An interesting design alternative to replication-based chip multiprocessors is to create heterogeneous chip multiprocessors composed of several different cores, with one or more of them running the operating system and orchestrating execution, and the others serving as accelerators where parts of the application are off-loaded. We are developing a simulator for this kind of heterogenous architectures, using the Cell Broadband Engine as a first model and the UNISIM modular infrastructure. Thanks to UNISIM, the modules composing the simulator can be easily changed and replaced by others, allowing us to customize the processor and explore the design space for these emerging architectures.