Apollo Guidance Computer (AGC) Restoration for 50th Anniversary of Apollo 11
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A deep dive into the Apollo Guidance Computer, and the hack that saved Apollo 14
How on Earth do you patch the software on a computer orbiting the Moon? Very carefully.
30 Jan 2020
https://arstechnica.com/science/202...e-computer-and-the-hack-that-saved-apollo-14/
Excerpt:
However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control, the LM's Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button—which meant there was a short-circuit or other electrical issue somewhere inside the LM's complicated guts.
This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.
Under hard time pressure, the ground had to quickly figure out what was wrong and devise a workaround. What they came up with was the most brilliant computer hack of the entire Apollo program, and possibly in the entire history of electronic computing.
To explain exactly what the hack was, how it functioned, and the issues facing the developers during its creation, we need to dig deep into how the Apollo Guidance Computer worked. Hold onto your hats, Ars readers—we're going in.
The Apollo Guidance Computer laid bare
It’s common to find that the AGC is often described as a mere calculator, or compared to a controller chip suitable for a watch or microwave. Looking at your watch, it tells the time and little else. The chip that drives the microwave blindly starts and stops the magnetron to heat expired Kung Pao chicken. In these devices, there is a very limited interaction with the surrounding hardware, no sophisticated computation, and no decision-making of any note.
In describing a "computer," one expects that the system would include the abilities we attribute to contemporary computers—the ability to run several programs at once, for example, or to present a simple yet intuitive interface, to control a wide variety of devices, and to gracefully recover from application errors. “Ha!” you might exclaim, “I carry a computer like that in my pocket!”
The idea of such capabilities being available nearly 60 years ago stretches credulity, but the Apollo Guidance Computer had these features and more. An interpreter to process “virtual” machines, similar to Java byte code? Check. The ability for remote data updates? Yup. Given all of these capabilities and more, it’s quite reasonable to argue that the AGC compares favorably with a modern smartphone. Yes, the AGC is slower and has far less memory, but that is only due to its unfortunate timing at birth, being at the wrong end of the Moore’s Law curve.
Although the processor at approximately 80,000 instructions per second was not especially fast, it is impossible to overemphasize the impact that its scarce memory had on AGC software developers. Consider the limits the programmers were under: all the software for the flight to the Moon and back had to fit in 36K words (15 bits long, plus 1 bit for parity) of read-only core rope memory. As “bytes” were not a concept in the AGC, all 15 bits of a word were accessed at once with no easy way to break the word into smaller divisions.
How on Earth do you patch the software on a computer orbiting the Moon? Very carefully.
30 Jan 2020
https://arstechnica.com/science/202...e-computer-and-the-hack-that-saved-apollo-14/
Excerpt:
However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control, the LM's Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button—which meant there was a short-circuit or other electrical issue somewhere inside the LM's complicated guts.
This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.
Under hard time pressure, the ground had to quickly figure out what was wrong and devise a workaround. What they came up with was the most brilliant computer hack of the entire Apollo program, and possibly in the entire history of electronic computing.
To explain exactly what the hack was, how it functioned, and the issues facing the developers during its creation, we need to dig deep into how the Apollo Guidance Computer worked. Hold onto your hats, Ars readers—we're going in.
The Apollo Guidance Computer laid bare
It’s common to find that the AGC is often described as a mere calculator, or compared to a controller chip suitable for a watch or microwave. Looking at your watch, it tells the time and little else. The chip that drives the microwave blindly starts and stops the magnetron to heat expired Kung Pao chicken. In these devices, there is a very limited interaction with the surrounding hardware, no sophisticated computation, and no decision-making of any note.
In describing a "computer," one expects that the system would include the abilities we attribute to contemporary computers—the ability to run several programs at once, for example, or to present a simple yet intuitive interface, to control a wide variety of devices, and to gracefully recover from application errors. “Ha!” you might exclaim, “I carry a computer like that in my pocket!”
The idea of such capabilities being available nearly 60 years ago stretches credulity, but the Apollo Guidance Computer had these features and more. An interpreter to process “virtual” machines, similar to Java byte code? Check. The ability for remote data updates? Yup. Given all of these capabilities and more, it’s quite reasonable to argue that the AGC compares favorably with a modern smartphone. Yes, the AGC is slower and has far less memory, but that is only due to its unfortunate timing at birth, being at the wrong end of the Moore’s Law curve.
