university of the west of england bristol 1 webb g146/mapld 2004 simulation of apollo guidance...

1WEBB G146/MAPLD 2004

University of the West of

EnglandBRISTOL

Simulation of Apollo Guidance Computer

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”!



EnglandBRISTOLMotivations

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?)



EnglandBRISTOLThe AGC

AGC in context AGC used in both Apollo command (CMC) and

lunar (LGC) modules First true Digital Autopilot Core of PGNS:

alignment and control of IMU via AOT DAP including autostabilisation modes via RCS as

part of PGNCSautoflight capabilities including descent, landing,

ascent and rendezvousUpdates AGS to provide backup abort capability in

case of PGNS failure



EnglandBRISTOLHighlights of LGC

Hardware Memory word size: 15 bit + 1 parity bit 36 banks comprising 1K words per bank 2K erasable memory Main accumulator (A) and subsidiary registers (L and Q) 24 Channels provide communication to and from external

equipment (plus L and Q registers are also addressable as channels)

Modified ones-complement integer arithmetic (except for counters) so +0 and -0 exist and are used heavily in software

Fixed-point FP arithmetic (programmer must handle scaling) Double precision FP (28 data bits) also available

Accumulator uses two sign bits to detect overflow conditions Cycle time: 11.7s - most basic instructions use two cycles (max:

6 cycles for DIV instruction) Interpretative instructions provide advanced mathematical

manipulations (e.g. vector and matrix manipulations) – speed: around 1ms per instruction



EnglandBRISTOLHighlights of LGC

Software Time-shared, priority-driven OS – up to 7 (6 for

CMC) jobs can scheduled at once Interrupt-driven – 10 interrupt levels

associated with timers and external hardware Basic instructions implemented in hardware More complex instructions (e.g. vector and

matrix manipulations) provided by an interpreter to allow ease of application programming – can be combined with basic instructions for greater programming flexibility



EnglandBRISTOL

A few interesting AGC instructions (1)

CCS Compare, Count and Skip 4-way branch - provides a convenient

loop control mechanismCCS k

Inst-a if (k)>0 A:=(k)-1 and branch to Inst-aInst-b if k=+0 A:=0 and branch to Inst-bInst-c if k<0 A:=ones-comp((k))-1 and branch to Inst-

cInst-d if k=-0 A:=+0 and branch to Inst-d

N.B. Not all branches need to be used (e.g. if (k) always >= +0)



EnglandBRISTOLAGC instructions (2)

TS Transfer to storageTS k

Inst-a

Inst-b

k := A if A is in positive or negative overflow, skip to Inst-b,

otherwise continue from Inst-a (implication of this instruction is that the programmer must always be aware whether A could be in a overflow condition)

INDEX index the next instruction Used to access array structures, e.g.:CA value Load accumulator with contents of value

INDEX offset Add contents of offset to next instructionAD base Add accumulator to base(offset)

Can also be used in sequence with other INDEXes



EnglandBRISTOLSample AGC program

EBANK= 5

SETLOC 4000

INHINT

CA FIVE

TS EB

START CA NINE

LOOPINI TS COUNT

CA VALS

INDEX COUNT

TS TABL1

CCS COUNT

TCF LOOPINI

CA NINE

LOOPAGIN TS COUNT

INDEX COUNT

CA TABL1

TS SR

CA SR

INDEX COUNT

TS TABL1

LOOPTEST CCS COUNT

TCF LOOPAGIN

RELINT

...

SR = 21

EB = 3

FIVE DEC 5

NINE DEC 9

VALS OCT 52525

SETLOC 2400

COUNT ERASE

SETLOC 300

TABL1 ERASE +9D

Equivalent to… (assuming 15 bit words)

#DEFINE vals 052525

// inhibit interrupts

for (count=9;count>=0;count--) {

tabl1(count)=vals;

}

for (count=9;count>=0;count--) {

tabl1(count)=(tabl1(count)>>1) |

((tabl1(count) & 1) << 14);

}

// enable interrupts

... Note that the Shift-Right register rotates any value

written to it



EnglandBRISTOLSimulator 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 11.7s clock rate



EnglandBRISTOLSources 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 for more details



EnglandBRISTOLConstruction

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



EnglandBRISTOLProgress so far

LGCFull instruction set operationalTimers and external interrupts operationalFully functional DSKY

Associated spacecraft systemsMajor associated LM switches and circuit-breakers

implementedMechanically functional AOT (no simulated starfield)Limited ACA/TTCA hand-controller simulation and RCS

graphicLimited IMU functionality

External systemsBasic telemetry downlink displayExecutive and Waitlist displayBasic statistical display



EnglandBRISTOLWork to do

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

Fully implement IMUSimulate vehicle dynamicsFinish implementation of AOT to permit star-based

alignmentsEmulation of microcode instructionsImprove statistical performance analysis

AcknowledgmentsThanks 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



EnglandBRISTOLAcronyms

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)



EnglandBRISTOLReferences

Some resources of interest to AGC historians... History of Recent Science and Technology - 77 documents

more-or-less relating to the development of the AGChttp://www.hrst.mit.edu/hrs/apollo (N.B. site is not always available)(A set of convenient indices to these documents is available at my

website:http://www.cems.uwe.ac.uk/~jtwebb/agc)

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

http://www.hq.nasa.gov/alsj Eldon C. Hall, Journey to the Moon, AIAA, Reston, VA, 1996

Should be read in conjunction with: Hugh Blair-Smith, Annotations to Journey to the Moon, http://www.hrst.mit.edu/hrs/apollo/ public/blairsmith.htm

university of the west of england bristol 1 webb g146/mapld 2004 simulation of apollo guidance...

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