
How do you measure nuclear warhead yield with a PDP-11? - segfaultbuserr
https://web.archive.org/web/20160507110751/http://www.csd.uwo.ca/staff/magi/personal/humour/Computer_Folklore/How%20do%20you%20measure%20nuclear%20warhead%20yield%3F.html
======
eggy
I so desperately wanted to get in on this novel personal computing thing, and
I couldn't afford a computer. In 1976 I bought a book on machine and assembly
language programming not knowing a thing from a bookstore on Wall St.
somewhere. It was for the DEC PDP-11. I went through the first third or so of
it with pencil and paper and put boxes on the side to show me the status of
registers. Not productive, but not a waste of time. I then bought a Commodore
PET in 1978 and the rest is history! That early exercise made it easy for me
to learn 6502 assembly and work on the Vic-20, C64, C128, and PowerPC Macs.

~~~
nicolashahn
Hey, this is not related to the original submission, but I just wanted to know
what keeps someone like you (who's been interested in computers for much
longer than I've been alive) going. It seems like it'd be very easy to tire of
the industry/hobby after that long, I want to know what you're currently
interested in and if you're comfortable sharing, what you're currently working
on day-to-day in your job (if you're not retired by now).

~~~
eggy
Funny, but I took only a few jobs where coding was more than 50% of my day
job, but for the most part I've held very eclectic jobs in a predominantly
technical or artistic career.

I was reading about neural networks and genetic algorithms in the late 80s and
programming in Lisp and C. Mark Watson's book (he's on HN)

"Common Lisp Modules: Artificial Intelligence in the Era of Neural Networks
and Chaos Theory"

got me started in coding them rather than just reading the more academic tomes
I read in the late 80s.

I am glad I didn't just focus on that then, although, I'd probably have more
of a savings now and higher salary. But not the adventures I managed to have
by meeting people in all different types of jobs. And there's something about
physical labor that calms me. Sitting at a desk all day seems so antithetical
to actually living a life out in the real world and feeling the earth between
your toes, the sun and wind on your body while taking in the green things
around you!

I was dumpster diving and taking stepper motors out of dot matrix printers in
the early 90s down on Wall street while working a 12 to 9 shift taking care of
a VAX/VMS and Banyan Vines PC network for an environmental law firm. I used to
write reports in Paradox, work on Concordance - a document management system -
BRS search routines and queries. I used the steppers for my amateur robotics
influenced by Rodney Brooks' subsumption architecture and Mark Tilden's BEAM
stuff. I played Doom for the first time at that job!

I was an art handler (Brooklyn Museum), paintings conservator's assistant, 3D
FX and animation on SGI intern using Prisms (now Houdini) at a NYC Ad
agency/FX company, welder, machinist, CNC coder, technical diver and ropework
technician (think IRATA/SPRAT), among the memorable items. I remember being
one of the early funders for getting Blender3D open sourced way back when
(2001/02), and writing a Python script to turn photos into wooden carvings
back in 2002/03 with it on my CNC table. The software was based upon a paper
by NASA about 'Shape from Shading'. it alsow churned out G-code for our table
after downloading the client's file, and gave me an estimate of machine time
and overall cost. This is before 3D printers, and tables used to be in the 6
figures, but this was an $8k 4 ft x 8 ft router table. My partner and I used
it to cut out kayak designs we made out of 4 mm Okume plywood and the carvings
I did in solid maple in bas relief. We did some urns for pets and pet
cemeteries (really! [http://thewoodenimage.com](http://thewoodenimage.com).
Used to come up on the Wayback Machine without pics or other content). Sad to
say, people loved our carvings at boat shows, etc., but nobody bought them. We
had crowds at our table, but not a single purchase. Funny cemeteries were our
market, and a gentlemen offered me money for the software to make bronze
tombstone plaques from family photos!

I worked at a company as a sys admin, and did some coding in C, C#, Java, but
mainly my strength was writing front ends for old Italian sheet metal punch
machines and translating modern G-code into the 'Octo' machines' proprietary
code. This was in C. Putting a temperature sensor in the server room, and have
it ping a pager of me and the other sys admins.

