Why did the Apollo 11 computer flash alarms during the Moon landing?
Minutes from touchdown, the lander's computer said it was overloaded. In Houston, an engineer had to decide whether that mattered.
▶ Start the storyBecause the guidance computer was being asked to do more than it had time for. About five minutes into the descent burn, at 6,000 feet, the lander's computer gave the first of several unexpected 1201 and 1202 program alarms. They were "executive overflows": the computer could not complete all its tasks in real time and had to postpone some of them.
Two things piled up. The computer was normally almost 85% loaded at that stage of the approach, and a stream of spurious signals from the rendezvous radar, which by the main account had been left on standby on purpose in case of an abort, added about 13% more. Then Buzz Aldrin entered the command 1668, which asked for a repeating altitude-difference display, adding another 10%. That tipped it over.
Step 1: Normal load
Almost 85% busy during the approach
Step 2: Radar noise
Spurious cycle steals add about 13%
Step 3: Command 1668
A repeating display adds another 10%
Step 4: 1202 alarm
Executive overflow
Step 5: Recovery
Low-priority tasks dropped, guidance continues
The reason this did not end the landing is a design decision. The software used priority scheduling, so when it overflowed it automatically dropped lower-priority tasks, including that display, and carried on with the critical guidance and control work. In Mission Control, computer engineer Jack Garman told Guidance Officer Steve Bales it was safe to continue, and that "go" was relayed to the crew.
Margaret Hamilton, who directed Apollo flight computer programming at MIT, later put it this way: the computer, rather than almost forcing an abort, prevented an abort. A machine with 36,864 words of read-only memory saved the landing by knowing what to drop.
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Recap
The computer survived the overload because it was built to drop the least important tasks first and tell the crew.
💡 A trick to remember it · Overloaded, the computer did not freeze: it dropped the small jobs, kept the landing, and asked humans to decide.
Surprising fact · The cause is still told two ways: a flawed interface specification, or a radar switch left on, a claim that is disputed.
Sources (3)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.