D-Jetronic was developed by Bosch as early as the sixties. Citroën had already been using it for the DS IE, and launched the SM IE with it in 1972.
The 'D' in D-Jetronic stands for "Drück" and it's a Speed-density system, meaning that no air flow meter is required. Instead a MAP (Manifold Absolute Pressure) sensor is used to measure load. Speed-Density systems are quite common even today.
The ECU is competely analog and is a pretty sturdy unit. Injectors are low impedance, requiring ballast resistors in the ECU.