系统与基础设施chrisfenton.com
自制 Cray-1A 超级计算机
Homebrew Cray-1A
2011 — 2019，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
作者 Chris Fenton 用 FPGA 自制了一台 1/10 比例、功能完整的 Cray-1A 超级计算机复刻品，从逻辑设计到机箱外观都尽量还原。项目后来还促成了 Cray 操作系统 COS 的数字考古与恢复，这台缩小版 Cray 最终被收藏在计算机历史博物馆中，与真机并列展出。
为什么经久不衰：它把传奇超算的硬件设计完整复现并开源，兼具工程深度与历史保存价值，多年来反复登上 HN 首页，至今仍是自制计算机与复古计算领域的经典参考。
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作者透露复刻机已被计算机历史博物馆收藏，与真机并列展出。
这帖子确实被发过很多次——给关注此事的朋友一个近况更新：今年春天，这台 1/10 比例的 Cray-1A 已经被安置在山景城计算机历史博物馆里那台真机的旁边！
fentonc查看英文原文
This really has been posted here a lot - Well, for those following along at home, this story actually got a relatively recent update - as of this spring, the 1/10-scale Cray-1A is now residing next to its bigger self in the Computer History Museum in Mountain View!
Cray-1 的倒数近似技巧与快速平方根倒数同源，都基于浮点数位模式近似对数。
Cray-1 没有除法指令，因为除法耗时不确定；做法是先用恰好 13 个周期算出分母的倒数近似值，再与分子相乘。这背后的技巧与著名的快速平方根倒数相同：浮点数的整数位可以很好地近似其以 2 为底的对数，而对数运算能实现任意 [-1,1] 范围内的常数次幂。我不太确定这种知识为何曾在 Cray 存在、后来又'失传'，猜测与用计算器取代习惯用计算尺（本质就是操作对数）的工程师有关。
nwallin查看英文原文
> This has some interesting implications – there’s no ‘divide’ instruction, for instance, because it can take a variable amount of time to finish. To perform a divide, you need to first compute the ‘reciprocal approximation’ (something we can do in exactly 13 cycles, it turns out) of the denominator value, and then perform a separate multiply of that result with the numerator. Note that the special sauce behind this is the same as the special sauce behind fast inverse square root. The integer bitwise components of a floating point number permit a really good approximation of the log base 2 of it. And once you have the log base 2 of a number simple arithmetic does interesting operations. log2(x^-.5) = -.5 \* log2(x) log2(x^-1) = -log2(x) The same trick can be used to calculate any constant power over the range [-1,1]. -.5 is the most famous for whatever reason, probably because of the "what the fuck?" comment in the open source quake code. I'm not entirely sure why this knowledge was known at Cray, then lost, and had to be rediscovered at SGI or whatever. And I'm not entirely convinced it ever was lost. But if it was lost, I imagine it had something to do with replacing engineers whose bread and butter was done on a sliderule (which is inherently built on the premise of manipulating logs of numbers) with engineers who always had a pocket calculator.
如今入门级 FPGA 性价比远超当年，复刻这类项目门槛已大幅降低。
文中说用的是 Xilinx Spartan-3E 1600 开发板，'这是你能买到的最大且不用花几千美元的 FPGA'——这一点已经过时了。入门级开发板（如 Artix-7 35T）的 FPGA 规模相当且快得多，预算稍宽还能更大；只要敢折腾，不到 200 美元就能买到十倍规模的 FPGA。
duskwuff查看英文原文
> The actual design was implemented in a Xilinx Spartan-3E 1600 development board. This is basically the biggest FPGA you can buy that doesn’t cost thousands of dollars for a devkit. Note that this bit is rather outdated. :) The FPGA on an entry-level development board (like a Xilinx Artix-7 35T) is comparable in size -- as well as much faster! -- and you can go even bigger without stretching your budget too much. If you're willing to get a little adventurous, in fact, you can get an FPGA that's over ten times the size for under $200: https://www.aliexpress.com/item/32907109444.html
项目衍生出磁盘读取、COS 操作系统恢复和可运行的模拟器等后续成果。
Chris Fenton 在此——真的有人寄给我一盒 80MB 的'磁盘组'软件，我改装了一台老式磁盘驱动器把它读了出来，并借此恢复了一份 Cray 操作系统 COS 的拷贝。我和 Andras Tantos 一直在合作，他还写了一个可下载的 Cray X-MP 数据中心模拟器，可以在里面运行我找到的 COS 镜像。
fentonc查看英文原文
Chris Fenton here - someone did actually send me an 80MB 'disk pack' with software, and I was able to modify a super old disk drive to actually read it: http://www.chrisfenton.com/cray-1-digital-archeology/ And thanks to some awesome help from others, I was able to actually recover a copy of COS (Cray OS): http://www.chrisfenton.com/cos-recovery/ Andras Tantos and I have been collaborating on this together for a while now, and he actually has a downloadable Cray X-MP datacenter simulator he wrote, where you can run the COS image I found: http://www.modularcircuits.com/blog/articles/the-cray-files/
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