Bendix G-15

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The Bendix G-15 was an early low-cost (and thus low-performance) computer, produced by Bendix Computer Division (later bought by Control Data), from a design by Harry Huskey. His design was based on the ACE, on which Huskey had worked while he was in the UK. It was first delivered in 1956; around 400 were sold (legend has it that the last one went to Harry Huskey personally).

It was a serial machine, which used a drum for its main memory; like the ACE on which it was based, it was capable of optimum programming. The drum rotated at 1500 RPM, and held 20 'long' tracks, each containing 108 words, which were 29 bits wide; the words within a track were numbered with 'timing numbers'. The logic was built out of vacuum tubes; the G-15 could perform 1,850 additions per second.

Standard input/output equipment included an electric typewriter, and a paper tape reader and punch; up to 4 magnetic tape drives were available as an option, along with a punched card reader and punch. A digital differential analyzer was available as a co-processor for solving certain types of problems.

Some sources consider the Packard Bell PB 250 to be a descendant of the G-15, but the strength of the connection (if any) is not yet known.


The G-15's instructions were also 29 bits wide. They contained 8 fields:

Field Name Width Description
SD 1 Single- or double-length
D 5 Destination; track where result is to be stored
S 5 Source; track holding source operand
CH 2 Characteristic; selected behaviour of transfers
N 7 Next; timing number of next instruction
B 1 Breakpoint
T 7 Transfer timing number
ID 1 Deferred instruction (if '1')

For operands in long tracks, the timing number used was selected by the 'Transfer' field. Instructions contained up to 3 addresses; a source, a destination, and the address of the next instruction. (The G-15 had no Program Counter.) The 'next' instruction was in the same track (hence only its timing number is given); control could be passed to a different track (the equivalent of a jump) with a specific instruction.

Note that there is no operation code in the instruction; instead, combinations of certain source and destination values and characteristics had side-effects. (A complete list may be found on pg. 7 of the 'Coding Manual for the Bendix G-15', below.)

In addition to the 20 long tracks, the drum also had 4 quad-word lines, 3 double-word lines (for double-length operands, the "ID = 'cand denominator, MQ = multiplier quotient, and PN = product numerator" - the G-15 had hardware multiplication and division), and 1 single-word line (the accumulator, AR). The shorter ones had the role of registers in most computers, and came with lower access times); to achieve that, the read and write heads were placed closer together around the circumference of the track.

Having a breakpoint bit in every instruction simplified debugging; no prior preparation was needed, to begin at any point in time.

Further reading

  • Harry D. Huskey, David C. Evans, The Bendix G-15 General Purpose Computer - the best of a bad lot, as far as comprehensible documentation goes
  • Harry D. Huskey, Granino A. Korn, (editors), Computer Handbook, McGraw-Hill, New York, 1962 - the G-15 is covered on pp. 20-15—20-19
  • Harry D. Huskey, From ACE to the G-15, Annals of the History of Computing, Vol. 6, No. 4, Oct0ber, 1984, pp. 350-371

External links