Although the processor at approximately 80,000 instructions per second was not especially fast, it is impossible to overemphasize the impact that its scarce memory had on AGC software developers. Consider the limits the programmers were under: all the software for the flight to the Moon and back had to fit in 36K words (15 bits long, plus 1 bit for parity) of read-only core rope memory. As “bytes” were not a concept in the AGC, all 15 bits of a word were accessed at once with no easy way to break the word into smaller divisions.
Winston
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APOLLO 11 TRIG WAS BRIEF
https://hackaday.com/2020/06/12/apollo-11-trig-was-brief/
In this day and age where a megabyte of memory isn’t a big deal, it is hard to recall when you had to conserve every byte of memory. If you are a student of such things, you might enjoy an annotated view of the Apollo 11 DSKY sine and cosine routines. Want to guess how many lines of code that takes? Try 35 for both.
Figuring out how it works takes a little knowledge of how the DSKY works and the number formats involved. Luckily, the site has a feature where you can click on the instructions and see comments and questions from other reviewers.
According to commenter [Luís Batalha], the code uses a polynomial approximation, but not part of the Taylor series as you might expect. Apparently, the polynomial used had less error over the expected range of inputs than a similar number of Taylor terms. He even includes some graphs comparing different methods of computing the functions.
When we land on the moon again, soon, it will be amazing and awe-inspiring. Now think of doing it in a world where the best computer on the planet couldn’t keep up with the PC you have in your garage gathering rust. Even then, the DSKY wasn’t even that computer. It ran with a 12 microsecond clock speed and had a whopping 72 kB of memory — most of it not writable.
https://fermatslibrary.com/s/apollo-11-implementation-of-trigonometric-functions
https://hackaday.com/2020/06/12/apollo-11-trig-was-brief/
In this day and age where a megabyte of memory isn’t a big deal, it is hard to recall when you had to conserve every byte of memory. If you are a student of such things, you might enjoy an annotated view of the Apollo 11 DSKY sine and cosine routines. Want to guess how many lines of code that takes? Try 35 for both.
Figuring out how it works takes a little knowledge of how the DSKY works and the number formats involved. Luckily, the site has a feature where you can click on the instructions and see comments and questions from other reviewers.
According to commenter [Luís Batalha], the code uses a polynomial approximation, but not part of the Taylor series as you might expect. Apparently, the polynomial used had less error over the expected range of inputs than a similar number of Taylor terms. He even includes some graphs comparing different methods of computing the functions.
When we land on the moon again, soon, it will be amazing and awe-inspiring. Now think of doing it in a world where the best computer on the planet couldn’t keep up with the PC you have in your garage gathering rust. Even then, the DSKY wasn’t even that computer. It ran with a 12 microsecond clock speed and had a whopping 72 kB of memory — most of it not writable.
https://fermatslibrary.com/s/apollo-11-implementation-of-trigonometric-functions
There is a good book on this computer that I just happen to be re-reading currently:
Link here:
https://arc.aiaa.org/doi/book/10.2514/4.868023
"Evolution of the Apollo Guidance Computer, Mr. Hall contends that the development of the Apollo computer supported and motivated the semiconductor industry during a time when integrated circuits were just emerging. This was the period just before the electronics revolution that gave birth to modern computers. In addition, the book recalls the history of computer technology, both hardware and software, and the applications of digital computing to missile guidance systems and manned spacecraft. The book also offers graphics and photos drawn from the Draper Laboratories’ archives that illustrate the technology and related events during the Apollo project. Written for experts as well as lay persons, Journey to the Moon is the first book of its kind and a must for anyone interested in the history of science and the relevance of computer technology to space exploration. "
Link here:
https://arc.aiaa.org/doi/book/10.2514/4.868023
"Evolution of the Apollo Guidance Computer, Mr. Hall contends that the development of the Apollo computer supported and motivated the semiconductor industry during a time when integrated circuits were just emerging. This was the period just before the electronics revolution that gave birth to modern computers. In addition, the book recalls the history of computer technology, both hardware and software, and the applications of digital computing to missile guidance systems and manned spacecraft. The book also offers graphics and photos drawn from the Draper Laboratories’ archives that illustrate the technology and related events during the Apollo project. Written for experts as well as lay persons, Journey to the Moon is the first book of its kind and a must for anyone interested in the history of science and the relevance of computer technology to space exploration. "
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