Kenbak-1

Kenbak-1

A Kenbak-1 at Deutsches Museum, Munich
Developer	John Blankenbaker
Manufacturer	Kenbak Corporation
Type	Personal computer
Release date	1971; 54 years ago (1971)
Introductory price	US$750 (equivalent to $5,640 in 2023)
Discontinued	1973 (1973)
Units sold	44[1]
Memory	256 bytes of memory

A program running in a Kenbak-1 IDE/emulator. Click to start animation. Note that the program's sole use is to show lights being shifted.

Kenbakuino, an Arduino-based Kenbak-1 emulator

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

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, 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

Add, Subtract, Load, Store, Load Complement, 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]

Opcode matrix for the Kenbak-1 instruction set
High octal digits	Low octal digit
	0	1	2	3	4	5	6	7
00	HALT	SFTR A1	SET 0 b0 XXX	ADD A #XXX	ADD A XXX	ADD A (XXX)	ADD A XXX, X	ADD A (XXX), X
01	HALT	SFTR A2	SET 0 b1 XXX	SUB A #XXX	SUB A XXX	SUB A (XXX)	SUB A XXX, X	SUB A (XXX), X
02	HALT	SFTR A3	SET 0 b2 XXX	LOAD A #XXX	LOAD A XXX	LOAD A (XXX)	LOAD A XXX, X	LOAD A (XXX), X
03	HALT	SFTR A4	SET 0 b3 XXX	STORE A #XXX	STORE A XXX	STORE A (XXX)	STORE A XXX, X	STORE A (XXX), X
04	HALT	SFTR B1	SET 0 b4 XXX	JPD A ≠0 XXX	JPD A =0 XXX	JPD A <0 XXX	JPD A ≥0 XXX	JPD A >0 XXX
05	HALT	SFTR B2	SET 0 b5 XXX	JPI A ≠0 XXX	JPI A =0 XXX	JPI A <0 XXX	JPI A ≥0 XXX	JPI A >0 XXX
06	HALT	SFTR B3	SET 0 b6 XXX	JMD A ≠0 XXX	JMD A =0 XXX	JMD A <0 XXX	JMD A ≥0 XXX	JMD A >0 XXX
07	HALT	SFTR B4	SET 0 b7 XXX	JMI A ≠0 XXX	JMI A =0 XXX	JMI A <0 XXX	JMI A ≥0	JMI A >0 XXX
10	HALT	ROTR A1	SET 1 b0 XXX	ADD B #XXX	ADD B XXX	ADD B (XXX)	ADD B XXX, X	ADD B (XXX), X
11	HALT	ROTR A2	SET 1 b1 XXX	SUB B #XXX	SUB B XXX	SUB B (XXX)	SUB B XXX, X	SUB B (XXX), X
12	HALT	ROTR A3	SET 1 b2 XXX	LOAD B #XXX	LOAD B XXX	LOAD B (XXX)	LOAD B XXX, X	LOAD B (XXX), X
13	HALT	ROTR A4	SET 1 b3 XXX	STORE B #XXX	STORE B XXX	STORE B (XXX)	STORE B XXX, X	STORE B (XXX), X
14	HALT	ROTR B1	SET 1 b4 XXX	JPD B ≠0 XXX	JPD B =0 XXX	JPD B <0 XXX	JPD B ≥0 XXX	JPD B >0 XXX
15	HALT	ROTR B2	SET 1 b5 XXX	JPI B ≠0 XXX	JPI B =0 XXX	JPI B <0 XXX	JPI B ≥0 XXX	JPI B >0 XXX
16	HALT	ROTR B3	SET 1 b6 XXX	JMD B ≠0 XXX	JMD B =0 XXX	JMD B <0 XXX	JMD B ≥0 XXX	JMD B >0 XXX
17	HALT	ROTR B4	SET 1 b7 XXX	JMI B ≠0 XXX	JMI B =0 XXX	JMI B <0 XXX	JMI B ≥0 XXX	JMI B >0 XXX
20	NOOP	SFTL A1	SKP 0 b0 XXX	ADD X #XXX	ADD X XXX	ADD X (XXX)	ADD X XXX, X	ADD X (XXX), X
21	NOOP	SFTL A2	SKP 0 b1 XXX	SUB X #XXX	SUB X XXX	SUB X (XXX)	SUB X XXX, X	SUB X (XXX), X
22	NOOP	SFTL A3	SKP 0 b2 XXX	LOAD X #XXX	LOAD X XXX	LOAD X (XXX)	LOAD X (XXX)	LOAD X (XXX), X
23	NOOP	SFTL A4	SKP 0 b3 XXX	STORE X #XXX	STORE X XXX	STORE X (XXX)	STORE X XXX, X	STORE X (XXX), X
24	NOOP	SFTL B1	SKP 0 b4 XXX	JPD X ≠0 XXX	JPD X =0 XXX	JPD X <0 XXX	JPD X ≥0 XXX, X	JPD X >0 XXX
25	NOOP	SFTL B2	SKP 0 b5 XXX	JPI X ≠0 XXX	JPI X =0 XXX	JPI X <0 XXX	JPI X ≥0 XXX	JPI X >0 XXX
26	NOOP	SFTL B3	SKP 0 b6 XXX	JMD X ≠0 XXX	JMD X =0 XXX	JMD X <0 XXX	JMD X ≥0 XXX	JMD X >0 XXX
27	NOOP	SFTL B4	SKP 0 b7 XXX	JMI X ≠0 XXX	JMI X =0 XXX	JMI X <0 XXX	JMI X ≥0 XXX	JMI X >0 XXX
30	NOOP	ROTL A1	SKP 1 b0 XXX	OR #XXX	OR XXX	OR (XXX)	OR XXX, X	OR (XXX), X
31	NOOP	ROTL A2	SKP 1 b1 XXX	—	—	—	—	—
32	NOOP	ROTL A3	SKP 1 b2 XXX	AND #XXX	AND XXX	AND (XXX)	AND XXX, X	AND (XXX), X
33	NOOP	ROTL A4	SKP 1 b3 XXX	LNEG #XXX	LNEG XXX	LNEG (XXX)	LNEG XXX, X	LNEG (XXX), X
34	NOOP	ROTL B1	SKP 1 b4 XXX	JPD UNC XXX	JPD UNC XXX	JPD UNC XXX	JPD UNC XXX	JPD UNC XXX
35	NOOP	ROTL B2	SKP 1 b5 XXX	JPI UNC XXX	JPI UNC XXX	JPI UNC XXX	JPI UNC XXX	JPI UNC XXX
36	NOOP	ROTL B3	SKP 1 b6 XXX	JMD UNC XXX	JMD UNC XXX	JMD UNC XXX	JMD UNC XXX	JMD UNC XXX
37	NOOP	ROTL B4	SKP 1 b7 XXX	JMI UNC XXX	JMI UNC XXX	JMI UNC XXX	JMI UNC XXX	JMI UNC XXX

