Talk:Apollo Guidance Computer

message: The "single" and "double" precision mentioned in the article links to IEEE definitions, but surely the author meant one or two of the 15/16-bit words the machine used?  --Anonymous, 21 Apr 2004
:Thanks for mentioning it, I have to admit I didn't think of that when inserting the links.   However, the opening paragraph of the respective 'IEEE articles' does actually define sgl/sbl precision fairly generally (i.e. using one vs two words), so hopefully we'll avoid misleading the readers too much. Eventually I think we should 1) make a separate article on general single/double precision numbers, and 2) incorporate even clearer introductory info, w/links to the general article, in the IEEE definition articles. --Wernher 16:13, 21 Apr 2004 (UTC)
thread_title: Single/double precision

message: For contemporary readers who may not have been alive at the time, or who may not be intimately familiar with computer architecture, the Apollo Guidance Computer could use a comparison to later well-known computers or calculators.  For example, how would it compare to the HP-65 programmable calculator, or a personal computer, such as the Apple II or IBM PC that came a decade or so later? Quicksilver 02:13, 11 November 2005 (UTC)
replies: text: Good point! Perhaps we should also put in the TI-83/89 and HP-48/49g+ modern calculators, since those would be quite well-known to many college students (and engineers, scientists) today. --Wernher 17:10, 13 November 2005 (UTC)

text: This the exact reason that I came looking for the Apollo computer information. We've heard for years that the cheap $4 calculator in the checkout line has more computer power than the one that went to the moon. So how much did it really have? I missed any reference to RAM or ROM. No changes in four years may not be a good sign. :) -- Kearsarge03216 (talk) 00:39, 27 March 2009 (UTC)
replies: text: It's clearly put forth in the article (Apollo_Guidance_Computer#Description) but the architecture and terminology were not altogether the same as today. A quick reading of the text shows the computer's RAM was about 2K and ROM was about 36k, along with memory add-ons. This was (barely) enough for the straightforward, skived-down guidance math it was built to do, but packed into a startlingly small area for the time. The interface was all hardware. It was indeed less powerful than advanced handhelds made only a few years later, much less powerful than the earliest IBM PCs but highly reliable. The AGC was more or less hand built, very expensively. $4? More like $1-2 when on sale at an office supply store: Most throwaway calculators in 2009 would be more powerful over all, but maybe less reliable. Any article comparison with calculators or PCs would need to be reliably sourced, though. Gwen Gale (talk) 04:33, 27 March 2009 (UTC)

text:  As a point of comparison, emulating the AGC using the Virtual AGC software takes about 2% of CPU time on a 3GHz Pentium-4 (i.e. that CPU can emulate the AGC at around 50x realtime). And given that it's probably running a hundred or more x86 instructions to emulate each AGC instruction, the AGC is less than 0.1% of the performance of the P4. Which makes you think, really, when you consider that it got men to the moon and back, with assistance from the ground. Mark Grant (talk) 07:45, 28 March 2009 (UTC)
replies: text: This does seem to line up neatly with the 2.048 MHz clocking speed of the AGC noted in the article. I've got spreadsheets which, from my human outlook, seem to do complex calculations "instantly," though these would have taken several seconds on the AGC (given it had enough memory to hold the data, which it did not), never mind the staggering overhead of the X Window GUI along with all those daemons and such. There is likely a source floating about somewhere which talks about this and could be cited in the text. Gwen Gale (talk) 08:30, 28 March 2009 (UTC)
thread_title: Points of reference

message: We should probably add more information about the decision to use ICs in the AGC rather than discrete transistors. I've added links to some documents on the klabs.org site discussing this decision and it would appear to have been highly contentious at the time but extremely sensible in hindsight. MarkGrant 02:11, 9 July 2006 (UTC)
thread_title: Integrated Circuits