I did Christmas display window animations for a display company in NYC as a
welder and later as the head of their creative technologies. One Christmas
(1996-7?). We had pneumatically operated 4 to 5 ft high Nutcrackers whose arms
would go up with a bell in it if a child touched the window where there were
five large buttons painted on the back of the window with a capacitive sensor
behind it, a BasicStamp (think today's Arduino) running it, queuing the music
and triggering the pneumatic valves. Remember right up until this, the
internet and information from companies was not as accessible. I had to write
letters, emails, and phone call for weeks.

I will probably never retire. I just put a deposit down on a live/work space
to use my desktop cnc mill/lathe, 3D printer and electronics equipment (a CNC
sewing/embroidery machine too).

I currently work at an engineering firm that specializes in stage machinery
and anything entertainment-related. Because I don't code full-time for a
living, I code up things in what works best for me and the company I work for.
I write them in C, python, factor, J, forth, assembler (micros), and I play
with a lot more PLs (Zig, Rust, Haskell, R/RStudio), but I am mainly sticking
with C and J for now with Zig as a cross compiler. I avoid python, not because
it isn't good or I don't like it, but it doesn't fit in with my preference for
terseness and shedding bloat. I am having a renewed interest in C and
assembler. I am getting older for some of the more physical work I did, so I
am now coding more to try and see if I can make something of it.

I was working in Macau for 6-plus years for the The House of Dancing Water
show up until the end of 2015. I have hundreds of technical dives working on
hydraulics, air FX, and electrical systems under water. It is almost 10 m deep
and was the world's largest commercial indoor pool!

Programming is not so much a career for me as it is simply a tool. The same
for my electronics and metalworking experience, or any other skill. Critical
thinking, problem solving, and knowing how to provide what's needed or desired
are more important. I've always followed my heart or gut on what's next and it
is landed me in a good place at my age. Follow your bliss to quote Joseph
Campbell!

[1]
[https://link.springer.com/book/10.1007/978-1-4612-3186-8](https://link.springer.com/book/10.1007/978-1-4612-3186-8)

~~~
hadsed
How involved is it to get the training for doing some of these technical diver
jobs?

~~~
eggy
I have found getting certified in a skill like diving or ropework is not the
point. It's what skills do you bring to the thing you dove or climbed to fix?
Do you know hydraulics, basic electrical work, troubleshooting, etc. Learn
those first. An old dive buddy of mine used to say you can teach a monkey to
dive, but what can he do when he gets there? I need skilled people. Also there
are many types of technical diving: commercial diving and deep sea diving,
wreck and cave diving, underwater welding, salvage, show divers (fairly new -
since the show 'O' in Las Vegas - divers that escort performers underwater for
water shows), and many more.

------
pnw_hazor
At LANL they put many experiments/sensors in the bomb stack.

A group I was with ran an experiment called PINEX (Pinhole Imaging Neutron
Experiment), they used high speed video cameras to capture light/photons from
a light intensifier. A meter diameter lead sphere was placed between the bomb
and the light intensifier to attenuate the neutron signal. The length of the
video cable (coax) was measured in time units.

They could get about a frame and half of video off the CCD and out of the bomb
hole before everything was vaporized. The video was captured using analog-to-
digital signal converters and then interpreted later. The ADC, memory, pulse
generators (to trigger light intensifier, start the CCD sweep, etc.) were
sitting in a CAMAC crate attached to a customized VAX. They even has a
customized VAX and monitor to visualize or analyze the captured data. The
captured frame would be rendered into a gray scale image of a concentric
circles that meant something to the bomb physicists.

My intern project involved showing how the ridiculously expensive and fragile
VAX setup could be replaced by a modern workstation running off the shelf
software. Of course, this was not well received, that is why my mentor had me
do the work and presentation - he would never have gotten away with it
himself. No matter though, bomb testing was stopped soon enough.

------
ChuckMcM
BTW, this is article is not actually correct[1]. The technique is however. The
measurement used an HP pulse generator to send a pulse down a piece of coax
and that pulse, and the "echo" or return were both attached to Ch1 and Ch2 of
an oscilloscope in the measurement trailer.

