ILLIAC IV

The ILLIAC IV was one of the first attempts at a massively parallel computer. Key to the design as conceived by Daniel Slotnick, the director of the project, was fairly high parallelism with up to 256 processors, used to allow the machine to work on large data sets in what would later be known as array processing. The machine was to have 4 quadrants. Each quadrant had a Control Unit (CU) and 64 Processor Elements (PEs). Originally Texas Instruments made a commitment to build the Processing Elements (PEs) out of large scale integrated (LSI) circuits. Several years into the project, TI backed out and said that they could not produce the LSI chips at the contracted price. This required a complete redesign using medium scale integrated circuits,leading to large delays and greatly increasing costs. This also led to scaling the system back from four quadrants to a single quadrant, owing to the fact that the MSI version was going to be many times larger than the LSI version would have been. This led to the CU having pull out 'cards' that were on the order of two feet square. For the PEs what should have been chips about 1 inch in diameter were now roughly 6 by 10 inches. Space, power and air conditioning (not to mention budget) did not allow for a four quadrant machine. The machine was 10' high, 8' deep and 50'long. There could be 10-12 instructions being sent from the CU on the wires to the PEs at anytime. The power supplies for the machine were so large that it required designing a single tongue fork lift to remove and reinstall the power supply. The power supply buss bars on the machine spanned distances greater than three feet, and were octopus-like in design. Thick copper, the busses were coated in epoxy that often cracked resulting in shorts and an array of other issues. ILLIAC IV was designed by Burroughs Corporation and built in quadrants in Great Valley, PA during the years of 1967 through 1972. It had a traditional one address accumulator architecture, rather than the revolutionary stack architecture pioneered by Burroughs in the 5500/6500 machines. Illiac IV was designed in fact to be a "back end processor" to a B6700. The cost overruns caused by not getting the LSI chips and other design screw ups by Burroughs (the control unit was built with positive logic and the PEs with negative logic, etc) made the project untenable.

Starting in 1970, the machine became the subject of student demonstrations at Illinois. First, that the project had been secretly created on campus. When this claim proved to be false, the focus shifted to the role of Universities in secret military research. Slotnick was not in favor of running classified programs on the machine. ARPA wanted the machine room encased in copper to prevent off site snooping of classified data. Slotnick refused to do that. He went further and insisted that all research performed on Illiac IV would be published. If the machine had been installed in Urbana this would have been the case. However, two things caused the machine to be delivered to NASA Ames. One was that Slotnick was concerned that the physical presence of the machine on campus might attract violence on the part of student radicals. This and the requirement to do secret research with the machine lead ARPA to move the machine to NASA Ames Research Center, where it was installed in a secure environment. The machine was never delivered to Illinois, arriving in 1972. Rumor has it that simulations run on the machine made the nuclear test band treaties possible. In 1972, when the first (and only quadrant) was operational at NASA, it was 13 times faster than any other machine operating at the time. The Control Unit and a few PEs may be seen today at the Computer History Museum in California.