message: This is an excellent article, but I question the assertion that it was the program alarms that caused Neil Armstrong to go to manual control of the Apollo 11 landing. Is there a source? As far as I know, all of the astronauts went to manual control during lunar landings, and I've never seen Armstrong's decision singled out like this before. --MLilburne 09:16, 13 July 2006 (UTC)
replies: text:  It does say 'more manual', but it seems odd to me too. BTW, I've also added a comment on the root cause of the 1201 alarms, which I didn't see mentioned anywhere else. Mark Grant 16:53, 13 July 2006 (UTC)
replies: text: There's a good discussion of manual control and lunar landings (though on a message board) here.  What "manual control" would mean in this context is, I believe, going to P66, which all of the commanders seem to have done. So I do think that the article is wrong. But I'm going to ponder a bit more before changing anything. --MLilburne 17:06, 13 July 2006 (UTC)
thread_title:  PGNCS trouble 

message: Somebody needs to correct this.  It can't have been 5 to 10 kW reduction.  More like tens of Watts.
:http://history.nasa.gov/ap16fj/csmlc/a16Lemer1-7.gif says that some sort of standby (I don't know if it's the same) reduced consumption by 3 Watts. Does someone know if it's the same as the one mentioned in article?
Presumably it's the same. The text has been corrected already to read W not kW.--agr 14:44, 8 December 2006 (UTC)
thread_title:  Standby Mode 

message: The photo is nice but gives no indication of scale. Could anyone add to the description section indications of Power usage and Physical dimensions? Garrie 03:01, 12 December 2006 (UTC)
thread_title:  Description section 

message: I added the technical and generalize tags. This page is far too technical. This is obviously an amazingly important technology, but I don't know that describing clock cycles, circuit design, and other technical specifications are the best ways to convey it. Can we get more about the historical context (state of IC computers in the era), advancements that the computer made that can still be seen in modern computers, how the computer affected or resolved missions-in-flight (ie Apollo 13)?Madcoverboy 16:02, 11 March 2007 (UTC)
thread_title: Technical & Generalize tags

message: * http://www.ibiblio.org/apollo/  —Preceding unsigned comment added by 82.83.112.185 (talk) 00:50, 28 December 2007 (UTC)
replies: text: This is already in the external links section. Gwen Gale (talk) 01:17, 28 December 2007 (UTC)
thread_title:  Misc 

message: Can you tell whether it is this Apollo Guidance Computer (AGC) that displayed an "error 1202" during Apollo 11 landing on the moon and forced Armstrong to moonland manually ?
: Yes. See, for example, http://history.nasa.gov/alsj/a11/a11.1201-pa.html Mark Grant (talk) 00:41, 25 July 2009 (UTC)
replies: text:  The 1201/1202 program alarms didn't have any bearing on Armstrong's decision to go to the "more manual" landing mode (in fact, every moon landing used that mode, which did still rely heavily on the computer). The alarms were a distraction, certainly, but beyond that didn't have any impact on the computer's functionality.  Kaleja (talk) 17:10, 21 April 2015 (UTC)
thread_title:  Error 1202 

message: The MIT AGC Project link is effectively dead --
The Burndy Library has moved to the Huntington Library, Art Collections, and Botanical Gardens in San Marino, California. The Dibner Institute, formerly on the MIT campus, is now closed. Information regarding the Burndy Library and Dibner Fellowships may now be found at http://huntington.org/burndy.htm. Inquiries may be sent to publicinformation@huntington.org.
Some of the material is archived at http://authors.library.caltech.edu/5456/, but I haven't tracked down all of the items from the external links yet.
Autopilot (talk) 00:13, 5 March 2008 (UTC)
thread_title:  External links: MIT AGC Project link rot 

message: Are there any examples of how the verb / noun command inputs worked? I can see the codes in the picture of the side panel, but its not clear what they are for or how they are used.
-Bill  —Preceding unsigned comment added by 75.180.8.80 (talk) 12:10, 26 July 2009 (UTC)
replies: text: See the external links for the article but page 17 of this pdf puts forth a quick overall take on the noun/verb buttons. Gwen Gale (talk) 12:55, 26 July 2009 (UTC)
replies: text: Looking at the "helpful" list of verbs and nouns, now I think Robert Heinlein was closer to the mark than I thought in his story  Misfit, where the navigators have to convert everything to binary - by hand! - before entering it into the computer. The DSKY must have been a brutal man-machine interface to use, by today's standards. --Wtshymanski (talk) 16:04, 2 April 2010 (UTC)
thread_title:  Verb/noun commands 

