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Cray-1 fantasies


EricBall

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Something about the Cray-1 keeps dragging my attention back to it - mainly wondering whether it would be possible to create one using modern chips etc. Of course some one has already done it: http://chrisfenton.com/homebrew-cray-1a/ but that doesn't stop me from thinking about how I would do it.

 

From a hardware perspective, there's a certain elegance about the design methodology Seymour Cray used building all of the logic from a single chip which contained a 4 input AND and a 5 input AND with normal and inverted outputs. If the circuit diagrams were available it would be trivial to convert into modern logic.

 

One big improvement would be in main memory. The Cray-1 arranged memory in 16 banks with each bank holding up to 64K words; each word being 64 bits wide with 8 bits of error correction (a total of 8Mbytes+ECC) - built from 1K by 1 bit chips!. Today a single chip from IDT provides 64K by 16 bits. So the main memory would only require 64 chips (not including ECC).

 

But this reveals one of the main challenges in recreating the Cray-1 (other than lack of detailed design information) - bus width and interconnection. Each Cray-1 module had a 96 pin connector with individual twisted pairs interconnecting modules. So while each module PCB only needs two signal planes the modules interconnection is a three dimensional rat's nest. Using wires also meant timing could be equalized by the simple method of making all of the wires the same length.

 

However, this is really a think exercise as even if I were to spend the time, money & effort to recreate a Cray-1, I'd then have to spend even more time & effort creating software to make use of my toy.

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