Software archaeology - Learning from the landing on the moon! by Tobias Voss

The Apollo Guidance Computer (AGC) was one of the first embedded computers, extremely lightweight (32kg) and compact for its time while running on only 55 watts

Hardware limitations were severe: 4KB RAM, 72KB ROM, 16-bit architecture, and only 2MHz clock speed - yet achieved 9 digits of decimal precision for critical calculations

The AGC pioneered several key software concepts:

• Real-time operating system with priority-based task scheduling
• Virtual machine architecture with an interpreter
• Hardware error checking through parity bits
• Cooperative multitasking
• Memory virtualization

The software development team led by Margaret Hamilton:

• Coined the term “software engineering”
• Prioritized robust error handling and fault tolerance
• Conducted extensive simulation testing
• Implemented iterative design improvements across missions

The AGC interface (DSKY) used a verb-noun input system allowing astronauts to:

• Monitor critical flight data
• Input commands and override automated systems
• Respond to program alarms
• Switch between different navigation modes

Famous 1202/1201 program alarms during Apollo 11 landing demonstrated the computer’s ability to handle errors gracefully by discarding low-priority tasks

The project’s success was enabled by:

• Clear mission goals
• High level of funding (4% of US budget)
• Extensive testing and simulation
• Focus on reliability and fault tolerance
• Skilled engineering teams

Many concepts pioneered in the AGC development are still relevant:

• The importance of robust error handling
• Value of abstraction and virtualization
• Need for thorough testing
• Significance of human oversight of automated systems