ILLIAC II

The ILLIAC II was a revolutionary super-computer built by the University of Illinois that became operational in 1962.

The concept, proposed in 1958, pioneered Emitter-coupled logic (ECL) circuitry, pipelining, and transistor memory with a design goal of 100x speedup compared to ILLIAC I.

ILLIAC II had 8192 words of core memory, backed up by 65,536 words of storage on magnetic drums. The core memory access time was 1.8 to 2 µs. The magnetic drum access time was 8.5ms. A "fast buffer" was also provided for storage of short loops and intermediate results (similar in concept to what is now called cache). The "fast buffer" access time was 0.25 µs.

The word size was 52 bits.

Floating point numbers used a format with 7 bits of exponent (power of 4) and 45 bits of mantissa.

Instructions were either 26 bits or 13 bits long, allowing packing of up to 4 instructions per memory word.

Rather than naming the pipeline stages, "Fetch, Decode, and Execute" (as on Stretch), the pipelined stages were named, "Advanced Control, Delayed Control, and Interplay".

During check-out of the ILLIAC II, before it became fully operational, faculty member Donald B. Gillies programmed ILLIAC II to search for mersenne prime numbers. The check-out period took roughly 3 weeks, during which the computer verified all the previous Mersenne primes and found three new prime numbers. The results were immortalized for more than a decade on a UIUC Postal Annex cancellation stamp, and were discussed in the New York Times, recorded in the Guinness Book of World Records, and described in a journal paper in Mathematics of Computation.

The ILLIAC II computer was disassembled roughly a decade after its construction. By this time the hundreds of modules were obsolete scrap; many faculty members took components home to keep. Donald B. Gillies kept 12 (mostly control) modules. His family donated 10 of these modules and the front panel to the University of Illinois CS department in 2006. The photos in this article were taken during the time of donation.

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