The Incredible Lightness of Software

NASA\´s Voyager is an exquisite object of exploration and of engineering. From an explorer\´s point of view, Voyager I is the first human artifact to have waded into the shores of interstellar space. From an engineer\´s perspective, Voyager is a beautiful case study in the development of complex software-intensive systems. Voyager\´s computers were an evolution of a proven system, the Viking Command Computer Subsystem (CCS). Because of the increased computational demands for its scientific mission, Voyager placed two instances of CCS on board-one for basic spacecraft hygiene and the other for attitude control-plus a third custom computer for flight data. As is typical for high-reliability real-time systems, each of these three systems was architected with two processors and built out of approximately 150 standard transistor-transistor logic integrated circuits. There existed a total of less than 70 Kbytes of memory for all three subsystems, and-in a first for space systems- the custom system used dynamic CMOS memory. Another important architectural decision-also novel for space systems-was the use of what at the time was called “soft logic." Today, we call it firmware.