In this paper, the authors propose a novel software Transaction-Level Modeling (TLM) approach for efficient HW/SW co-simulation. In HW/SW co-simulation, timing synchronization should be involved between the hardware and software simulations for keeping their concurrency. However, improperly handling timing synchronization either slows down the simulation speed or scarifies the simulation accuracy. Their approach performs timing synchronization only at the points of HW/SW interactions, so the accurate simulation result can be achieved efficiently. Furthermore, they define three abstraction levels of software TLM models based on the type of interactions captured.