When a pulse was fired a Polaroid picture was taken of the screen at the same
time. By measuring the difference between the leading edge of the initial
pulse and the returning pulse the length of the cable could be computed. With
a _lot_ of pulse generators and oscilloscopes you could synchronize them to
each fire slightly later than the next and get a number of solid data points
on the cable being vaporized.

The Polaroids all included on the picture the rack and rack slot where they
were located, and the sequencing was known, so you could digitize the pictures
and extract the data.

[1] I actually worked in one of these trailers as part of a summer job during
college writing code on said PDP 11/34.

~~~
kps
This paper lists the gear used (at some point) for the measurements, on p16:
[https://www.osti.gov/biblio/7352892](https://www.osti.gov/biblio/7352892)

    
    
        2 - 1.5 MHz tape recorders
        2 - Oscilloscopes with cameras (HP-180 w/1805A and 1821A)
        2 - Digital freq. and time interval counters/dig. volt meters HP5326B
        2 - Sandia SLIFER chassis, shop made
        2 - Pulse generators, HP214A
        2 - Engine generators, 25 kW
        1 - Trailer or shelter, portable, with utilities
        2 - Time code generator
        2 - WWV receivers
        4 - Radios for local communication
        2 - Countdown receivers
        2 - Hi potters, HP4329A
        1 - Time mark generator (Cal scopes) HP226A
        2 - Variable oscillators, HP3312A
        2 - Time delay generators, Rutherford
        3 - Counter (HP5345A)
        2 - Program timer, shop made

~~~
ChuckMcM
Wow, that is awesome. I wish they had a picture from inside the trailer.
Basically a Decwriter + PDP-11/34 at one end, and two back to back rows of 19"
"telco" racks with scopes, pulse generators, and cameras.

------
hazeii
I have a number of PDP11's (that escaped nuclear obliteration) and have the
skills to keep your nuclear plant running until 2050 [0]. The instruction set
is really worth studying to this day, and the fact it has both pre and post-
increment/decrement surely informed the design of C (and its predecessors). So
orthogonal a single instruction could replicate itself through memory:-

MOV (-PC),(-PC)

As the instruction counter (PC, AKA R7) auto-incremented on each instruction,
that instruction first decrements the PC (so it points back to the current
instruction) and then copies the contents of the memory location (i.e. the
current instruction) to the next lower memory address. And since the PC has
now been decremented twice, the operation can repeat until it hits somewhere
without any physical RAM (as PDP-11's have a bus signal to tell them whether
the addressed memory exists or not).

Also easy to program in octal; knowing that MOV is 01MRMR, addressing mode M
is 4 for pre-decrement and R is the register number, no-one even needed an
assembler to write that instruction as:-

014747

The ultra-regular ISA made it easy to learn and remember bootstraps, it'd just
take a few passes along the toggle switches to key in a loader. It became
rather easier with boot ROMs and serial terminals, e.g. on the LSI-11 just
typing '173000G' was enough to bring up the system (back then I got to code a
boot loader for 5" disk drives, 2 different 8" disk driver and a user
interface with a prompt into 256 words of ROM).

Those were the days, my friend.

Funnily enough I booted one of the PDP's a few days ago and it wasn't even
hard to remember op-codes learnt 40+ years ago.

[0]
[https://news.ycombinator.com/item?id=5904191](https://news.ycombinator.com/item?id=5904191)

~~~
giardini
from the stackoverflow topic titled:

"What are the historical reasons C languages have pre-increments and post-
increments?":

[https://stackoverflow.com/questions/30429753/what-are-the-
hi...](https://stackoverflow.com/questions/30429753/what-are-the-historical-
reasons-c-languages-have-pre-increments-and-post-increme)

A quote from that topic:

> _' Ritchie, The Development of the C Language:

"People often guess that they [ED. the increment and decrement operators] were
created to use the auto-increment and auto-decrement address modes provided by
the DEC PDP-11 on which C and Unix first became popular. This is historically
impossible, since there was no PDP-11 when B was developed." The PDP-7 seems
to have some features that might have played a role, albeit I don't think it's
entirely clear from the document that they were the main reason for the
existence of both prefix and postfix ++'_<

~~~
hazeii
That's interesting, as I'd assumed the BCPL/B/C progression happened alongside
the PDP-7 to PDP-11 progression with the PDP-11 being the design that
regularised things.

