Kenbak-1

The Kenbak-1 is considered by the Computer History Museum,[2] the Computer Museum of America[3] and the American Computer Museum[4] to be the world's first "personal computer",[5] invented by John Blankenbaker (born 1929) of Kenbak Corporation in 1970 and first sold in early 1971.[6] Less than 50 machines were ever built, using Bud Industries enclosures as a housing.[1] The system first sold for US$750.[7] Today, only 14 machines are known to exist worldwide,[8][9] in the hands of various collectors and museums. Production of the Kenbak-1 stopped in 1973,[10] as Kenbak failed and was taken over by CTI Education Products, Inc. CTI rebranded the inventory and renamed it the 5050, though sales remained elusive.[11]

Kenbak-1
A Kenbak-1 at the Computer History Museum
DeveloperJohn Blankenbaker
ManufacturerKenbak Corporation
TypePersonal computer
Release date1971 (1971)
Introductory priceUS$750 (equivalent to $5,420 in 2022)
Discontinued1973 (1973)
Units sold44[1]
Memory256 bytes of memory

Since the Kenbak-1 was invented before the first microprocessor, the machine didn't have a one-chip CPU but was instead based purely on small-scale integration TTL chips.[12] The 8-bit machine offered 256 bytes of memory,[13] implemented on Intel's type 1404A silicon gate MOS shift registers.[14] The clock signal period was 1 microsecond (equivalent to a clock speed of 1 MHz), but the program speed averaged below 1,000 instructions per second due the many clock cycles needed for each operation and slow access to serial memory.[12]

The machine was programmed in pure machine code using an array of buttons and switches. Output consisted of a row of lights.

Internally, the Kenbak-1 has a serial computer architecture, processing one bit at a time.[15][16]

Technical description

Registers
Kenbak-1 registers
07 06 05 04 03 02 01 00 (bit position)
Main registers
A A
B B
X X (Index)
P Program Counter
Flags
000000 C O A flags
000000 C O B flags
000000 C O X flags
Input/Output
Output Lights
Input Switches

The Kenbak-1 has a total of nine registers. All are memory mapped. It has three general-purpose registers: A, B and X. Register A is the implicit destination of some operations. Register X is also known as the index register and turns the direct and indirect modes into indexed direct and indexed indirect modes. It also has program counter, called Register P, three "overflow and carry" registers for A, B and X, respectively, as well as an Input Register and an Output Register.[17]

Addressing modes

Add, Subtract, Load, Store, Load Compliment, And, and Or instructions operate between a register and another operand using five addressing modes:

  • Immediate (operand is in second byte of instruction)
  • Memory (second byte of instruction is the address of the operand)
  • Indirect (second byte of instruction is the address of the address of the operand)
  • Indexed (second byte of instruction is added to X to form the address of the operand)
  • Indirect Indexed (second byte of instruction points to a location which is added to X to form the address of the operand)

Instruction table

The instructions are encoded in 8 bits, with a possible second byte providing an immediate value or address. Some instructions have multiple possible encodings.[17]


See also
  • Datapoint 2200, a contemporary machine with alphanumeric screen and keyboard, suitable to run non-trivial application programs
  • Mark-8, designed by graduate student Jonathan A. Titus and announced as a "loose kit" in the July 1974 issue of Radio-Electronics magazine
  • Altair 8800, a very popular 1975 microcomputer that provided the inspiration for starting Microsoft
  • Gigatron TTL, a 21st-century implementation of a computer using small-scale integration parts

References
  1. "Oral History of John Blankenbaker" (PDF). Computer History Museum. June 14, 2007.
  2. "What was the First PC?". Retrieved October 25, 2022.
  3. "PastExibits LINK - History of the PC". Retrieved October 13, 2022.
  4. "The George R. Stibitz Computer Pioneer Award". Archived from the original on September 13, 2008. Retrieved August 5, 2008.
  5. "Timeline of Computer History". Computer History Museum. Retrieved July 22, 2008.
  6. BBC News, November 6, 2015
  7. "Kenbak-1 The Training Computer". Computerworld. November 17, 1971. p. 43. Retrieved May 25, 2014.
  8. "List of Extant Kenbak-1 Computers". Kenbak.com. Retrieved 13 October 2022.
  9. "Kenbak-1". Computer Museum of Nova Scotia. Retrieved 19 November 2015.
  10. p. 52, "The First Personal Computer", Popular Mechanics, January 2000.
  11. Robert R Nielsen, Snr (2005). "Inside the Kenbak-1". YouTube. Archived from the original on 2021-12-13. Retrieved 8 November 2015.
  12. Erik Klein. "Kenbak Computer Company Kenbak-1". Old-computers.com. Retrieved May 25, 2014.
  13. Bill Wilson (6 November 2015). "The man who made 'the world's first personal computer'". BBC News.
  14. "Technical".
  15. "Kenbak Theory of Operation Manual". p. 16.
  16. "Official Kenbak-1 Reproduction Kit".
  17. "Programming Reference Manual KENBAK-l Computer"
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