Source Code
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These source-code files are part of a reconstructed copy of Skylark 048, the
Block II Command Module (CM) Apollo Guidance Computer (AGC) software for the
Skylab-2, Skylab-3, Skylab-4, and Apollo-Soyuz Test Project missions.
They have been created via disassembly of binary dumps of the original core rope
memory modules actually flown on Skylab-2, part numbers 2010802-541, 2010802-551,
2010802-561, 2010802-571, 2010802-581, and 2010802-591. Access to these modules
was provided by the New Mexico Museum of Space History, who we are much indebted to.
The source code for the Apollo 15, 16, and 17 software, Artemis 072, was used as
a starting point. Heavy use was made of TRW 4900.5-244, Programmed Guidance Equations
for Skylark Command Module Earth Orbital Program, dated 14 February 1972. This document
contains pseudocode of essentially all of Skylark, using original program labels which
have been copied for this source reconstruction.
Since only binary dumps (rather than listings) of Skylark are available as source material,
all comments and labels are approximate.
They have been taken from the Programmed Guidance Equations or other AGC programs where
possible, or, in some places, written from scratch to match what we believe would have
been in the original listing.
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017595,000002: ## Copyright: Public domain.
017596,000003: ## Filename: GIMBAL_LOCK_AVOIDANCE.agc
017597,000004: ## Purpose: A section of Skylark revision 048.
017598,000005: ## It is part of the source code for the Apollo Guidance Computer (AGC)
017599,000006: ## for Skylab-2, Skylab-3, Skylab-4, and ASTP. No original listings of
017600,000007: ## this software are available; instead, this file was created via
017601,000008: ## disassembly of dumps of the core rope modules actually flown on
017602,000009: ## Skylab-2. Access to these modules was provided by the New Mexico
017603,000010: ## Museum of Space History.
017604,000011: ## Assembler: yaYUL
017605,000012: ## Contact: Ron Burkey <info@sandroid.org>.
017606,000013: ## Website: www.ibiblio.org/apollo/index.html
017607,000014: ## Mod history: 2023-09-04 MAS Created from Artemis 072.
017608,000015: ## 2024-03-04 MAS Updated for Skylark 48.
017609,000016:
017610,000017:
017611,000018: 22,2000 SETLOC KALCMON1
017612,000019: 22,2000 BANK
017613,000020:
017614,000021: 22,2745 COUNT* $$/KALC
017615,000022: 22,2745 E6,1712 EBANK= BCDU
017616,000023:
017617,000024: # DETECTING GIMBAL LOCK
017618,000025: 22,2745 46135 LOCSKIRT SLOAD BHIZ
017619,000026: 22,2746 03076 WHICHDAP
017620,000027: 22,2747 45011 WCALC
017621,000028:
017622,000029: 22,2750 41335 SLOAD DMP
017623,000030: 22,2751 03074 DKRATE
017624,000031: 22,2752 05103 TUFITDK
017625,000032: 22,2753 45002 SR4 CALL # COMPUTE THE INCREMENTAL ROTATION MATRIX
017626,000033: 22,2754 44522 DELCOMP # DEL CORRESPONDING TO A 1 SEC ROTATION
017627,000034: # ABOUT COF
017628,000035: 22,2755 41335 SLOAD DMP
017629,000036: 22,2756 03074 DKRATE
017630,000037: 22,2757 05103 TUFITDK
017631,000038: 22,2760 74206 PUSH VXSC
017632,000039: 22,2761 02314 COF
017633,000040: 22,2762 77721 MXV
017634,000041: 22,2763 05057 QUADROT
017635,000042: 22,2764 17342 STODL BRATE
017636,000043: 22,2765 03352 AM
017637,000044: 22,2766 56205 DMP DDV
