After Hours

Geek Trivia: Cast to the 404 winds

What kind of error did a 404 code signify within the operating system of the Apollo Guidance Computer, the first embedded system ever put to practical use, and the computer that accompanied the first human beings to land on the moon?

One of the prevailing criticisms of the U.S. space shuttle program is that it is comprised of antiquated equipment. Designed in the 1970s, built in the 1980s, and with two decades of operational fatigue on the fleet, Discovery, Endeavor, and Atlantis aren't cutting edge technology in any respect -- especially their computer systems. The space program is an exercise in proven, cost-effective, work-horse tech, rather than the latest and greatest innovations.

We've come a long way since the days of Apollo.

When Apollo 11 set forth for the moon in 1969, it carried with it what were then arguably the two most advanced computers ever built. The Apollo Guidance Computer -- two of which went on every manned moon mission -- was the first computer to use integrated circuitry and was thus the first modern embedded computer system ever put to use. Custom-designed by MIT and purpose-built by Raytheon, the Apollo Guidance Computer was the basis for the world's first fly-by-wire systems, which in turn directly influenced the design of the space shuttle's flight control systems.

Not bad for a 2 MHz, 16-bit system with a four kilobit RAM allotment and 32K of fixed memory. (And that was the Block II advanced model; earlier Apollo shakedown missions used an even more limited Block I design.) The Apollo Guidance Computer had a 19-key calculator-style keyboard that astronauts used to program the system with two-digit commands. Bear in mind, the first commercially available pocket calculator wasn't sold until 1967, when Texas Instruments debuted a basic function model (add, subtract, multiply, and divide) for about $100. A calculator-esque interface was cutting edge for 1969.

That isn't to say the Apollo Guidance Computer -- advanced as it was for its era -- was error free. The AGC that ran the Apollo 11 lunar module's Primary Navigation, Guidance, and Control System (PNGCS, pronounced pings) malfunctioned during the first lunar descent. Fortunately, the error codes 1201 and 1202 didn't faze Buzz Aldrin and Neil Armstrong when they popped up, informing the astronauts of a critical buffer overflow. (Two radar systems were feeding the Apollo Guidance Computer data simultaneously due to an error in flight protocol.) The Apollo Guidance Computer simply popped into a failover mode and -- because the system was designed with 15 percent spare memory capacity in case of error -- the mission went off without a hitch. If the Apollo Guidance Computer had thrown a 404 error, it might have been a different story.

WHAT DID A 404 ERROR CODE SIGNIFY IN THE APOLLO GUIDANCE COMPUTER SOFTWARE SYSTEM?

Get the answer.

About

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger -- amusing and abusing IT pros. Read his full profile. You can a...

12 comments
pgit
pgit

If you look at the post flight public pronouncements of the Apollo astronauts they bend over backward to explain why you "can't see stars" when in space. (such as on the moon) Really. A lot of reporters asked why none of the photos from the moon showed any stars. NASA offered technical mumbo-jumbo to explain it away, so the reporters asked the astronauts themselves for personal reports of the incredible view they must have had walking on an orb absent a distorting atmosphere. The Apollo 11 3 were caught off guard, Aldrin and Armstrong locked up in classic HAL 9000 "601" fashion, (a bit of trivia, that) and Michael Collins, who (allegedly) didn't descend to the moon but remained in orbit reported "I don't recall ever seeing any stars." Google up the video of their first post flight press conference. The whole affair smells like the Cuyahoga river... well, circa 1969 anyway.

JohnMcGrew
JohnMcGrew

...although I really don't know much about it beyond its very primitive digital display. Anyone here know more about that?

armc
armc

TI may have released a four function pocket calculator in 1967, but it damn sure cost more than $100! I remember the first kid in my high school physics class (Hi, Glen!) to get a TI pocket calculator, and I'm pretty sure it cost more than $100 in late 1972 or early 1973.

hal001
hal001

I always wondered what kind of computer system was on the Appollo mission. Now I know. That is a great article.

frankobrien3764
frankobrien3764

I'm afraid that the 0404 error is not the "IMU not found"... There was a similiar error, a 0210 (all numbers octal) which was "IMU not operating". A 0404 error in the Lunar Module was when it could not see two stars in any of the 6 possible postions of its "Alignment Optical Telescope" (think sextant). In the Command Module, a 0404 error was when the LM was outside the field of view of its sextant (which served a dual role for rendezvous navigation). Important other quibbles: The system was 16 bit, but that was including a parity bit. That's like saying my Wintel server is a nine bit machine... Memory was 2K of 15 bit words, 36K of ROM. Most importantly, the computer didn't malfunction, it was getting a hot I/O from a radar interface (a design flaw, not a procedural problem),and the errors were saying that it ran out of available processor blocks, not buffers (the computer had no buffers) There was no 15% spare memory capacity in the computer, most missions flew with something like 5 or 10 free words of storage. Hope this helps! Frank O'Brien Author, The Apollo Guidance Computer: Architecture and Operation (Springer-Praxis, mid-2009 publishing date)

metilley
metilley

I remember the HP-65 and still have mine... somewhere. I wonder if it's worth anything now? I loved HP's RPN (Reverse Polish Notation). Quite an idea!

twells
twells

The Apollo Guidance Computer did not run at 2Mhz. It had a 2Mhz clock but split the signal and ran at 1Mhz. The ship itself ran at 512Khz.

TechnoDoc
TechnoDoc

You are indeed the Paul Harvey ("The Rest of the Story") of tech, and I have to read your posts even if they make me late for meetings. Thanks! I worked for General Electric, contracting to NASA, back in the late 70s, still analyzing data from those all those missions from the 1970s. Despite public perceptions of hardware computing power then, brainpower was >>> hardware in those days both in space and back on the ground...

#1 Kenster
#1 Kenster

My TI pocket calculator, purchased in 1973, cost me $99.99 at the college book store. It was the least expensive calculator that had a square root function. The next day it went down $10 and the book store would not budge and give me the cheaper price. When the rechargeable battery finally died, the replacement battery cost was something like $15. The only store that I found that carried the battery had the same calulator with battery (last one they had) in the window for $12. I bought a newer and better calculator for around $20 instead.

lost in Texas
lost in Texas

on commercial programs for pay while the primary did its thing. I explain to the kiddies around here now that they haven't felt real pressure: having to thread the latest back up tapes onto that secondary mainfame during something critical like a landing whan the primary went all casters up will make your voice go high. But we always had a great splash-down party at the Flintlock at the end of each mission.

rubmop
rubmop

I wish there was more of Kranz' character and determination around today.

MurphysAcolyte
MurphysAcolyte

Talk about working in the clutch. Your milkshake, may I partake out of respect?

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