 As if landing on the moon for the first time in human history wasn't dramatic enough, Apollo 11 also experienced a series of 1202 program alarms and a 1201 program alarm. So, here's what they mean. To talk about these program alarms, we have to dive into the Apollo guidance computer just a little bit. By the time the Apollo missions flew to the moon, the software program that ran the missions could fit, though only just fit, into a set of read-only magnetic cores. This meant that the pieces of information the computer needed to run a mission could be called up at any time. The computer could also run multiple programs simultaneously to get multiple data points at one time, which is really important. Let's take, for example, the moment of landing on the moon. The computer has to know where the lunar module is with reference to the lunar surface. It also has to know its attitude, its speed, as well as engine performance. These are all data points that the astronauts need to call up or input into the computer and read off the disk key. If you'd like to know a little bit more about the disk key and how the mountain verb situation worked on Apollo, I do have a video about that right here. So let's go back to the Apollo guidance computer and our example of landing on the moon. We know that we need all these different pieces of information flowing into the computer at one time, but the Apollo guidance computer was a single processor system. So how could it actually manage all of these things simultaneously? Well, it actually couldn't. It could process each one individually but extremely quickly. Take that fast processing power and mix it with slow human reaction time and you've got something that looks like it's running program simultaneously even though they're not actually exactly simultaneous. But still, there's a bit of a problem. We have all these different pieces of information that need to be processed and we can't do them all at the same time. We have to put them in a priority order. So how do you know which one to take first and where do they go when they queue up to be next in line? The Apollo guidance computer had a 12-word data area called the core set. This contained all the information to execute a given program. There were six of these core sets in the command module and seven in the lunar module. In each 12-word core set, five words were taken up by processing information. The seven remaining words were left for temporary variables or extra storage, whatever the need might be. So five words of memory in the core set to execute a program and seven words for all the variables and temporary information associated with that program. But it gets a little bit more complicated. Scheduling the next program fell to the executive. It looked at the task's starting address and its priority and then passed that information to a no-back routine. This scanned the core sets to see where there was any available space for the program to execute and if so, where that space was. So basically, it found an empty space in one of those core sets and put that program set to run in that space. Through exhaustive testing, teams at MIT's instrumentation lab that developed software came up with a way such that no core set would ever be completely filled. There would always be a space for the next task. And of course, there was also the variable that if the task that needed to be done had a higher priority than one running, it could be interrupted to run the new program and then pick back up where they left off. So the program was never designed to be overloaded. There were enough core sets such that everything that would have to happen during a mission at any given time could be run nearly simultaneously or else scheduled to run as soon as there was free space. So where did the program alarms from the Apollo 11 mission come in? Well, it seemed that the engineers at MIT thought they had developed a system that could not be overloaded. But on Apollo 11, the computer did get overloaded. In preparation for their lunar landing, Neil Armstrong and Buzz Aldrin turned on their rendezvous radar to track the command service module. This was a safety measure. It tracked the CSM so it knew where it was in case they needed to do an abort in the lunar module. The crew left the radar on in slew mode, meaning it had to be manually positioned by an astronaut. And this also meant that it wasn't sending data to the computer. But what neither the astronauts nor the guys in mission control knew was that radar coupling data units were flooding the Apollo guidance computer with counter-interrupt signals. This was due to an oversight in the computer's power supply design structure. These signals were taking up just a little bit of the computer's processing time. And the spurious job kept running in the background. So unbeknownst to anyone, this signal prevented vital programs associated with the landing from completing. So when a new task was sent to the computer, there was nowhere for it to go. The running and scheduled jobs were holding their core set and their back areas. There was no clear space for a new job to be scheduled. When the executive found there was no place to put new programs, it triggered the 1201 alarm signaling executive overflow no core sets and also the 1202 alarm signaling executive overflow no back areas. This in turn triggered a software reboot. All of the dots were canceled regardless of priority and started up again as per their table order. So quickly that no guidance or navigational data was lost. Which is lucky. Apollo 11 had multiple of these alarms. Four 1202 alarms and one 1201 alarm. Eventually Buzz Aldrin realized there was a correlation between one of the programs he was trying to run and the 1202 alarms. Every time he tried to run the program that would call up range data to the landing area, he got that alarm. So he stopped running it and asked Houston for the data instead. Of course engineers from MIT knew the 1201 and 1202 program alarms were innocuous, especially in the case of Apollo 11. And as we know, the alarms did not force an abort. Neil Armstrong and Buzz Aldrin were able to land the eagle on the Sea of Tranquility less than 10 minutes after that first 1202 program alarm. I've got a more detailed look at how the computer works with the core sets in the back areas as well as the story of Apollo 11 over on my companion blog at Discover. So I urge you guys to go check that out. The link is in the description and hopefully it will answer any lingering questions you might have. I also want to remind you that I am able to take sponsors on Vintage Space now. This opens up some new interesting perks for you guys and also helps make Vintage Space possible. And those of you interested in the Pete Conrad Vintage Space t-shirt, I do have a link to my store down in the description. So a lot of you guys have asked me questions about the Apollo Guidance Computer and I am not the expert. My friend Frank O'Brien is. He literally wrote the book on the Apollo Guidance Computer. I've got the link to that from Amazon in the description as well. If you want to know everything you never needed to know about how the computer worked, check it out. It is a brilliant resource and definitely something those of you with deep questions should have. So let me know if you have any other questions about the Apollo Guidance Computer or Apollo 11 or just anything you'd like to see covered in future episodes. Be sure to follow me on Twitter and Instagram for daily content and of course with new episodes going up right here every single week including some fun live streams. Be sure to subscribe so you never miss an episode.