message: I'm in no great hurry, but at some time I'd like to move the focus of the article to be on the Block II design (which actually flew the manned missions), with the Block I taking a lesser role. At present the text describes the Block I by default, with references to changes for the block II made later. The design history is interesting, and the evolution from Block I to Block II is important, but the design of interest is in fact the Block-II version. Are there good reasons for not doing this? Lissajous (talk) 05:30, 11 October 2009 (UTC)
thread_title:  Block I vs Block II 

message: These links actually seem to be working now. Added 'nb.' on size of downloads.
--220.101.28.25 (talk) 11:48, 31 October 2009 (UTC)
replies: text: They work for me too, so I removed the 'dead link' tags. I also changed the 'section headers' for the links section to wiki headings since they're not very obvious when they're just part of the generic text; that's bugged me for a while and I think this is an improvement. Mark Grant (talk) 20:16, 31 October 2009 (UTC)
thread_title:  Links to Software Source Code 

message: "The software could be overwritten by the astronauts using the DSKY interface. As as done on Apollo 14." - this seems rather dubious to me as the software was in fixed core memory, and the only explanation I've found of AGC hacks on Apollo 14 is this:
http://www.ibiblio.org/apollo/#Solution_to_the_Apollo_14_Final_Exam
So in that case it would appear that the astronauts were changing variables in the erasable memory rather than code.
I'm sure I do remember a mission installing a software 'patch' in erasable memory but can't find any reference to it now. And that's still not really 'overwriting' the software. Mark Grant (talk) 05:17, 15 September 2010 (UTC)
thread_title:  Overwriting software 

message: At the top of the article it mentions that the dimensions of the AGC are 24" x 12.5", however from the pictures it looks more squarish, and I recall it being more like 8" x 8". Is there a source to confirm dimensions? Or maybe I am thinking of just the DSKY part of it? Logicman1966 (talk) 23:48, 9 February 2011 (UTC)
replies: text: You're thinking of the DSKY. Here is a ref for the Block I AGC dimensions, which are a little different  from the dimensions given in our article: http://www.nasm.si.edu/collections/artifact.cfm?id=A19720340000  -agr (talk) 01:10, 10 February 2011 (UTC)
thread_title:  Dimensions 

message: > Block II had 32 kilowords of fixed memory and 4 kilowords of erasable memory.
I'd like to explain this in terms an ordinary person will understand - would it be fair to say: "approximately as much memory as a Comodore 64" ?  
Regards, Ben Aveling 10:57, 7 October 2011 (UTC)
thread_title:  In layman's terms... 

message: This is kinda confusing. So according to what it says now, it sounds as if full on used 70W and standby used 65W or 60W -- and I'm not sure that's correct. Also, can we get an inline reference there? 31.16.108.201 (talk) 23:38, 26 February 2012 (UTC)
thread_title:  Standby 

