The Apollo Guidance Computer: Architecture and Operation

by Frank O'Brien

Designing a mission for a flight to the Moon requires balancing the demands of a wide array of spacecraft systems, with the details of tending each component generating complex and often contradictory requirements. More than any other system in the Apollo spacecraft, the Apollo Guidance Computer drove the capabilities of the lunar missions. In the 1960's, most computers filled an entire room yet the spacecraft's computer was required to be compact and require little power.  When compared to modern systems, the AGC's design limitations and lack of speed presented formidable challenges. Yet, hardware and software engineers overcame these difficulties, and their creation was able to guide a new and complex spacecraft and its precious human cargo away from the safety of Earth and towards a new world. Although people today find it difficult to accept that it was possible to control a spacecraft using such a 'primitive' computer, it nevertheless had capabilities that are advanced even by today's standards.

The Apollo Guidance Computer: Architecture and Operation is the first comprehensive description of the Apollo computer, beginning with its internal organization to its user interface and flight software. Particular emphasis is placed on the instruction set, Executive capabilities, the Interpreter and the detailed procedures for mission application software. Launch, landing on the Moon and entry back on Earth are explained in rich detail and show how the computer was an integral part of the spacecraft operation. As a comprehensive account, it spans the disciplines of computer science, aerospace engineering and spacecraft operations. The Apollo Guidance Computer: Architecture and Operation is an essential reference for space historians and engineers, and serves well as a complementary text for computer science courses.

Genres: Space, History, Science, Nonfiction, Programming, Computers, Computer Science

430 pages, Paperback

First published July 1, 2010

Reviews

[Torben Koch · Rating 5 out of 5]

Very exciting :)

[Brian Page · Rating 4 out of 5]

I’ll begin with the obvious: The Apollo Guidance Computer: Architecture and Operation by Frank O’Brien is not an easy read. But for the reader who comes to this book properly prepared, it is wonderful. The detail is nearly mind-boggling and thus, by properly prepared, I believe that for a reader to fully appreciate the content, they ought to have a significant understanding of computer architecture and operating system design. The Apollo program in its entirety was an enormously, almost unimaginably, complex assembly of technologies. Without exaggeration, a major part of the complexity, the heart & soul, was the AGC and the software written to control the mission. Unfortunately, the need for nearly infinite cleverness by the programmers was the result of a self-inflicted wound. The original design specification called for 12-bit addressability and only eight low-level instructions (op codes). To no one’s surprise today (and it shouldn’t have been a surprise in the late 1950s when the Apollo concept began to gel), such a limited architecture with addressability into 4K blocks of 15-bit words, is totally inadequate. In retrospect, it’s apparent that the limitations of the AGC could easily have significantly delayed the moon landing. That it did not, and the techniques developed to overcome the architecture deficiencies, is what this book is all about. O’Brien guides the reader through a microscopic examination of the architecture to the bit level (about 1/3 of the book) and then operating system (Executive & Interpreter) structure & flow (another 1/3), and finally a program-by-program run through of a moon landing mission profile. I’m quite willing to call this book definitive, and the best account for communicating the actual complexity of an Apollo mission. It’s difficult to imagine the effort O’Brien expended in comprehending the arcane intricacies of the AGC architecture & operation, but historians of technology and fans of the Apollo space program will be grateful for his labors.

[Adam J Cecchetti · Rating 4 out of 5]

A good book but strangely pedantic and strangely vague at times. If your idea of a good time doesn't include reading a one off CPU Instruction Set Architecture and text descriptions of program execution related to orbital mechanics and flight operations this might not be for your.

[Peter Caron · Rating 4 out of 5]

I guess I never considered the obvious fact that rockets can be steered! This is a tough read but I found it worth the effort. The development of the navigation computers was as much about creative problem solving as it was about technology so, there is relevance for such work even today.

[Andy Cromer · Rating 4 out of 5]

Frank O'Brien is a gifted engineer who introduces the Apollo Guidance Computer step by step or should I say register by register.

After the core knowledge of the processing system the book switches to an overview of the program sequence and basics to guidance and navigation of the Apollo spacecraft. Overall a great book about the Apollo missions.

[Nikky · Rating 4 out of 5]

If you've read books like Digital Apollo and wanted more information about the hardware and software that went into the Apollo Guidance Computer, this is the book for you. Thick with nuanced details about the inner workings of the AGC, you'll learn a lot about how a highly resilient and capable system was made in the late 1960s.

What you won't learn, however, is the design process or the writing of the software. It reads much like a technical manual for the AGC rather than a detailed history.

[Greg Parrott · Rating 5 out of 5]

Though at times very dry, and a bit surprising at the number of typos, this book is a must read for engineers of all types. Before there were very many integrated circuit chips floating around, NASA took a gamble on them to meet the size, weight, and power requirements for getting men to the moon and back. Even back in the 1960's, engineers were having to more with less. A situation we still find today when "COGs" is often waved around by the bean counters.

[Matthew Wilson · Rating 5 out of 5]

Fascinating look, from both a computer engineering and computer science perspective, at the details of the computer that made the moon missions possible. Amazing what the engineers came up with, both working within the constraints of the available technology while also pushing the boundary of technology.

[Peter Joseph · Rating 5 out of 5]

A towering master work on what, admittedly is a fairly niche subject but if you want the real details on the computers that played a vital role in putting humans on the moon then read this. This book is more of a computer science text than a general historical book however, so beware of this.

[Neal W · Rating 3 out of 5]

Bit esoteric for my taste. While I enjoy a technical read this one went a bit far over my head. Later chapters more of what I hoped for but a little short on analysis

[Neal Aggarwal · Rating 5 out of 5]

A fantastic trip down memory lane.