The Charming Genius of the Apollo Guidance Computer
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The Apollo Project was the first flight system to deploy with a digital, general-purpose computer made of integrated circuits at its core: the Apollo Guidance Computer (AGC). It was a complete research project: no IC computer had run consecutively for more than a few hours, sophisticated programming techniques were unknown and the interactive human/computer interface had to be invented and made to appeal to astronauts opposed to machine interference in flight operations. In this talk I'll give the historical context for the AGC, discuss its initial design and the evolution of this design as the Apollo Project progressed. We'll do a deep-dive on the machine architecture and note how tight integration with a special-purpose vehicle admitted incredibly sophisticated behaviour from a primitive machine. We'll further discuss the human/computer interface for the AGC, how the astronaut's flight roles dictated the computer's role and vice versa. Motivating examples from select Apollo flights will be used. Throughout, we'll keep an eye on lessons to be gleaned from the experience of engineering the AGC and how we can adapt these lessons to modern computer systems in mission-critical deployments.
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The Charming Genius of the Apollo Guidance Computer
- 1. The Charming Genius of the Apollo Guidance Computer
- 5. Apollo is carried into orbit by the three stage Saturn V.
- 8. Final stage boosts Apollo into Free Return orbit.
- 15. This is Apollo.
- 16. Space travel is a tricky business.
- 17. Orbital paths are a narrow balance of accelerations.
- 18. Too little, you miss your goal.
- 19. Too much, you miss your goal.
- 20. The guidance control system of a spacecraft must answer three questions:
- 21. 1. Which way is up? 2. Where am I? 3. Where am I going?
- 22. Apollo has triply redundant means to answer these questions.
- 24. Global network of long-range radar stations.
- 26. High Gain Antena
- 33. Dead Reckoning
- 38. The Apollo Guidance Computer integrates all of these systems.
- 40. It also is the fly-by-wire control computer.
- 42. It also manages both the analog and digital displays.
- 45. It does this with:
- 46. five interrupts,
- 47. 1MHz clock,
- 48. 16-bit words,
- 49. 2048 words RAM,
- 50. 36K words ROM,
- 52. It weighs 70lbs.
- 53. It is 1 meter3 .
- 54. It draws 55 watts.
- 55. Real Time Computing Complex
- 56. AGC is a digital computer.
- 57. There have been others.
- 58. It’s the first to use Integrated Circuits.
- 59. Core-rope memory is used as well.
- 61. One of the first interactive computers.
- 63. DSKY
- 64. Allows AGC to give fast updates to Astronauts.
- 67. What is the computer doing back there?
- 68. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec
- 69. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec } Mission Time
- 70. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec } Wait List
- 71. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec } Wait List T4RUPT
- 72. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec } Wait List T4RUPT Digital Autopilot
- 73. 10050 150 200 250 300 3500 400 450 500 600 700 800 900 1000550 650 750 850 950 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 AGC Interrupts, 1sec } Wait List T4RUPT Digital Autopilot Fine Scale Clock
- 74. TIME3, TIME4 and TIME6 are programmable.
- 75. Nowadays, we would call the AGC a “priority-scheduled real-time embedded” computer.
- 78. Priority Scheduling Two tables exist for jobs in the AGC.
- 81. Priority Scheduling Waitlist •<4ms execution •9 task limit •basic instructions
- 82. Priority Scheduling Waitlist •<4ms execution •9 task limit •basic instructions •no rescheduling
- 83. Priority Scheduling Waitlist •<4ms execution •9 task limit •basic instructions •no rescheduling •no Executive
- 84. Executive?
- 85. The AGC software provides two operating abstractions.
- 87. Executive • low-level routines • system restarts Operating Abstractions
- 88. Executive • low-level routines • system restarts • supervision Operating Abstractions
- 89. Executive • low-level routines • system restarts • supervision • keeps CoreSet Operating Abstractions
- 91. There’s nothing to stop the CoreSet from filling up.
- 92. Most invariant guarantees are left to analysis and testing.
- 94. Priority Scheduling CoreSet •12 task limit •priority ordered •20ms interrupt
- 95. Priority Scheduling CoreSet •12 task limit •priority ordered •20ms interrupt •option to use Interpreter
- 96. Interpreter?
- 97. The native instructions available from the AGC are very primitive.
- 98. AGC word-size of 15 data bits has insufficient accuracy for spaceflight.
- 100. Interpreter Operating Abstractions • high-level routines • rich instruction set
- 101. Interpreter Operating Abstractions • high-level routines • rich instruction set • extra-wide words
- 102. Interpreter Operating Abstractions • high-level routines • rich instruction set • extra-wide words • radically simpler programming
- 103. Free Return Orbit
- 104. Capture Braking
- 105. Capture Braking
- 106. Lunar Orbit
- 107. Up to this point, loss of the CM computer hasn’t been abort- worthy.
- 108. Different matter for the LM.
- 109. The LM computer is absolutely essential to a controlled landing on the Moon.
- 111. Program 63 fires LM engines.
- 112. Program 63 fires LM engines. (Routine in AGC source is BURNBABY.)
- 113. Program 63 fires LM engines. (Apollo 11’s CoreSet overflowed here.)
- 115. Program 64 pitches craft. (All LMs had a potentially fatal bug.)
- 117. Program 66 steadies thrust vector. (Early LMs had a nearly fatal bug.)
- 119. The AGC is interesting, but why study it?
- 121. Few believed it could ever be dependable.
- 122. Yet, it was.
- 123. How?
- 125. The techniques developed for the AGC are in use today.
- 126. The past…
- 127. The past…
- 128. The past…
- 129. The past…
- 130. informs our present.
- 131. We sit inside of a great project.
- 132. The techniques that we develop today…
- 133. are the foundation of the future.
- 134. So build something fucking amazing.
- 135. Thank you! <3
- 136. BIBLIOGRAPHY •The Apollo Guidance Computer: Architecture and Operation Frank O’Brien •Journey to the Moon: The History of the Apollo Guidance Computer Eldon C. Hall •Stages to Saturn Roger E. Bilstein •How Apollo Flew to the Moon W. David Woods •Digital Apollo: Human and Machine in Spaceflight David A. Mindell