PROTEUS is a high-performance simulator for MIMD multiprocessors. It is fast, accurate, and flexible: it is one to two orders of magnitude faster than comparable simulators, it can reproduce results from real multiprocessors, and it is easily configured to simulate a wide range of architectures. PROTEUS provides a modular structure that simplifies customization and independent replacement of parts of architecture. There are typically multiple implementations of each module that provide different combinations of accuracy and performance; users pay for accuracy only when and where they need it. Finally, PROTEUS provides repeatability, nonintrusive monitoring and debugging, and integrated graphical output, which result in a development environment superior to those available on real multiprocessors.