Cray-1
Entry posted by EricBall
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Even though I didn't get into computers until several years after the Cray-1 first came to market, it was still a legendary computer.
Just the look and shape of it was awe inspiring; looking like something from 2001: A Space Odyssey (or any other SF film). No simple box, but a C shaped tower with a "love seat". Then you learned that "love seat" was the refrigeration system - it was cooled with Freon - how cool was that? A computer that got so hot it needed a cooling system. (Which actually wasn't that uncommon for large systems of the time. But when your point of reference is a fanless home computer....)
The actual design of the Cray-1 is interesting. Seymour Cray took the knowledge he gained working on the CDC supercomputers and built something very advanced.
One major design change he made was adding a huge number of registers for temporary storage, including vectors. Up to that point vector processors processed data stored in main memory. Seymour realized memory was too slow so a significant performance improvement could be gained by having a large number of registers and performing all operations on those registers. This is the same "load/store architecture" most RISC processors use. The Cray-1 had over 5 kilobytes of register storage!
The Cray-1 also had 12 independent functional units. Each was pipelined so they could process multiple instructions without delays. The vector and floating point units also had the ability to automatically forward the result to another functional unit, called chaining. With judicious use of this with vector processing the Cray-1 could hit 250MFLOPS under ideal conditions. (Although I'm not certain how, since it only had 3 floating point functional units and ran at 80MHz, thus 240MFLOPS.)
Yep, the Cray-1 had a 80MHz system clock; which doesn't sound that fast today. However, the Apple 2, available at the same time had a 1MHz system clock and the original IBM PC, which came 5 years later, had a 4.77MHz clock. So for the time 80MHz was insanely fast. Also consider the Cray-1 was made from LSI chips: 1Kbit SRAM for main memory (max 8Mbyte+1Mbyte ECC), 16x4 bit 6ns registers, and fast & slow NAND chips.
But while a microprocessor is typically less than an inch square, the Cray-1 "processor" was spread over 340 square feet circuit board (512 6x8 PCBs). So the design of the Cray-1 had to account for propagation delays, and ensure the delays were equalized as much as possible. (Either by cutting wires to precise lengths or adding extra capacitance to PCB traces.) The curve of the physical frame was also to minimize distance.
In order to be as fast as possible, the Cray-1 was made using ECL (emitter coupled logic) circuits. ECL circuits are very fast, but require a lot of power because they never "turn off". This led to the built-in cooling system, which always sounded so impressive.
Part of me wonders how small and how fast a Cray-1 made with modern technologies would be. There are certainly Cray emulators out there. But they probably only worry about duplicating the ISA, and are not clock accurate. Unfortunately, even if schematics were available it wouldn't be possible to simply "port" the design to modern technologies as much of the Cray-1's design was overcoming physical limitations. But it's fun to dream.
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