[TUHS] Re: early unix rand

[Jonathan Gray <jsg@jsg.id.au>]

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

> 
> 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/rand.3
> [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/s3/rand.s