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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.