The Kenbak-1 is considered by many to be the world's first "Personal Computer." The Computer History Museum granted it this designation when they were still located in Boston in 1986. More specifically, the machine represents the first commercially available Von Neumann (stored program) computing device intended and priced for personal use.
John V. Blankenbaker designed the Kenbak-1 and marketed in the pages of Scientific American in 1971. The machine's name was taken from the middle of John's last name.
The Kenbak-1 was designed in 1970 and pre-dated microprocessors. The Intel 4004 (the worlds first microprocessor) was introduced in 1971. Instead of being microprocessor based the Kenbak-1 was built almost entirely from TTL components.
Unlike many earlier machines and calculating engines, the Kenbak-1 was a true stored-program computer that offered 256 bytes of memory, a wide variety of operations and a speed equivalent to nearly 1MHz.
Approximately 40 of these machines were built and sold before they were discontinued. The world just wasn't quite ready for personal computing and the Kenbak-1 lacked some critical capabilities (such as expandability and I/O) that were needed to foster the revolution. 14 are currently known to exist with few more likely to be discovered.
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Contributors: Erik Klein
Julian Skidmore (Manchester, UK) comments:
Although the Kenbak-1's oscillator is 1MHz, it's clock speed was half that at 500KHz and simple calculations show it must execute instructions at an astonishingly low rate 300 instructions per second or less. This is because:
1. Memory is 1024 bits of serial memory, so random access will take 512 cycles on average - 1000 access per second.
2. CPU registers are placed in main memory too, and a typical instruction involves reading/writing the program counter, the accumulator and the instruction itself. Thus a maximum 1000/3 = 333 instructions can execute per second.
Having said that, the architecture is impressive. It looks like a typical 8-bit CPU: with several registers, multiple addressing modes including indexed addressing and variable length instructions.
Please consider donating your old computer / videogame system to Old-Computers.com or one of our partners from anywhere in the world (Europe, America, Asia, etc.).
I own the only prototype, visited John Blankenbaker several times and he even signed the computer for me. I have some information, pictures and even a video on my website.
In 2022 John gave me the permission to preserve his legacy and continue his former website kenbak-1.net. It is now hosted here: kenbak-1.info.
Although the Kenbak-1's oscillator is 1MHz, it's clock speed was half that at 500KHz and simple calculations show it must execute instructions at an astonishingly low rate 300 instructions per second or less. This is because
1. Memory is 1024 bits of serial memory, so random access will take 512 cycles on average - 1000 access per second.
2. CPU registers are placed in main memory too, and a typical instruction involves reading/writing the program counter, the accumulator and the instruction itself. Thus a maximum 1000/3 = 333 instructions can execute per second.
Having said that, the architecture is impressive. It looks like a typical 8-bit CPU: with several registers, multiple addressing modes including indexed addressing and variable length instructions.
Sunday 1st July 2007
Julian Skidmore (Manchester, UK)
Julian Skidmore has some great comments about speed, but I have a few corrections. While the nominal clock speed is 1 MHz as stated, and there is a divider which divides it down to two 500 KHz clock phases signals for the shift registers, these Intel 1404A shift registers are actually two lines multiplexed, so each of the clock phases cause one input and output data, so the bit rate still stays at 1 million bits per second. But she''s off by a factor of 3 on the instruction rate.
And while Mike Corregan has some great analysis, and did some python emulations, you can''t beat measuring the instruction time on a real original Kenbak-1 computer. On my computer, which has a clock rate a bit low (800 KHz, maybe due to aging of resistors and capacitors) the standard "count and display" program executes at 768 instructions/second, and those are all 2-byte instructions. If my clock rate was increased to the nominal 1 MHz, I''d expect the instruction rate to be close to 1000 instructions//second. The program on the website kenbak.com when run, flashes the high-order bit at once/second, which is 256 loops, and 3 instructions/loop, so 768 instructions/second.)
