View source for KENBAK-1

The '''KENBAK-1''' (many Web sites give the name as 'Kenbak-1', but scans of original manuals show the capitals were thee usual) was the first hobbyist [[personal computer]]. (Depending on one's exact definition of 'personal computer', it has been argued that it was the first personal computer, overall, but that was likely the [[LINC]]; with the added 'hobbyist' qualifier, it probably is accurate.) It was first produced in 1971, by John V. Blankenbaker, and intended for use in teaching [[programming]]. It was not commercially successful - only about 52 were made - and did not introduce any important technical ideas, but it was still historic.

Like other early personal computers (such as the [[Datapoint 2200]]), the Kenbak-1 preceded [[microprocessor]]s being available. Like them, its [[Central Processing Unit|CPU]] is made out of [[SSI]] [[transistor-transistor logic|TTL]] [[integrated circuit|ICs]], from the [[74 series]] family. It is a [[serial]] computer (which, like many of the [[:Category:Early Computers|earliest computers]], which reduced the cost, by reducing the component count), with a 1-bit [[arithmetic logic unit|ALU]]. It had a clock rate of 1 megahertz, but the serial nature meant it [[execute]]d about 480 [[instruction]]s per second (for typical 2-byte instructions).

Its [[main memory]] is built out of [[shift register]]s. It had no [[input/output|I/O]] capability; it was programmed by storing data in memory with the [[front panel]].

==Architecture==

It had a word size of one byte (8 bits), the ALU's width. It was basically a [[load-store architecture|load-store]] machine; most instructions had 5 [[addressing mode]]s:

* immediate (constant)
* memory
* indirect (deferred)
* indexed
* indirect indexed

(Bit instructions and [[jump]]s did not support all 5 modes.) Most instructions were two bytes long: the second byte's contents depended on the addressing mode; the first byte contained two bits of register specification, three bits of [[operation code|opcode]], and three bits of addressing mode. There were roughly 27 different instructions (ones marked with '!' did not use the format described above; the digit is the number of different versions):

* ADD
* SUB
* LOAD
* STORE
* AND !1
* OR !1
* LNEG !1
* JUMP !10
* SKIP !2
* SET !2
* SHIFT !2
* ROT !2
* NOP !1
* HALT !1

All jumps were [[conditional branch]]es. The KENBAK-1 had 4 [[register]]s:

* A - the [[accumulator]]
* B - a secondary accumulator
* X - the [[index register]]
* P - the [[Program Counter]]

It had two [[condition codes]], set by the ADD and SUB instructions. 

==External links==

* [http://bitsavers.org/pdf/kenbak/ Kenbak] - documentation at [[Bitsavers]]
** [http://bitsavers.org/pdf/kenbak/KENBAK-1_TheoryOfOperation_Sep72.pdf Installation, Maintenance and Theory of Operation KENBAK-1 Computer]
** [http://bitsavers.org/pdf/kenbak/KENBAK-1_PgmgRef_Apr71.pdf Programming Reference Manual KENBAK-1 Computer]
* [https://www.kenbak.com/ Kenbak-1 Computer]
** [https://www.kenbak.com/kenbak_registry Kenbak Registry] - approximately 14 originals are left
** [https://www.kenbak.com/design Design of the Kenbak-1] - detailed internal description
** [https://www.kenbak.com/footinycadschematics TinyCAD Schematics] - full schematics
* [http://kenbak-1.com/ Welcome to KenbakKit.com!] - "Home of the Official Kenbak-1 Reproduction Kit!"
* [https://bytecollector.com/kenbak_1.htm Kenbak-1] - no longer maintained, but useful info and links
* [https://www.kenbak-1.net/index.htm KENBAK-1 Computer]
* [https://onlineonly.christies.com/s/firsts-history-computing-paul-g-allen-collection/kenbak-1-digital-computer-121/230059 Kenbak-1 digital computer] - from [[Paul Allen]]'s collection at the [[Living Computer Museum|LCM]], includes several images
* [https://oldcomputermuseum.com/kenbak-1.html Kenbak-1]

[[Category: Personal Computers]]