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The Charming Genius of the Apollo Guidance Computer

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The Charming Genius of the Apollo Guidance Computer


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This is the Earth


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This is the Moon


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This is the Moon 6 5 6 9 0 4 This is the Earth m k


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Apollo is carried into orbit by the three stage Saturn V.


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System checks happen in LEO.


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Final stage boosts Apollo into Free Return orbit.


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6k m 56 9 40 n oo M Trans-lunar Injection


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Free Return Orbit


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This is Apollo.


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Space travel is a tricky business.


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Orbital paths are a narrow balance of accelerations.


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Too little, you miss your goal.


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Too much, you miss your goal.


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The guidance control system of a spacecraft must answer three questions:


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1. Which way is up? 2. Where am I? 3. Where am I going?


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Apollo has triply redundant means to answer these questions.


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Deep Space Network


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Global network of long-range radar stations.


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High Gain Antena


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Celestial Navigation


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High Gain Antena Telescope


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High Gain Antena Sextant Telescope


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Mission timed star charts.


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Celestial Navigation


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Dead Reckoning


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Inertial Guidance


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Three gimbals save weight.


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Gimbal lock is the tradeoff.


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Celestial Navigation Dead Reckoning


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The Apollo Guidance Computer integrates all of these systems.


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It also is the fly-by-wire control computer.


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It also manages both the analog and digital displays.


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It does this with:


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five interrupts,


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1MHz clock,


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16-bit words,


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2048 words RAM,


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36K words ROM,


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and 17 registers.


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It weighs 70lbs.


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It is 1 meter . 3


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It draws 55 watts.


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Real Time Computing Complex


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AGC is a digital computer.


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There have been others.


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It’s the first to use Integrated Circuits.


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Core-rope memory is used as well.


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One of the first interactive computers.


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DiSplay & KeYboard


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DSKY


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Allows AGC to give fast updates to Astronauts.


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Allows astronaut running of select programs.


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What is the computer doing back there?


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 } Mission Time 1000


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 } 500 550 600 650 700 750 800 850 Wait List 900 950 1000


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 } 500 550 600 650 700 750 800 850 Wait List T4RUPT 900 950 1000


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 } 500 550 600 650 700 750 800 850 900 950 1000 Wait List T4RUPT Digital Autopilot


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0 TIME2 TIME1 TIME3 TIME4 TIME5 TIME6 50 AGC Interrupts, 1sec 100 150 200 250 300 350 400 450 } 500 550 600 650 700 750 800 850 900 950 1000 Wait List T4RUPT Digital Autopilot Fine Scale Clock


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TIME3, TIME4 and TIME6 are programmable.


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Nowadays, we would call the AGC a “priority-scheduled real-time embedded” computer.


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Priority Scheduling


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Priority Scheduling (Invented for the AGC)


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Priority Scheduling Two tables exist for jobs in the AGC.


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Waitlist Priority Scheduling •<4ms execution


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Waitlist Priority Scheduling •<4ms execution •9 task limit


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Waitlist •<4ms execution •9 task limit Priority Scheduling •basic instructions


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Waitlist •<4ms execution •9 task limit Priority Scheduling •basic instructions •no rescheduling


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Waitlist •<4ms execution •9 task limit Priority Scheduling •basic instructions •no rescheduling •no Executive


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Executive?


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The AGC software provides two operating abstractions.


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Executive • Operating Abstractions low-level routines


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Executive • Operating Abstractions • low-level routines system restarts


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Executive • Operating Abstractions • • low-level routines system restarts supervision


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Executive • Operating Abstractions • • • low-level routines system restarts supervision keeps CoreSet


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CoreSet Priority Scheduling •12 task limit


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There’s nothing to stop the CoreSet from filling up.


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Most invariant guarantees are left to analysis and testing.


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CoreSet Priority Scheduling •12 task limit •priority ordered


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CoreSet •12 task limit •priority ordered Priority Scheduling •20ms interrupt


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CoreSet •12 task limit •priority ordered Priority Scheduling •20ms interrupt •option to use Interpreter


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Interpreter?


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The native instructions available from the AGC are very primitive.


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AGC word-size of 15 data bits has insufficient accuracy for spaceflight.


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Interpreter • high-level Operating Abstractions routines


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Interpreter • high-level Operating Abstractions routines • rich instruction set


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Interpreter • high-level routines • rich instruction set Operating • extra-wide words Abstractions


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Interpreter • high-level routines • rich instruction set Operating • extra-wide words Abstractions • radically simpler programming


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Free Return Orbit


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Capture Braking


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Capture Braking


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Lunar Orbit


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Up to this point, loss of the CM computer hasn’t been abortworthy.


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Different matter for the LM.


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The LM computer is absolutely essential to a controlled landing on the Moon.


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Program 63 fires LM engines.


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Program 63 fires LM engines. (Routine in AGC source is BURNBABY.)


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Program 63 fires LM engines. (Apollo 11’s CoreSet overflowed here.)


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Program 64 pitches craft.


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Program 64 pitches craft. (All LMs had a potentially fatal bug.)


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Program 66 steadies thrust vector.


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Program 66 (Early LMs had a steadies nearly fatal bug.) thrust vector.


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The AGC is interesting, but why study it?


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The AGC was barely possible.


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Few believed it could ever be dependable.


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Yet, it was.


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How?


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Careful, pragmatic and empirical engineering.


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The techniques developed for the AGC are in use today.


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The past…


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The past…


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The past…


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The past…


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informs our present.


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We sit inside of a great project.


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The techniques that we develop today…


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are the foundation of the future.


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So build something fucking amazing.


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Thank you! <3


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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


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