The full set of addressing modes uses 3 bits, thus 8 addressing modes. Using
these addressing modes on the program counter (R7) gave a lot of flexibility
and was how constants and memory were loaded and stored. It might sound
complex but in practice was incredibly easy:-

    
    
        mov #141,@#177566   ; Output 'a' to the terminal
    

That's a "MOV (PC)++,@(PC)++" instruction that loads the next word from memory
(octal 141) and increments the PC and stores in the address given by the next
word (after the constant, as the PC has just been auto-incremented) and
finally autoincrements the PC again to leave it pointing to the next
instruction. The layout in memory would thus be:-

    
    
      12737  # Instruction: MOV (PC)++,@(PC)++
      00141  # ASCII 'a'
      177566 # Address of terminal outer buffer
    

Given these games using the PC to address data and memory, PDP-11 programmers
had little difficulty when it came to understanding C pointers.

------
antman
Related: Fermi's report on estimating yield by dropping small pieces of paper
during a nuclear blast and measuring their displacement.

[http://www.dannen.com/decision/fermi.html](http://www.dannen.com/decision/fermi.html)

~~~
semi-extrinsic
Also related: G.I. Taylor who developed the theory of blast waves to the point
where he could estimate yield from photographs published in newspapers, and
wrote papers that published the classified yield of the first US nuclear
tests.

Edit: His estimate was between 17 and 23 kilotons of TNT, the real yield was
22 kilotons.

[https://royalsocietypublishing.org/doi/10.1098/rspa.1950.004...](https://royalsocietypublishing.org/doi/10.1098/rspa.1950.0049)

[https://royalsocietypublishing.org/doi/10.1098/rspa.1950.005...](https://royalsocietypublishing.org/doi/10.1098/rspa.1950.0050)

~~~
qubex
This remakrable achievement is often used as the setting-off point to explain
dimensional analysis.

~~~
jedimastert
> This remakrable achievement is often used as the setting-off point to
> explain dimensional analysis.

I was always a fan of dimensional analysis as it tickled "puzzle solving"
aspect of my lizard brain, but if it were introduced via nuclear blast
analysis in my classes I bet it would have grabbed a lot more attention.

------
dekhn
I'm pretty sure this is apocryphal. It seems more likely that somebody used
cables with TDR to measure yield at some point, and somebody noticed that the
ethernet interfaces on their PDPs had similar equipment, and made a story that
combined them.

------
qubex
There’s a lot of discussion here on how (or even whether) a ‘reflectometer’ (I
had to look up what that even meant) works.

However, I’m more interested in the things I _do_ understand, and in this
case, my question is: considering the fact that there needs to be another
(remote) computer to receive the readings, and considering that we’re speaking
of processes that ‘degrade’ the cable dangling into the nuclear borehole at an
alarming rate, just how does one go about performing an ordinary electrical
transmission of data over those distant ranges in those minute times? The
latencies are enormous and therefore clearly we’re not talking something like
TCP and it’s three-way-handshake SYM/ACK kind of response.

And once the blast approaches the surface (assuming it doesn’t breach it)… is
it sufficient for the computer to (comparatively) leisurely transmit its
findings whilst it’s in the process of falling into the enormous crater that’s
opening up beneath it? Can one presume that the power remains stable long
enough for this to occur? (Assuming that the power supply has been destroyed
and/or the cables jolted loose, how long does the machine “stay up whilst
falling down”?)

~~~
raldi
Underground nuclear tests don’t make huge holes in the surface. That’s the
whole point.

Here’s what they look like at ground level:

[https://www.atlasobscura.com/articles/heres-what-an-
undergro...](https://www.atlasobscura.com/articles/heres-what-an-underground-
nuclear-test-actually-looks-like)

Plenty of room for the computer to survive.

~~~
qubex
Enormous subsistance craters are what I had in mind when I wrote “enormous
crater”.

~~~
raldi
They’re not enormous relative to the length of LAN cables.

