In the past our research group has used SimpleScalar for architectural research. However, that project is obsolete and the simulator is slowly becoming obsolete with respect to real-world systems. Also, it does not provide full system simulation, although a set of patches by Jack Whitham merges RTEMS and SimpleScalar to provide full system simulation for RTEMS. I tried to use Jack's patches, but I could not quite get everything working. When I sent him an e-mail to ask about it, he suggested that I should look at M5 as a more realistic simulator instead. So I did.
M5 provides two types of simulators: Full System (FS) and Syscall Emulation (SE).