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Simulation of the Apollo Guidance Computer. Julian Webb University of the West of England, Bristol, UK [email protected] Introduction. Main aim of project:

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Simulation of the apollo guidance computer
Simulation of the Apollo Guidance Computer

Julian Webb

University of the West of England, Bristol, UK

[email protected]

G146/MAPLD 2004


  • Main aim of project:

    • To produce a fully functional Block II LGC (Lunar Module version of the AGC) simulator/emulator running the original MIT-produced code and interacting with the user via reasonably accurate simulations of spacecraft controls

  • Though currently based on the LGC, the simulator would, with some alteration, also be able to represent the CMC

  • The AGC simulator is very much a “work in progress”!

G146/MAPLD 2004


  • Why?

    • To provide a working AGC based on original materials rather than a functional simulation

    • To provide an interesting and accessible history of computing educational resource

    • To honour the efforts of the programmers, engineers and managers involved at MIT/Draper Labs

    • To preserve some AGC materials in a machine-readable form

    • To facilitate statistical analysis of AGC performance

    • To allow better comparison with other 1960’s computers (has the AGC had a bad press?)

G146/MAPLD 2004

Simulator basics
Simulator basics

  • The simulator provides a real-time emulation of the LGC, DSKY and other associated LM systems in a package for use under Windows XP

  • Written in C++ (the AGC itself) and Visual Basic (DSKY and other systems)

    • these two major components communicate via DLLs

    • also possible to interface other programs to AGC

  • The AGC simulator adjusts its speed automatically to give the correct 11.7s clock rate

G146/MAPLD 2004

Sources used
Sources used

  • Available sources

    • Documentation

      • History of Recent Science and Technology project (mostly ex-MIT/Draper Labs documents)

      • Apollo Lunar Surface Journal (operational documentation, checklists etc.)

    • Source code listings (pdf files)

      • Luminary 1C (for LGC - two versions prior to Apollo 13 flight release)

      • Colossus 249 (for CMC - as flown on Apollo 9)

    • See References slide for more details

G146/MAPLD 2004


  • Method

    • Luminary source code entry (c1500 pages)

      • available as MS Excel and text files

    • ‘YUL assembler’ written (in Perl)

    • Assembled machine code checked against original core listing (and later against another AGC enthusiast’s version)

    • AGC written and tested

    • Interface DLL written

    • DSKY and other components written

    • Testing against original LM checklists

G146/MAPLD 2004

Difficulties encountered
Difficulties encountered

  • Difficulties - documentation

    • Lack of coherent documentation - many documents available but spread over a period of 10 years

    • Some documentation misleading (to the historian) due to numerous version changes - care required to interpret the documents in relation to their context

    • Documentation mainly aimed at informing MIT staff (for obvious reasons) - hence much background research required

    • Little documentation for YUL assembler

    • Lack of machine-readable source code

G146/MAPLD 2004

Technical difficulties
Technical difficulties

  • Difficulties - technical

    • Precise understanding of operation of several instructions required investigation of the microcode and...

    • some microcode control pulses are not well documented

    • A few questions remain concerning the YUL assembler output

    • Uplink loads not documented - necessary to simulate these to provide (e.g.) appropriate mission time, vehicle mass

G146/MAPLD 2004

System boundaries
System boundaries

  • Difficulties - system boundaries

    • LGC linked to other LM systems and to the mission control environment

    • Necessary to provide at least a basic simulation of many of these external systems

    • Full emulation limited (at present) to LGC, DSKY and AOT

    • Other systems provided in simulation as required by LGC functionality

    • A major omission is the AGS which seems to have little internal technical documentation remaining (especially regarding the AEA)

    • Microcode - simulation of this is a proposed later stage of this project

G146/MAPLD 2004

Educational features
Educational features

  • One aim of this project is to provide an educational facility

  • Demonstration mode:

    • allows pre-prepared scripts to interact with the simulation whilst providing an explanatory commentary

    • features of the AGC can be shown either in isolation or in the context of a mission

    • this facility can be seen in the poster presentation

  • Also available is a fault-creation facility to demonstrate the effect of (e.g.) parity errors, DSKY display malfunctions etc.

G146/MAPLD 2004

Progress so far
Progress so far

  • LGC

    • Full instruction set operational

    • Timers and external interrupts operational

    • Fully functional DSKY

  • Associated spacecraft systems

    • Major associated LM switches and circuit-breakers implemented

    • Mechanically functional AOT (no simulated starfield)

    • Limited ACA/TTCA hand-controller simulation and RCS graphic

    • Limited IMU functionality

  • External systems

    • Basic telemetry downlink display

    • Executive and Waitlist display

    • Basic statistical display

G146/MAPLD 2004

Work to do
Work to do

  • The following features are planned for the future (not in any particular order):

    • Fully implement IMU

    • Simulate vehicle dynamics

    • Finish implementation of AOT to permit star-based alignments

    • Emulation of microcode instructions

    • Improve statistical performance analysis

G146/MAPLD 2004


  • The current simulator provides enough functionality to gain useful insights into the operational use of the LGC and associated environment

  • Future work will extend the simulation to provide for spacecraft manoeuvring

  • The intention is to allow a full LM descent, landing, ascent and rendezvous to be demonstrated

G146/MAPLD 2004


  • Thanks are due to Ron Burkey (who has been working independently on an AGC simulator) for providing a series of stimulating questions during my work and for permitting the checking of the assembled Luminary code against his own machine code created in an alternative way

G146/MAPLD 2004


  • ACA – Attitude Controller Assembly

  • AEA – Abort Electronics Assembly (the AGS digital computer system)

  • AGC – Apollo Guidance Computer (cf CMC and LGC)

  • AGS – Abort Guidance System – backup to PGNS to allow rendezvous

  • AOT – Alignment Optical Telescope

  • CMC – Command Module Computer

  • DSKY – DiSplay and KeYboard for AGC

  • IMU – Inertial Measurement Unit

  • ISS – Inertial SubSection

  • LGC – Lunar module Guidance Computer

  • PGNCS – Primary Guidance, Navigation and Control Section

  • PGNS – Primary Guidance and Navigation Section

  • RCS – Reaction Control System (on LM 16 jets arranged in two systems)

  • TTCA – Throttle/Translation Controller Assembly

  • YUL – AGC assembler (original written at Christmas)

G146/MAPLD 2004


  • Some resources of interest to AGC historians...

    • History of Recent Science and Technology - 77 documents more-or-less relating to the development of the AGC (N.B. site is not always available)

      (A set of convenient indices to these documents is available at my website:

    • Apollo Lunar Surface Journal - contains several interesting LM operational documents plus complete surface transcripts

    • Eldon C. Hall, Journey to the Moon: The History of the Apollo Guidance Computer, AIAA, Reston, VA, 1996

      Should be read in conjunction with: Hugh Blair-Smith, Annotations to Journey to the Moon, public/blairsmith.htm

G146/MAPLD 2004