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K > KENBAK COMPUTER COMPANY > Kenbak-1   


KENBAK COMPUTER COMPANY
Kenbak-1

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.

__________

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.


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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)

          
Tuesday 7th September 2021
Mike Corrigan (United States)

Regarding Julian Skidmore''s post: The typical instruction really involves four memory accesses. the program counter, the instruction itself, a register and a memory operand. (Only those with immediate operands and shifts do not have a separate memory operand. So assuming all acccess are random, this would only give 250 instructions per second. The clock was indeed about 512KHz, and the serial memory was two shift registers of 1024 bits each (which were shifted in parallel). This means that one pass through the memory takes about two milliseconds. But, because of the layout of the registers, code and data in memory$ the time between executions of consecutive instructions was almost always about 2 ms. (This is true for all but except store and jmp-and-mark instructions which typically take 4 ms, and indirect and indexed memory modes which could take even more) This presumes that data bytes are placed at higher memory addresses than the code. The machine would access the program counter, the instruction, the memory operand and finally the target register all in a single pass through the memory. At that point it is in position to do it again (the program counter comes right after the registers) The store instruction needed to access the memory operand last causing a second pass through the memory. So the typical instruction took about 2 ms, with some taking 4. Many programs could come near to executing 500 instructions per second.

          
Monday 6th September 2021
Mike Corrigan (United States)

 

NAME  Kenbak-1
MANUFACTURER  Kenbak Computer Company
TYPE  Home Computer
ORIGIN  U.S.A.
YEAR  1971
END OF PRODUCTION  1973
BUILT IN LANGUAGE  None
KEYBOARD  Front panel keys only
CPU  None - TTL based logic
SPEED  approx 1 MHz
RAM  256 bytes of shift-register memory
TEXT MODES  None
GRAPHIC MODES  None
COLORS  None
SOUND  None
SIZE / WEIGHT  18
I/O PORTS  None
POWER SUPPLY  Linear
PRICE  $750 new




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