message: In the 1960's I was an engineer who worked on the AGC at Raytheon.  I noticed four minor factual errors:
1.  There were actually 2 different integrated circuits in the AGC.  As stated, one was the dual RTL 3-input nor gates used for the logic (made by Fairchild).  In addition the menory used an integrated circuit sense amplifier (made by Norden).
2.  The packaging of the ICs was not in flat packs, but metal TO-5 can-style packages (I forget the number of leads).  I believe this was done for cooling reasons.  The AGC was conduction cooled (no air in space) and the individual modules had magnesium headers with holes for the TO-5 cans, resistors, and other components.
3.  Although wire wrap was used in the backplane connections (for module-to-module connections), within the (epoxy) potted modules the interconnections were welded wire.  (Solder connections were viewed as too unreliable).
4.  I challange the photo of the erasable core memory.  The erasable memory was not built of individual planes as pictured, but was a "folded" design which was also potted to form a module like the others, but in a "silastic" type material since the epoxy was too rigid and the cores couldn't stand any compressive force.  Interestingly, each core had 4 tiny magnet wires threading through it.  The requirement of no solder connections also applied to those magnet wires each of which needed to thread several hundred tiny cores.  In the process of manufacturing the "folded" stack, sometimes cores needed to be replaced (because they were defective), which meant painstakingly removing and replacing the 4 wires involved.  In commercial memories where solder splices are allowed it's easy, but for the AGC memory the 4 wires needed to be replaced and re-strung.  — Preceding unsigned comment added by 72.28.170.2 (talk) 19:01, 8 July 2012 (UTC)
replies: text: Regarding item 2, apparently the Block I version used can-style packages and the Block II used flat packs. See this document: http://klabs.org/history/history_docs/mit_docs/1675.pdf "Case History of the Apollo Guidance Computer" by Eldon Hall of MIT, which says on page 12 "Table II is a comparison of the Block II computer characteristics with those of Block I. ... it became [in the Block II] possible to increase the complexity of the equivalent circuit in each microcircuit element [integrated circuit] and also to change from the TO-47 package [a metal can with 6 leads coming out the bottom] to the flat package. ... The reduction in size [from the Block I to the Block II] is attributed mostly to the decrease in the number and size of the logic-gate packages, that is, from 4,100 TO-47 [can] packages to 2,800 flat packages." Also see figure 9 on page 18 which illustrates the packages and the internal circuitry. Differtus (talk) 07:21, 5 March 2015 (UTC)

text: I mistrust the photo of the erasable memory as well. Neither the photo itself nor any NASA document seems to confirm, that this is actually a module from the AGC. To me it looks like the photo has been published here to stimulate ebay bids (see listing #332010866033). Opawaltrop (talk) 13:32, 31 October 2016 (UTC)
thread_title:  Factual errors 

message: Found this from page 95 of the book "The Apollo Guidance Computer: Architecture and Operation, ed:Frank OBrien"
"Communicationg with the outside world :the IO system , The EDRUPT instruction" 
It appears to explain how the EDRUPT Instruction was used. For disabling interupt, and appears to have been used for implimenting self diagnostic tests.
When EDRUPT is run, all other intrupts are disabled,  and ZRUPT register loaded with value of Z (program counter) , and during the LM autopilot , code for terminating the DAP cycle is run, then RESUME instructoin is run at the end to re-enable interupts.
so the like (not exact word-to-word, just extracting how the Instruction is __supposed_ to work)
Not verified on the AGC emu yet, but this appears reasonable.
27.253.192.65 (talk) 15:21, 3 April 2013 (UTC)
thread_title:  EDRUPT Instruction

message: The wikipedia article claims there are only 11 opcodes. However two of the links in the external section clearly contradict that:
* Virtual AGC Home Page – Ronald Burkey's AGC simulator, plus source and binary code recovery for the Colossus (CSM) and Luminary (LEM) SW  ( http://www.ibiblio.org/apollo/index.html )
* AGC4 Memo #9, Block II Instructions – The infamous memo that served as de facto official documentation of the instruction set ( http://authors.library.caltech.edu/5456/01/hrst.mit.edu/hrs/apollo/public/archive/1689.pdf  page 7 and onwards, for instance)  — Preceding unsigned comment added by 2.221.28.211 (talk) 17:21, 27 May 2013 (UTC)
replies: text: I believe 11 opcodes is for Block I; the Block II section of the article says 34 opcodes. Kaleja (talk) 17:07, 21 April 2015 (UTC)
replies: text: The op-code used a 3-bit field, so there were actually only 8 legitimate opcodes. This was extended to 11 using some sort of hack involving deliberate register overflows. The remainder of the 34 were instructions for the Colossus/Luminary virtual machine. Rhoark (talk) 22:18, 4 June 2015 (UTC)
thread_title:  Only 11 opcodes? 

