This paper was published in the Proceedings of the 1997
Winter Simulation Conference (S. Andradóttir, K. J. Healy,
D. H. Withers, and B. L. Nelson, editors), and the copyright is held
by the Board of Directors. It appears on pages 397-404.
Abstract The paper describes a simulation substrate
that allows thinking agents to interact with a world. The world
is simulated by standard discrete event simulation, but the
timing of an agent's behavior is determined by the amount of
computation it performs. Therefore, if an agent thinks a lot
about what to do given a situation in the world, the duration of
its thinking results in a delay to its subsequent actions.
Thus, the thinking of the agent is time pressured. The
computation time of the agent is automatically assessed by the
substrate in a way that is independent of the computer running
the simulation. This is done by implementing the thinking of
the agents in a variant of Common Lisp called Timed Common Lisp,
in which each function advances a clock by an appropriate,
user-specifiable amount of time. This renders agent thinking and
behaviors deterministic, making results comparable and
replicatable across platforms. The simulation substrate also
supports the interaction of continuous activities, in addition
to executing discrete, point-like events. This substrate has
been used to implement an Artificial Intelligence Planning
system that simulates multiple agents fighting forest fires in
Yellowstone National Park.