Superscalar in-order processors form an interesting alternative to out-of-order processors because of their energy efficiency and lower design complexity. However, despite the reduced design complexity, it is nontrivial to get performance estimates or insight in the application-microarchitecture interaction without running slow, detailed cycle-level simulations, because performance highly depends on the order of instructions within the application's dynamic instruction stream, as in-order processors stall on interinstruction dependences and functional unit contention. To limit the number of detailed cycle-level simulations needed during design space exploration, we propose a mechanistic analytical performance model that is built from understanding the internal mechanisms of the processor.The mechanistic performance model for superscalar in-order processors is shown to be accurate with an average performance prediction error of 3.2% compared to detailed cycle-accurate simulation using gem5. We also validate the model against hardware, using the ARM Cortex-A8 processor and show that it is accurate within 10% on average. We further demonstrate the usefulness of the model through three case studies:(1) design space exploration, identifying the optimum number of functional units for achieving a given performance target; (2) program-machine interactions, providing insight into microarchitecture bottlenecks; and (3) -We added modeling of an arbitrary number of functional units of any type in contrast to a fixed number in the ISPASS paper (i.e., 4 ALUs and 1 unit for all other types). -We completely revised the modeling of interinstruction dependences and unified it with the functional unit contention modeling. -We added modeling of memory-level parallelism, which has a nonnegligible impact on performance for some benchmarks that were not evaluated in the ISPASS paper. -We validated the model against hardware.-We added a case study on sizing the number of functional units. -We reevaluated all other case studies using the new model and revealed new insights about the interaction between dependences and functional unit contention.Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies show this notice on the first page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee.