Bendix G-15 - Architecture

Architecture

The G-15 was a serial-architecture machine, one of several inspired by the ACE. It used a magnetic drum to simulate the recirculating delay line memory of other serial designs. Each track had a set of read and write heads; as soon as a bit was read off a track, it was re-written on the same track a certain distance away. The length of delay, and thus the number of words on a track, was determined by the spacing of the read and write heads, the delay corresponding to the time required for a section of the drum to travel from the write head to the corresponding read head. Under normal operation, data were written back without change, but this data flow could be intercepted at any time, allowing the machine to update sections of a track as needed.

This arrangement allowed the designers to create "delay lines" of any desired length. In addition to the twenty "long lines" of 108 words each, there were four more short lines of four words each. These short lines recycled at 27 times the rate of the long lines, allowing fast access to frequently needed data. Even the machine's accumulators were implemented as drum lines: three double-word lines used for intermediate storage and double-precision addition, multiplication, and division in addition to a one single-word accumulator. This use of the drum rather than flip-flops for the registers helped to reduce tube count.

A consequence of this design was that, unlike other computers with magnetic drums, the G-15 did not retain its memory when it was shut off. The only permanent tracks were two timing tracks recorded on the drum at the factory. The second track was a backup, as the tracks were liable to erasure if one of their amplifier tubes shorted.

The serial nature of the G-15's memory was carried over into the design of its arithmetic and control circuits. The adders worked on one binary digit at a time, and even the instruction word was designed to minimize the number of bits in an instruction that needed to be retained in flip-flops (to the extent of leveraging another one-word drum line used exclusively for generating address timing signals).

The G-15 had 180 vacuum tube packs and 300 germanium diodes. It had a total of about 450 tubes (mostly dual triodes). Its magnetic drum memory held 2,160 words of twenty-nine bits. Average memory access time was 14.5 milliseconds, but its instruction addressing architecture could reduce this dramatically for well-written programs. Its addition time was 270 microseconds (not counting memory access time). Single-precision multiplication took 2,439 microseconds and double-precision multiplication took 16,700 microseconds.