------
barbegal
There is some truth to this account:
[https://books.google.co.uk/books?id=6t-wCwAAQBAJ&pg=PA131&lp...](https://books.google.co.uk/books?id=6t-wCwAAQBAJ&pg=PA131&lpg=PA131)
It's called CORRTEX.

And I think you could probably set up a PDP-11 to sample roughly every
millisecond so this could work.

~~~
beamatronic
That doesn’t seem fast enough to be useful...

~~~
hwillis
The initial fireball of a nuke is caused by xrays absorbed by air and develops
in microseconds. Basically all at once, since the xrays move at the speed of
light. A spherical region of air just ionizes.

The expanding fireball moves quite a bit slower and increases 10s of meters
(from an initial ~10-100 m) over several milliseconds[1]. After that it slows
down even more, and if you measure every millisecond you'll see the fireball
expanding by meters.

[1]:
[https://pbs.twimg.com/media/Dbf8IK9V0AA38po.jpg](https://pbs.twimg.com/media/Dbf8IK9V0AA38po.jpg)

~~~
Tehchops
This phenomenon is referred to as a "double flash":

[http://wordpress.mrreid.org/2015/04/18/the-nuclear-double-
fl...](http://wordpress.mrreid.org/2015/04/18/the-nuclear-double-flash/)

------
kps
I read a different version of this story many years ago, but have not been
able to find it again. In it, the machine did not send measurements to a more
distant location, it recorded them in memory: core memory, with _each core
individually labelled_ , in case the machine didn't stay in one piece.

(If true, this probably implies the machine was not a QBUS PDP-11, and the
reflectometer was not an ethernet card.)

------
_sbrk
Great stuff from the golden years of the 'net. Archived websites like this
show what it was, before it became infected with consumers.

~~~
Swizec
My favorite was an in depth website about farts. It explained how it was
impossible to fart while you sleep because your “don’t pee myself” reflex
keeps everything sealed shut. This is why you often let a big one rip soon
after waking.

I believed that shit for 20 years ... then my girlfriend moved in and lemme
tell ya, humans definitely fart in their sleep.

------
loph
Having a DEQNA was the reason they did not love their PDP-11. They were
notoriously buggy. Replaced with DELQA. See
[http://www.bitsavers.org/pdf/dec/qbus/EK-DELQA-
UG-002.pdf](http://www.bitsavers.org/pdf/dec/qbus/EK-DELQA-UG-002.pdf)

------
tech-historian
FYI, the PDP-11 cost around USD $20,000 when introduced in the early 1970s.
That's equivalent to ~$135,000 today.

~~~
dylan604
Oh, so it was developed by SGI? ;-)

~~~
walshemj
If SGI built systems designed like a tank - I recall helping to install a dual
8 inch floppy drive took two guys to lift it into position

~~~
pmiller2
Have you seen a PDP-11? It probably weighs closer to a tank than anything
produced by SGI. You wouldn't be lifting a PDP-11 without equipment: it would
be the size of a refrigerator or 2.

~~~
kps
> _Have you seen a PDP-11?_

There's a PDP-11/73 about eight feet to my left; it's a 4U box, much smaller
than a little SGI Onyx, let alone an Origin.

~~~
pmiller2
Huh, I had no idea there was a PDP-11 that would fit on a desktop. I was
thinking of something more like this:
[https://gunkies.org/wiki/PDP-11/40](https://gunkies.org/wiki/PDP-11/40)

~~~
thequux
In the info box photo on that page, only the box at the bottom (with the red
and purple switches) is the processor; above it are two blanking plates and a
TU-56 tape drive. I suspect that KPS considers the machine to just be the
processor, whereas you're looking at the processor and surrounding
peripherals.

That said, there were PDP-11's that were desk-sized, such as the DEC
Professional (about the size of an IBM 5150), or even smaller, the VT240
(which is largely an embedded PDP-11, and it has about the footprint of a C64,
though somewhat thinner)

~~~
pmiller2
Ah, yes. I was indeed considering the whole system (processor + peripherals).
I guess it shouldn't surprise me that there's a PDP-11 that fits on a desk,
because I know at least some MicroVaxes fit on desks, and they're
contemporaneous with the PDP-11.

[https://en.wikipedia.org/wiki/MicroVAX](https://en.wikipedia.org/wiki/MicroVAX)

------
mhh__
Curious

I assume the TDR picks up induced EMF in the cable by the radiation from the
Detonation.

