The design of future parallel computers requires rapid simulation of target designs running realistic workloads. These simulations have been accelerated using two techniques: direct execution and the use of a parallel host. Historically, these techniques have been considered to have poor portability. This paper identifies and describes the implementation of four key operations necessary to make such simulation portable across a variety of parallel computers. These four operations are: calculation of target execution time, simulation of features of interest, communication of target messages, and synchronization of host processors. Portable implementations of these four operations have allowed us to easily run the Wisconsin Wind Tunnel II (WWT II)---a parallel, discrete-event, direct-execution simulator---across a wide range of platforms, such as desktop workstations, a SUN Enterprise server, a cluster of workstations, and a cluster of symmetric multiprocessing nodes. We plan to release ...