The Apollo missions had technology no more complicated than a pocket calculator. From link [**here**][1], there's an information about Apollo Guidance Computer (AGC) > The on-board Apollo Guidance Computer (AGC) was about 1 cubic foot with 2K of 16-bit RAM and 36K of hard-wired core-rope memory with copper wires threaded or not threaded through tiny magnetic cores. The 16-bit words were generally 14 bits of data (or two op-codes), 1 sign bit, and 1 parity bit. The cycle time was 11.7 micro-seconds. Programming was done in assembly language and in an interpretive language, in reverse Polish. So, I've stumbled upon some source code when I researched what was up there, and I've noticed great comments (eg. TEMPORARY, I HOPE HOPE HOPE) VRTSTART TS WCHVERT # Page 801 CAF TWO # WCHPHASE = 2 ---> VERTICAL: P65,P66,P67 TS WCHPHOLD TS WCHPHASE TC BANKCALL # TEMPORARY, I HOPE HOPE HOPE CADR STOPRATE # TEMPORARY, I HOPE HOPE HOPE TC DOWNFLAG # PERMIT X-AXIS OVERRIDE ADRES XOVINFLG TC DOWNFLAG ADRES REDFLAG TCF VERTGUID The actual programs in the spacecraft were stored in [core rope memory][1], an ancient memory technology made by (literally) weaving a fabric/rope, where the bits were physical rings of ferrite material. "Core" memory is resistant to cosmic rays. The state of a core bit will not change when bombarded by radiation in Outer Space. **[Virtual Apollo Guidance Computer (AGC) software is also on GITHUB!][2]** [Some part of documentation is here.][3] Another sample of source code with great comments. 033911,000064: 32,3017 06037 FLAGORGY TC INTPRET # DIONYSIAN FLAG WAVING 034090,000243: 32,3241 13247 BZF P63SPOT4 # BRANCH IF ANTENNA ALREADY IN POSITION 1 034091,000244: 034092,000245: 32,3242 33254 CAF CODE500 # ASTRONAUT: PLEASE CRANK THE 034093,000246: 32,3243 04616 TC BANKCALL # SILLY THING AROUND 034094,000247: 32,3244 20623 CADR GOPERF1 034095,000248: 32,3245 16001 TCF GOTOP00H # TERMINATE 034096,000249: 32,3246 13235 TCF P63SPOT3 # PROCEED SEE IF HE'S LYING 034101,000254: 32,3251 04635 TC POSTJUMP # OFF TO SEE THE WIZARD ... 034102,000255: 32,3252 74126 CADR BURNBABY [1]: https://en.wikipedia.org/wiki/Core_rope_memory [2]: https://github.com/rburkey2005/virtualagc [3]: http://www.ibiblio.org/apollo/ My question here is this: - How were the teams writing this much code able to make it functional given the tools at the time? Because if you compile so much code that was used on Apollo 11... it'd take days, even weeks. I seriously doubt that programmers back then left everything to happen by chance. [1]: http://www.hq.nasa.gov/alsj/a11/a11.1201-fm.html