~~~
ISL
The "time-domain reflectometer" will send a signal down the line and listen
for a reflection from its end.

This happens in much the same way a travelling wave will reflect from the free
end of a dangling rope. Even though a jiggling rope and an electrical signal
seem different, the underlying mathematics (mismatched impedance at the
rope/cable end) is essentially the same.

The time difference between when the signal is sent and the reflected signal
received, multiplied by the speed of the signal (over two), gives the distance
to the end of the cable.

If the reflectometer can make the measurement quickly-enough, the series of
measurements will trace out the decreasing length of the cable as the
detonation destroys it.

~~~
segfaultbuserr
This doesn't answer the original question: How could the experimenter ensure
the electromagnetic pulse from the nuclear denotation wouldn't create false
readings in the TDR measurements?

~~~
hwillis
Nuclear reactions don't just make EMPs; it's not like nuclear plants are
emitting continuous electromagnetic interference. Nuclear EMPs are created
when radiation ionizes susceptible atoms, creating charged particles via
Compton scattering. That's why high-altitude nukes create much more intense
EMPs; the low-density atmosphere is much easier to ionize.

Gases at low pressure are DRAMATICALLY easier to ionize than anything else-
think fluorescent tubes, etc. Dirt will not be highly ionized by an
underground explosion, although I can't actually find anything about
underground EMPs. It is safe to say that the EMP will be very greatly reduced,
though.

The EMP is dangerous to electronics because it creates a voltage gradient over
a large area. A 5 volt gradient across a few inches of ground plane will
destroy most electronics permanently. A semiconductor is painstakingly
embedded with atoms that create an intrinsic voltage difference across a tiny
sliver of silicon; a mild voltage in the wrong direction will erase that gap.
It will not destroy wires, since the pulse is extremely short.

Forget erroneous measurements. The EMP would kill the computer if it was just
a plain wire. You're talking about picking up tens of thousands of volts
across any kind of protection circuit. The only possible way this works is
with _some_ protection, ideally shielded wire like a coax cable. If that's the
case it'll just cause a brief (<10 microsecond, mostly) flash of current to
ground. Modern Ethernet is twisted pair, but the first ethernet was coax and
the first R58 was sometimes shielded.

There are other types and components of nuclear EMPs but they mostly aren't
important in this case. There are several highly questionable things about
this story- the PDP-11 would need a pretty skookum power supply to not bottom
out, the motivation is... strange, the ethernet wire would need to be
incredibly long, etc. etc. EMP is probably not one of them, at least as far as
the _quality_ of data. EMP becomes irrelevant if you can access raw data or
the ping uses anything but the simplest impulse waveform. Assuming you can
actually collect data in the first place, anyway.

------
WalterBright
My fluid mechanics professor showed how to measure the yield just by examining
a movie of the blast frame by frame, i.e. the size and rate of expansion of
the cloud.

A professor did this in the late 1940s and got himself investigated by the
military for espionage, until he showed them how to do the calculation.

------
posix_me_less
Here is how nuclear explosion vaporizing telephone cables was thought to sound
like on the open line in 60's. Absolutely terrifying scene.

[https://youtu.be/kw5o1eRhSiw?t=83](https://youtu.be/kw5o1eRhSiw?t=83)

------
WalterBright
I had a PDP-11 in the form of a Heathkit H11. I sorely regret selling it.

------
craftinator
I feel like this would be right at home in a Neal Stephenson novel.

------
raldi
(1992)

------
lmilcin
Not likely to be workable.

Nuclear explosions generate massive amount of electromagnetic interference
which is likely to fry or at least throw off any regular reflectometer that
relies on echo of the signal. Don't want to say it is technically impossible
but would probably require especially designed instrument.

~~~
pmiller2
I guess I wasn't the only one thinking the PDP-11 would likely be electrically
fried before it got blown to smithereens here.