message: Margaret Hamilton has become somewhat of an icon for women in STEM in the past year or so, which is all well and good, but it has caused some dubious claims about her accomplishments to creep in. She is often called, without qualification, the leader of Apollo software design. This was not a responsibility she held from the beginning of the program, rather only becoming the leader for command module software sometime after Apollo 8, and gaining responsibility for LM software later still. Major architectural elements she has been given credit for, namely the virtual machine interpreter and the executive/waitlist system were not her creations. The virtual machine was designed by Hugh Blair-Smith. The executive/waitlist system was designed by J. Halcombe Laning and Richard Battin, based on a paper they wrote in 1956 and their experience with the WHIRLWIND machine. Hamilton certainly worked on implementing and applying these ideas, but there is no evidence that she originated these or similarly influential concepts. The main idea she originated, "Higher-Order Systems" was dead-on-arrival in academia [] and industry []. HOS seems to bear some relation to the work of Dijkstra and Hoare that preceded it as well as systems like coq or Haskell's Arrows that came later, but neither building on the former nor influencing the latter.
The idea that she coined the term "software engineering" is possible, but dubious. When asked directly about it, she said only that she "began to use" the term, not that she began its use. The most widely recognized confirmed use of the term was in a 1965 letter by the president of the ACM, when Hamilton was 27 years old and not yet prominent in the Draper lab. Douglas T. Ross claims to have been using it as much as a decade before that, which Hamilton likely would have been exposed to while they both worked on the SAGE system. As eventual team leader, there's no doubt that Hamilton was involved in industrial aspects of software engineering like configuration management and schedule prediction. However, these things were quite notoriously imposed on the Draper lab by Bill Tindall around the time of Apollo 1. A sort of mythology has built up that software at that time was mostly done by women because it was denigrated by the male hardware engineers, but this doesn't really fit the facts. It conflates the kind of work Hamilton did with the rope memory weavers, who were seen as akin to telephone operators. Hamilton was among a tiny minority of women actually writing software at that time. It's true electrical hardware engineers tended to be dismissive of the challenges of software, but this was not a gender-based divide. Male computer scientists had been contending with it just as much as the few female ones for a decade by the time Hamilton began to work on Apollo. Software design and quality assurance became a program priority driven top-down by George C. Mueller and Bill Tindall, not something practiced surreptitiously and unappreciated. Rhoark (talk) 23:32, 4 June 2015 (UTC)
replies: text: Not to parse the article too closely, but the article says that under Hamilton's direction a "sophisticated software interpreter" was developed.  While the claims you are refuting may be being made elsewhere, only the interpreter leadership is made here.  I suggest that unless the claim regarding the interpreter is in dispute that this tag be removed from the first instance of the article.  Further she was an early drive force for systems and realtime engineering and indeed "The design principles developed by Hamilton for the AGC became foundational to "software engineering"" is a fair comment.  SO I believe that the second instance of the Dispute tage should be removed.  — Preceding unsigned comment added by 204.128.192.34 (talk) 21:20, 16 October 2015 (UTC)
replies: text:  I originally wrote to generally debunk questionable claims both in the article and in the sources these claims come from. With respect to the claim that the interpreter was developed under Hamilton's leadership:
* Hugh Blair-Smith claims to have independently written the YUL assembler for the interpreted language beginning in 1959 (before the Draper lab was even contracted to work on Apollo.) 
* That doesn't say who was responsible for the design of the interpreter of the resulting pseudocode, but does imply there was enough of a specification existing by 1959 for Blair-Smith to begin the task.
* Blair-Smith maintained YUL throughout the Apollo program.
* He also takes credit for "microprogramming" of the "instruction repertoire" which possibly refers to the interpreter backend, but I can't say for sure.
* This 1961 manual for the Mod 3C computer includes thorough description of the interpreter. It is authored by Ramon Alonso, J. Halcombe Laning Jr., and Hugh Blair-Smith.
* According to the book Digital Apollo the interpreter was the "brainchild" of Laning, as was the executive scheduler (which among other feats allowed the computer to not be overloaded by the Apollo 11 radar mishap.)
* I can't find an exact date for when Margaret Hamilton began to work on Apollo, but the outside bounds are placed by her working on weather-predicting software in 1960 and becoming Apollo software lead in 1965. Some of the intervening time was spent on the SAGE air defense system.
* According to this 1976 history published by the Draper lab Hamilton led a team that implemented designs handed down by senior engineers.
* Hamilton became responsible for all command module software after Apollo 8, by which time foundational elements of the software architecture had long been established.
Taken together, I think this thoroughly establishes that however invaluable Hamilton's contributions were to the program, the interpreter could not possibly have been developed under Hamilton's direction, and it's extremely dubious that she influenced the interpreter or executive design in any significant way. Rhoark (talk) 03:46, 17 October 2015 (UTC)
thread_title:  Margaret Hamilton Hagiography 

