This paper introduces VTsim, a device simulator for Xilinx Virtex-II FPGAs. VTsim is currently a globally synchronous event-driven device simulator modeled at the CLB level. Through use of JBits3 and ADB, VTsim provides designers with the ability to simulate all resources within an FPGA via a virtual device. The simulator has been designed for rapid response, low memory usage, and ease of interaction. VTsim is part of the JHDLBits open-source project geared at providing enhanced control of resource manipulation, placement, and routing.