Computer enthusiasts have recreated the legendary Apollo Guidance Computer, the on-board computer that was instrumental in Apollo missions and to which we dedicated a well-deserved special last year when it celebrated the 50th anniversary of man’s arrival on the Moon.

Unfortunately, the Apollo 10 mission assembly language software was lost and it was another important manned mission as a “dress rehearsal” for stepping on the Moon, which flew eight weeks before Neil Armstrong’s famous “Moonwalk” mission.

Space flight engineer Mike Stewart found a clever way to recreate this software. From a hard copy of an earlier version of the program and with the help of a small army of volunteers, he hand-transcribed the list of sources and all of his programs to complete 1,735 pages.

Recall that the Apollo Guidance Computer program was written in Assembly language and it took 25 minutes to compile it into a mainframe of the time, something that a modern laptop would do in less than a second.

Stewart also used NASA notes describing the changes in each version of the program, including a recently discovered agency document that provided the checksum for each version of the computer run. Stewart was able to cut and paste carefully chosen codes and variables from later versions of the program, and based on clues from NASA, he recreated a program that had the same checksum.

He has also posted a video of the Apollo 10 source code highlighting comments included by programmers. “Cheerful comments on a very serious code”, which basically warned of the repercussions of making changes. For example, a Latin phrase from a play that roughly translates to Don’t touch this. The final installed version had error tolerance, exception control and the possibility of actually a “soft reset”, restarting the machine while keeping the RAM information. It was not originally planned, but the programmers changed their minds when an astronaut mistakenly erased all the data on the computer during a test by mistaken command.

I don’t know

A ‘Chinese’ job to recover the code that governed the Apollo 10 main computer, which was fundamental in all space missions. And that was 2,000 times slower and had 1,000 times less memory than a current smartphone. To design the software that powered the Apollo Guidance Computer, it took the work of 350 engineers and the equivalent of the work of 1,400 years of a single person until you land on the Moon.

More information | Apollo Guidance Computer, the story of the computer that took us to the Moon

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