During the first Moon landing, the computer overloaded, and its design saved the mission.
Minutes before Apollo 11 touched down, the Apollo Guidance Computer flashed alarms 1202 and 1201: it had more work than time. Instead of crashing, it restarted, dropped its least important jobs and kept flying. Mission Control said 'Go', and the landing continued. The computer had roughly the power of a 1970s home computer.
The Apollo Guidance Computer was designed at the MIT Instrumentation Laboratory in the early 1960s and built by Raytheon. It was the first computer based on silicon integrated circuits, about a cubic foot in size, holding 4,100 chip packages. Its programs lived in core rope memory, made by threading wires through or around tiny magnetic cores, so the software was literally woven into the hardware. Astronauts talked to it by entering two-digit 'verb' and 'noun' codes on a keypad called the DSKY.
During Apollo 11's descent, the computer was already running at about 85 per cent capacity. A rendezvous radar left on as a precaution was sending a stream of spurious signals, eating another 13 per cent. When Buzz Aldrin asked for a display comparing radar and computed altitude, the extra load tipped it over, triggering executive overflow alarms. The root cause was a flawed interface specification that left two 800-hertz signals out of phase, making a stationary radar antenna look as if it were jittering. The issue had shown up once in testing, but flying with proven hardware had seemed safer than switching to a new radar.
The computer coped because of its operating system. J. Halcombe Laning's design scheduled work by priority, and when overloaded the machine performed a soft restart, discarding low-priority tasks such as that display and preserving guidance and control. Guidance controller Steve Bales and his team, including Jack Garman, judged it safe and called 'Go'. Had the computer simply failed, the landing would have been aborted.
The software effort took some 1,400 person-years. Its details were implemented by a team led by Margaret Hamilton, who paid close attention to the errors astronauts might make; she received the Presidential Medal of Freedom in 2016. The computer has been called the fourth astronaut, and its ideas shaped Skylab, the Space Shuttle and early fly-by-wire aircraft.
Source: Wikipedia — Apollo Guidance Computer · Text summarised from Wikipedia (CC BY-SA 4.0)