message: In what ways is the AGC4 Memo #9, Block II Instructions (listed as an external link), infamous?  Were there significant errors in the documentation?69.69.101.185 (talk) 21:36, 30 November 2017 (UTC)
replies: text: See annotation to Chapter 11, pages 133-134 (Hugh Blair-Smith's Annotations to Eldon Hall's Journey to the Moon) 
"So the (in)famous Memo #9, simply listing the instructions and what they did, was enough for the people that mattered, and not enough for the people who didn't matter."
--89.25.210.104 (talk) 19:25, 5 April 2018 (UTC)
thread_title:  Infamous AGC4 Memo #9, Block II Instructions? 

message: === Before Block I ===
* Transistor computers:
1959 - Mod 1A construction started
1960 - Construction of Mod 1B started, Mod 1A abandoned
1961 - Mod 1B completed
1962 - Mod 3C built, modified and completed into AGC3
* IC computers:
1963 - AGC4
=== Block I ===
1964 - AGC5, AGC6
1965 - AGC7
Sources:
:
3. Ramon L. Alonso et al., A Digital Control Computer Developmental Model 1B (1962, 12 Mbytes).
15. Hugh Blair-Smith, Annotations for Eldon Hall's book, Journey to the Moon:  The History of the Apollo Guidance Computer (1997) ()
38. Eldon C Hall, MIT's Role in Project Apollo, Vol. III: Computer Subsystem (1972, 11 Mbytes).
,  (Additional materials) — Preceding unsigned comment added by 89.25.210.104 (talk) 18:48, 5 April 2018 (UTC)
replies: text: Includes a picture of AGC4.  — Preceding unsigned comment added by Iandiver (talk • contribs) 18:06, 27 May 2023 (UTC)
thread_title:  Before Block II 

message: 1964 - AGC5, AGC6
1965 - AGC7
Sources:
:
3. Ramon L. Alonso et al., A Digital Control Computer Developmental Model 1B (1962, 12 Mbytes).
15. Hugh Blair-Smith, Annotations for Eldon Hall's book, Journey to the Moon:  The History of the Apollo Guidance Computer (1997) ()
38. Eldon C Hall, MIT's Role in Project Apollo, Vol. III: Computer Subsystem (1972, 11 Mbytes).
,  (Additional materials) — Preceding unsigned comment added by 89.25.210.104 (talk) 18:48, 5 April 2018 (UTC)
replies: text: Includes a picture of AGC4.  — Preceding unsigned comment added by Iandiver (talk • contribs) 18:06, 27 May 2023 (UTC)
thread_title:  Block I 

message: The pink/peach alloy used for the case - was it magnesium-aluminium - as stated by WSJ  ? - Rod57 (talk) 00:12, 16 September 2019 (UTC)
thread_title:  What case material 

message: I tried to find this information in the article, but couldn't find it. Obviously there must have been two AGCs for every manned mission to the moon, but how many were tested before / flown on unmanned missions / used for something else later? And those few the article mentions that were used for later projects, were they surplus left over from the Apollo program, or did Raytheon continue to build some more specifically? --BjKa (talk) 19:13, 14 April 2020 (UTC)
thread_title:  How many were built? 

message: For anyone interested in the AGC in general, or any detail of its workings and operation, I can highly recommend the videos of CuriousMarc, documenting a restoration of a privately owned AGC to working condition. --BjKa (talk) 19:13, 14 April 2020 (UTC)
thread_title:  Youtube Recommendation 

message: the list of registers in this article is incomplete. 
for a more complete list of registers see Appendix C in the book: "The Apollo Guidance Computer: Architecture and Operation" by Frank O'Brien. ISBN: 1441908773, 9781441908773  — Preceding unsigned comment added by 61.69.177.79 (talk) 07:10, 6 December 2020 (UTC)
thread_title: Other registers Incomplete