13 Mar
2024
13 Mar
'24
10:41 p.m.
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 <rminnich(a)gmail.com> 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 <jsg(a)jsg.id.au> 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.
https://github.com/dspinellis/unix-history-repo/blob/Research-V3/man/man3/r…
From the man page description it sounds like this
has to be the
original generator, perhaps with different constants.
Thanks!
Best,
Russ
[1]
[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…