How Margaret Hamilton Saved Apollo 11
On July 20, 1969, just three minutes prior to the Apollo 11 Lunar Module touched down on the Sea of Tranquility, the onboard computer lit up with 1201 and 1202 Program Alarms. The rendezvous radar switch had been erroneously set in the wrong position, bombarding the AGC with 4.8 kHz counter interrupts and stealing nearly 15% of the computer's precious instruction cycles.
Because software pioneer Margaret Hamilton and her team at the MIT Instrumentation Laboratory (later Draper) had architected an asynchronous executive system with priority job scheduling, the computer did not freeze or reboot blindly. Instead:
- Priority Scheduling: High-priority tasks (burn calculations, attitude control, landing radar) took precedence over low-priority radar tracking.
- Phase Table Memory: In the event of core overload, the system purged transient job buffers (VAC areas) and resumed from the last known state vector without losing engine throttling.
- Asynchronous Tasking: Real-time computing was decoupled from fixed synchronous timers, birthing modern fault-tolerant software engineering.