The glowing screens of classic arcade cabinets housed worlds that felt alive, thanks to the seemingly intelligent behavior of non-player characters (NPCs). From the relentless ghosts in Pac-Man to the clever soldiers in Contra, these enemies exhibited dynamic behaviors that were groundbreaking for their time. Unlike modern games powered by complex algorithms and vast memory, arcade machines achieved this dynamism through clever, hardware-limited programming techniques.

The primary tool for creating NPC behavior was the Finite State Machine (FSM). An FSM is a programming model where an NPC can only be in one of a predefined set of states at any given time. Common states included “Patrol,” “Chase,” “Attack,” and “Flee.” Transitions between these states were triggered by specific conditions from the game world, such as the player entering a certain radius. For example, a guard NPC might patrol a set path (Patrol state) until the player is spotted, triggering a shift to a Chase state. This created the illusion of logical decision-making without overwhelming the machine's limited processor.

Furthermore, developers relied heavily on pattern-based and scripted behavior. To conserve precious memory, developers pre-programmed intricate movement patterns. The most famous example is the four ghosts in Pac-Man: Blinky (red) directly chases the player, Pinky (pink) ambushes by targeting ahead of Pac-Man, Inky (blue) uses a more complex, conditional strategy, and Clyde (orange) switches between chasing and scattering randomly. Each ghost’s behavior was a fixed pattern, but their combination against the player created emergent, unpredictable, and highly challenging gameplay that felt uniquely dynamic.

Hardware constraints of the era, particularly limited ROM and RAM, forced incredible ingenuity. Every byte of memory and CPU cycle was precious. Behaviors were often tied directly to the game’s clock cycle or frame count. This is why many enemies appear to move in specific rhythms or patterns. Sound and music chips were sometimes even used as timers to trigger state changes, offloading work from the main CPU. This efficient, hardware-tied programming is a hallmark of classic arcade design.

In conclusion, arcade machines handled dynamic NPC behavior not with true artificial intelligence, but with brilliantly designed systems of pre-scripted patterns, state machines, and hardware-level tricks. These methods efficiently created the perception of complex, reactive behavior within extreme technical limitations. The legacy of these techniques is profound, forming the foundational principles of game AI that continue to influence developers today, reminding us that innovation often blooms under constraint.