[TUHS] Re: early unix rand

Norm Schryer wrote a (nearly?) exhaustive floating-point tester that he ran when a new CPU arrived, always with wrong results. Doug McIlroy probably knows more about it than I do, who only observed it from afar. -rob On Thu, Mar 14, 2024 at 4:18 AM ron minnich &lt;rminnich(a)gmail.com&gt; wrote: > Got the name wrong: Computer Engineering: A DEC View of Hardware Systems > Design > > On Wed, Mar 13, 2024 at 9:41 AM ron minnich &lt;rminnich(a)gmail.com&gt; wrote: > >> by the way, I realize that random number urban legend sounds ridiculous, >> in light of how hardware design is done today, but those of you who did >> hardware design in those days (guilty!), and had access to -11 >> schematics and boards, might wonder if it's not possible. There was a >> habit, in those days, for performance reasons, of subbing transparent >> latches for flip-flops to gain a little time. An engineer I knew at Amdahl >> said that was a pretty hot topic there. Certainly, the technique of design >> for testability was not really in wide use in the -11 days. Gordon Bell's >> book "Computer Design" is particularly instructive. >> >> E.g., how did you verify the floating point on your new machine? Put an >> older machine next to a new machine, do lots of computation, see if there >> is disagreement, you've found a bug in the new machine, right? Maybe. >> Sometimes, you discover the older machine had a bug the newer one did not >> ... happened more than once, including on the 360 to 370 transition. >> >> On Tue, Mar 12, 2024 at 6:09 PM ron minnich &lt;rminnich(a)gmail.com&gt; wrote: >> >>> There used to be an urban legend about multiply overflow and the PDP 11. >>> >>> This would’ve been circa 1976. Someone from DEC told us that on a >>> multiply overflow, the contents of the destination register would be “kind >>> of” random. I was never able to verify that claim. But that might explain >>> this code. >>> >>> On Tue, Mar 12, 2024 at 16:05 Jonathan Gray &lt;jsg(a)jsg.id.au&gt; wrote: >>> >>>> On Tue, Mar 12, 2024 at 08:55:02AM -0400, Russ Cox wrote: >>>> > Hi all (and TUHS), >>>> > >>>> > The Third Edition rand(III) page [1] ends with >>>> > >>>> > WARNING The author of this routine has been writing >>>> > random-number generators for many years and has >>>> > never been known to write one that worked. >>>> > >>>> > My understanding is that Ken wrote the rand implementation. >>>> > But I'm curious about the origin of this warning. >>>> > I had assumed that Ken wrote it as a combination warning+joke, >>>> > but Rob suggested that to him it didn't sound like Ken and >>>> > perhaps Doug or Dennis had written it. Does anyone remember? >>>> > >>>> > Separately, I am trying to find out what the very first >>>> > Unix rand implementation was. In the TUHS archives, >>>> > the incomplete V2 sources contain a reference to srand >>>> > in cmd/bas0.s [2], but there is no definition in the tree. >>>> > The V3 man pages list it, but as far as I can tell full >>>> > library sources do not appear in the TUHS archives >>>> > until the V6 snapshot. The V6 rand [3] is: >>>> > >>>> > rand: >>>> > mov r1,-(sp) >>>> > mov ranx,r1 >>>> > mpy $13077.,r1 >>>> > add $6925.,r1 >>>> > mov r1,r0 >>>> > mov r0,ranx >>>> > bic $100000,r0 >>>> > mov (sp)+,r1 >>>> > rts pc >>>> >>>> matches V5: >>>> https://www.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s3/rand.s >>>> Distributions/Research/Dennis_v5/v5root.tar.gz >>>> <https://www.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s3/rand.sDistributions/Research/Dennis_v5/v5root.tar.gz> >>>> >>>> > >>>> > Perhaps this is the original rand as well? It is hard to imagine >>>> > a much simpler one, other than perhaps removing the addition, >>>> > but doing so would create a sequence of only odd numbers. >>>> > >From the man page description it sounds like this has to be the >>>> > original generator, perhaps with different constants. >>>> > >>>> > Thanks! >>>> > >>>> > Best, >>>> > Russ >>>> > >>>> > [1] >>>> > >>>> https://github.com/dspinellis/unix-history-repo/blob/Research-V3/man/man3/r… >>>> > [2] >>>> > >>>> https://github.com/dspinellis/unix-history-repo/blob/Research-V2/cmd/bas0.s >>>> > [3] >>>> > >>>> https://github.com/dspinellis/unix-history-repo/blob/Research-V6/usr/source… >>>> >>>