The Apollo Guidance Computer Architecture and Operation Frank O'Brien The Apollo Guidance Computer Architecture and Operation Published in association with Praxis Publishing Chichester, UK Mr Frank O'Brien West Windsor New Jersey USA SPRINGER±PRAXIS BOOKS IN SPACE EXPLORATION SUBJECT ADVISORY EDITOR: John Mason, M.B.E., B.Sc., M.Sc., Ph.D. ISBN 978-1-4419-0876-6 Springer Berlin Heidelberg New York Springer is a part of Springer Science + Business Media (springer.com) Library of Congress Control Number: 2009936113 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. # Copyright, 2010 Praxis Publishing Ltd., Chichester, UK. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Jim Wilkie Copy Editor: David M. Harland Typesetting: BookEns Ltd, Royston, Herts., UK Printed in Germany on acid-free paper Contents List of figures .................................................. ix Author's preface................................................ xiii Acknowledgments...............................................xvii 0 The state of the art .......................................... 1 From whence we came: early computing . ........................ 1 Outside the computer room: early computing in aviation and space ..... 2 Computing in manned spacecraft ............................... 3 Defining computer ``power'' ................................... 4 The evolution of computing ................................... 7 Technology acquisition: consumers vs the aerospace industry .......... 8 1 The AGC hardware ..........................................11 Introduction ...............................................11 Overview of Chapter 1 .......................................15 Physical characteristics of the AGC .............................16 Properties of number systems ..................................18 Double precision numbers.....................................27 FIGMENT ................................................29 Instructions: the basic units of computer operation ..................29 Memory management ........................................31 A tour of low core and the central registers .......................37 Keeping time in the AGC: timers and clocks ......................47 Counters ± CDUS (X, Y, Z, OPTS, OPTT) and PIPAS (X, Y, Z) ......51 Radar, engine and crew interfaces...............................55 Memory addressing and banking in the AGC ......................59 Interrupt processing .........................................68 The instruction set ..........................................70 Communicating with the outside world: the I/O system ..............87 vi Apollo Guidance Computer 2 The Executive and Interpreter ..................................99 Introduction to the Executive ..................................99 Scheduling: preemptive and cooperative multiprogramming ..........101 The Executive .............................................103 The astronaut interface: the display and keyboard .................123 Telemetry uplink...........................................140 Synchronous I/O processing and T4RUPT .......................143 High level languages and the Interpreter . .......................150 The Interpreter ............................................160 3 The basics of guidance and navigation ...........................199 Hardware unique to solving guidance and navigation problems .......199 The important questions in guidance and navigation................209 Question 1: Which way is up? .................................209 Question 2: Where am I? ....................................217 Question 3: Which way am I going? ............................222 4 Mission programs and operations ...............................231 Introduction ..............................................231 Launch from Earth.........................................231 The lunar landing ..........................................245 Lunar orbit rendezvous......................................287 The digital autopilot ........................................312 Erasable memory programs...................................334 AGC data uplink and downlink ...............................337 Command Module entry.....................................341 Computer problems during Apollo 11 and Apollo 14 ...............358 Epilogue .................................................365 Appendixes ...............................................369 A: AGC instruction set ......................................369 B: AGC interrupt vectors ....................................371 C: Layout of the special and control registers .....................372 D: Command Module I/O channels ............................373 E: Lunar Module I/O channels ................................379 F: Interpreter instruction set ..................................386 G: Command Module programs (Major Modes)...................391 H: Command Module routines ................................392 I: Command Module verbs ...................................393 J: Command Module nouns ..................................395 K: Command Module program alarms . .......................401 L: Lunar Module programs (Major Modes) ......................403 M: Lunar Module routines ...................................404 N: Lunar Module verbs .....................................405 Contents vii O: Lunar Module nouns .....................................407 P: Lunar Module program alarms..............................412 Q: Command Module and Lunar Module downlists ................415 R: AGC navigation star catalog ...............................417 S: Configuring the CSM and LM DAP (Routine 03) ...............418 Glossary of terms and abbreviations .................................421 Bibliography ..................................................423 Illustration credits ..............................................427 Index........................................................431 To my mother and father Mom is no longer with us, but I know that future explorers will use her star in Heaven to guide them safely home. List of figures 1 Apollo Guidance Computer and DSKY . ........................17 2 AGC component locations ....................................18 3 Numeric representation in the AGC word. ........................20 4 Fractional powers of 2 .......................................23 5 Integer vs fractional representation ..............................24 6 Negation of one and two's complement . ........................28 7 AGC instruction format ......................................30 8 Core memory ..............................................35 9 Schematic of core rope .......................................36 10 Flight version of core rope ....................................37 11 Arithmetic examples in modified one's complement .................40 12 Logical and cyclical shifts .....................................46 13 Inertial Measurement Unit ....................................51 14 Codes for radar data source selection ............................57 15 LM tapemeters: altitude and altitude rate . ........................59 16 Erasable storage and bits 10 and 9 ..............................61 17 Erasable storage banking .....................................62 18 Fixed storage and bits 12 and 11 ...............................63 19 Fixed storage banking........................................63 20 View of both erasable and fixed storage . ........................64 21 Instruction format including the quarter code ......................72 22 AND, OR and XOR logical operations . ........................77 23 The TS instruction and overflow processing .......................79 24 Summing five entries in a table .................................85 25 Instructions created from special cases of existing instructions .........86 26 Characteristics of counter registers and I/O channels ................90 27 The AGC and its I/O devices and interfaces .......................91 28 I/O instruction format .......................................92 29 I/O channel usage ...........................................93 x Apollo Guidance Computer 30 Core Set layout............................................105 31 Core Sets and job scheduling .................................106 32 Vector Accumulator area layout ...............................107 33 Allocated Core Sets and VAC areas ............................108 34 Waitlist tables.............................................115 35 Addition of a new waitlist task ................................116 36 Restart and phase tables .....................................118 37 The +phase / ±phase tables ..................................120 38 Display and keyboard.......................................124 39 Diagram of the DSKY ......................................126 40 Noun tables ..............................................139 41 Data