Jump to content


From Wikipedia, the free encyclopedia
A Kenbak-1 at the Computer History Museum
DeveloperJohn Blankenbaker
ManufacturerKenbak Corporation
TypePersonal computer
Release date1971; 53 years ago (1971)
Introductory priceUS$750 (equivalent to $5,640 in 2023)
Discontinued1973 (1973)
Units sold44[1]
Memory256 bytes of memory

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]

Since the Kenbak-1 was invented before the first microprocessor, the machine did not 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[edit]


Kenbak-1 registers
07 06 05 04 03 02 01 00 (bit position)
Main registers
X X (Index)
P Program Counter
000000 C O A flags
000000 C O B flags
000000 C O X flags
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, also known as the index register, turns the direct and indirect modes into indexed direct and indexed indirect modes. It also has a 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[edit]

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[edit]

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[edit]

  • 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


  1. ^ a b "Oral History of John Blankenbaker" (PDF). Computer History Museum. June 14, 2007.
  2. ^ "What was the First PC?". Retrieved October 25, 2022.
  3. ^ "PastExhibits - 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. ^ "The man who made 'the world's first personal computer'". November 6, 2015 – via www.bbc.com.
  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. ^ a b 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". www.kenbak-1.net.
  15. ^ "Kenbak Theory of Operation Manual". p. 16.
  16. ^ "Official Kenbak-1 Reproduction Kit".
  17. ^ a b "Programming Reference Manual KENBAK-l Computer"

External links[edit]