017638,000045: 22,2767 05055 ANGLTIME
017639,000046: 22,2770 77661 SR
017640,000047: 22,2771 20606 5
017641,000048: 22,2772 17350 STODL TM
017642,000049: 22,2773 03346 BRATE +4
017643,000050: 22,2774 41246 ABVAL DMP
017644,000051: 22,2775 03346 BRATE +4
017645,000052: 22,2776 51525 PDDL ABVAL
017646,000053: 22,2777 03344 BRATE +2
017647,000054: 22,3000 65205 DMP PDDL
017648,000055: 22,3001 03344 BRATE +2
017649,000056: 22,3002 03342 BRATE
017650,000057: 22,3003 41246 ABVAL DMP
017651,000058: 22,3004 03342 BRATE
017652,000059: 22,3005 74266 VDEF VXSC
017653,000060: 22,3006 02374 1/2ALPHA
017654,000061: 22,3007 37326 STCALL BIASTEMP
017655,000062: 22,3010 45037 WCALCOUT
017656,000063:
017657,000064: 22,3011 70740 WCALC LXC,1 DLOAD*
017658,000065: 22,3012 00767 RATEINDX
017659,000066: 22,3013 05045 ARATE,1
017660,000067: 22,3014 45002 SR4 CALL # COMPUTE THE INCREMENTAL ROTATION MATRIX
017661,000068: 22,3015 44522 DELCOMP # DEL CORRESPONDING TO A 1 SEC ROTATION
017662,000069: # ABOUT COF
017663,000070: 22,3016 74343 DLOAD* VXSC
017664,000071: 22,3017 05045 ARATE,1
017665,000072: 22,3020 02314 COF
017666,000073: 22,3021 77721 MXV
017667,000074: 22,3022 05057 QUADROT
017668,000075: 22,3023 17342 STODL BRATE
017669,000076: 22,3024 03352 AM
017670,000077: 22,3025 55605 DMP DDV*
017671,000078: 22,3026 05055 ANGLTIME
017672,000079: 22,3027 05045 ARATE,1
017673,000080: 22,3030 77661 SR
017674,000081: 22,3031 20606 5
017675,000082: 22,3032 27350 STOVL TM
017676,000083: 22,3033 03342 BRATE
017677,000084: 22,3034 77761 VXSC
017678,000085: 22,3035 05101 BIASCALE
017679,000086: 22,3036 03326 STORE BIASTEMP # ATTITUDE ERROR BIAS TO PREVENT OVERSHOOT
017680,000087: # IN SYSTEM
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The following label name is a guess.
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017682,000089: 22,3037 43014 WCALCOUT BON SETGO # STATE SWITCH CALCMAN2 (43D)
017683,000090: 22,3040 00313 CYC61FLG
017684,000091: 22,3041 71766 RCYCLR61
017685,000092: 22,3042 01035 CALCMAN2 # 0(OFF) = BYPASS STARTING PROCEDURE
017686,000093: 22,3043 45120 NEWANGL +1 # 1(ON) = START MANEUVER
017687,000094:
017688,000095:
017689,000096: 22,3044 00044 15053 ARATE 2DEC .0022222222 # = .05 DEG/SEC
017690,000097: 22,3046 00221 24255 2DEC .0088888889 # = .2 DEG/SEC
017691,000098: 22,3050 00554 02660 2DEC .0222222222 # = .5 DEG/SEC
017692,000099: 22,3052 02660 13301 2DEC .0888888889 # = 2 DEG/SEC $22.5 DEG/SEC
017693,000100: 22,3054 00003 04000 ANGLTIME 2DEC .000190735 # = 100B - 19
017694,000101: # MANEUVER ANGLE TO MANEUVER TIME
017695,000102: 22,3056 03146 14632 QUADROT 2DEC .1 # ROTATION MATRIX FROM S/C AXES TO CONTROL
017696,000103: 22,3060 00000 00000 2DEC 0 B-28 # AXES (X ROT = -7.25 DEG)
017697,000104: 22,3062 00000 00000 2DEC 0 B-28
017698,000105: 22,3064 00000 00000 2DEC 0 B-28
017699,000106: 22,3066 03131 11275 2DEC .099200 # =(.1)COS7.25
017700,000107: 22,3070 77461 47370 2DEC -.012620 # =-(.1)SIN7.25
017701,000108: 22,3072 00000 00000 2DEC 0 B-28
017702,000109: 22,3074 00316 30407 2DEC .012620 # (.1)SIN7.25
017703,000110: 22,3076 03131 11275 2DEC .099200 # (.1)COS7.25
017704,000111: 22,3100 00004 05253 BIASCALE 2DEC .0002543132 # = (450/180)(1/0.6)(1/16384)
017705,000112: 22,3102 12000 00000 TUFITDK 2DEC 0.3125
End of include-file GIMBAL_LOCK_AVOIDANCE.agc. Parent file is MAIN.agc