This paper examines the layered software modules of a heterogeneous multiprocessor simulator and debugger, and the design patterns that span these modules. Lucent's LUxWORKS simulator and debugger works with multiple processor architectures. Its modeling infrastructure, processor models, processor monitor / control, hardware control, vendor simulator interface and Tcl/Tk extension layers are spanned by the following design patterns: 1.) build and extend abstract virtual processors, 2.) build reflective entities, and 3.) build a covariant extensible system. Together these modules and patterns define a processor execution architecture that encourages reuse and dynamic extensibility.