couple people did from 2005 to 2009. As Erik Klein put details about his machine on the internet, and the schematics and documentation became available, at least two individuals made up reproduction Kenbak-1's. from the original plans.

In 2005 I made several Kenbak-1's and sold a couple of them privately and on eBay. Soon afterwards, Grant Stockly made and sold a "Kenbak-1 Series 2" kit which made it easy to make a high-quality replica. He stopped selling his kits or updating his site around 2008, but his website is still up, and has great information.

For information about making your own Kenbak-1, with downloadable PC board artwork, see this Making a Reproduction Kenbak-1 page.

But I Really Want an Original Kenbak-1!

The Nova Scotia Computer Museum's hoard of seven

computers, some labeled Kenbak-1, some labeled CTI,

before they were scattered all over the world.

(photo courtesy Herbert Eisengruber)

You aren't likely to stumble upon one at a flea market, but maybe. Of around 50 Kenbak-1's originally made, few still exist, and when they come up for sale, they are expensive. There are 14 documented computers in existence, but some private collectors (and even some museums) aren't forthcoming about what they have, or where they came from. Also, when sold at auction, both buyers and sellers can remain anonymous. This makes it difficult to track machines.

We began trying to track all original Kenbak-1 computers around 2005 with some success. For our best accounting of all of the existing Kenbak-1 computers are, see our LIST OF ALL REMAINING ORIGINAL KENBAK-1 COMPUTERS which lists all known original Kenbak-1 Computers.