History

The Kenbak-1, released in early 1971, is considered by the Computer History Museum to be the world's first personal computer. It was designed and invented by John Blankenbaker of Kenbak Corporation in 1970, and was first sold in early 1971. Unlike a modern personal computer, the Kenbak-1 was built of small-scale integrated circuits, and did not use a microprocessor. The system first sold for US$750. Only 44 machines were ever sold, though it's said 50 to 52 were built. In 1973, production of the Kenbak-1 stopped as Kenbak Corporation folded.

With a fixed 256 bytes of memory, input and output restricted to lights and switches (no ports or serial output), and no possible way to extend its capabilities, the Kenbak-1 was only really useful for educational use.

256 bytes of memory, 8 bit word size, and I/O limited to switches and lights on the front panel are also characteristics of the 1975 Altair 8800, whose fate was diametrically opposed to that of the Kenbak. However, there were three major differentiating factors between the Altair and the Kenbak which led to the later Altair 8800 selling over 25000 units and influencing many, while the Kenbak-1 only sold 44, and influenced mostly no one.

• The Kenbak-1, designed before the invention of the microprocessor, had a limited instruction set that was professionally considered "incompatible with microcomputer application goals", according to a citation pointing at the KENBAK-1 programming manual in the contemporary February 1974 issue of RCA Engineer Magazine.
• The Kenbak-1 had no ability for expansion. There were no expansion slots, and tragically, no serial port or any other way to get data out of the machine (other than the 8 lamps on the front). There was also no way to load data into the machine other than its physical switches. There was no ability to upgrade the capacity of the RAM, and even if there were, there would have been no way to simultaneously address more than 256 bytes of RAM due to limitations of the machine code language.
• The Kenbak-1 was not advertised outside of the educational market. It was advertised in Science magazine and in-person at a local teacher's convention. There was no attempt to market the machine at the hobbyist market as later successful computers did. John Blankenbaker would later cite this as the reason that his machine failed, as the educational market was "too slow" to adopt his machine while it could have been relevant. However, it is also worth noting that in the educational market, the Kenbak-1 was competing against timeshares of more capable and established computers such as the PDP-8.

If the Kenbak-1 were advertised better, and the machine had at least one serial port to make it more useful, it may have done very well at its price-point of $750 in 1971, which no other Turing-complete computer on the market came close to. However, it would not be very long before personal computers based on the much more capable Intel 8008 would come to market, followed shortly after once again by the ten-times-as-fast Intel 8080, in the highly-expandable Altair 8800.

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. "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. 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. "Programming Reference Manual KENBAK-l Computer"

External links

• Kenbak.com Comprehensive Kenbak-1 history and technical information
• KENBAK-1 Computer Article
• KENBAK-1 Computer – Official Kenbak-1 website at www.kenbak-1.info
• Kenbak-1 Emulator – Kenbak-1 emulator in JavaScript
• Kenbak-1 Emulator – Kenbak-1 Emulator download
• Kenbak 1 – Images and information at www.vintage-computer.com
• Kenbak documentation at bitsavers.org