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.

1. The Charming Genius
of the
Apollo Guidance
Computer

2. This
is the
Earth

3. This
is the
Moon

4. This
is the
Earth
This
is the
Moon
405696km

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

7. System
checks
happen
in LEO.

8. Final stage boosts
Apollo into Free
Return orbit.

9. M
oon
405696km
Trans-lunar
Injection

13. 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.

23. Deep
Space
Network

24. Global network
of long-range
radar stations.

26. High Gain Antena

27. Celestial Navigation

28. High Gain Antena
Telescope

29. High Gain Antena
TelescopeSextant

30. Mission
timed
star
charts.

32. Celestial Navigation

33. Dead
Reckoning

34. Inertial
Guidance

35. Three
gimbals
save
weight.

36. Gimbal
lock is the
tradeoff.

37. Celestial Navigation
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,

51. and 17 registers.

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.

62. DiSplay &
KeYboard

63. DSKY

64. Allows
AGC to
give fast
updates to
Astronauts.

65. Allows
astronaut
running of
select
programs.

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.

76. Priority
Scheduling

77. Priority
Scheduling
(Invented for the AGC)

78. Priority
Scheduling
Two tables
exist for jobs
in the AGC.

79. Priority
Scheduling
Waitlist
•<4ms execution

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

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.

86. Executive
• low-level
routinesOperating
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

90. Priority
Scheduling
CoreSet
•12 task limit

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

92. Most invariant
guarantees are left to
analysis and testing.

93. Priority
Scheduling
CoreSet
•12 task limit
•priority ordered

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.

99. Interpreter
Operating
Abstractions
• high-level routines

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.)

114. Program 64
pitches craft.

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

116. Program 66
steadies
thrust vector.

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

119. The AGC is interesting,
but why study it?

120. The AGC
was barely
possible.

121. Few believed it
could ever be
dependable.

122. Yet, it was.

123. How?

124. Careful,
pragmatic
and empirical
engineering.

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