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)
It turns out that I was wrong about the time a ''typical'' instruction takes. The average instruction seems to take about 4ms, with a few taking 2ms and a few taking 6ms. There are lots of reasons for variation, and it is hard to predict. So I wrote a python program which embodies the state machine described in the Kenbak-1 Theory of Operations document. This allows me to put in a Kenbak program and it gives all the details about the timings of each instruction executed. The first thing I noticed is that every instruction takes an integral multiple of 128 byte times (byte time is 16us). So the only possible instruction times are 2ms, 4ms, 6ms, 8ms, 10ms and 12ms (The longest running instruction I could find takes 6 byte times (12ms).
I ran a program which finds all of the fibonacci numbers between 0 and 65536 and the average instruction time was right about 4ms ... 256 instructions per second.
There are some interesting things I noticed:
$ Any instruction placed at address 4 (or 0x84) will take 2ms longer than anywhere else.
$ Any instruction that modifies the X register will take 2ms longer than the same instruction modifiying A or B.
$ Any instruction that has a memory operand whose address (module 128) is less that the instruction''s address (modulo 128) will take 2ms longer than if the memory operand''s address (modulo 128) is greater than the instruction''s address (modulo 128)
