This paper proposes a new cost effective approach for on-the-fly microarchitecture simulations on real size applications. The original program code is lightly annotated to provide a fast (direct) execution mode, and an embedded instruction-set emulator enables on-the-fly simulations. While running, the instrumented-and-emulated program can switch from the fast mode to the emulation mode, and vice-versa. The instrumentation tool, calvin2, and the instruction-set emulator, DICE, presented in this paper, exhibit low execution overheads in fast mode (1.31 on average for the SPEC95 benchmarks). This makes our approach well suited to simulate on-the-fly samples spread over an application.