Abstract: A method and apparatus is provided for integrating a logic level simulation with an instruction level simulation for more accurate and faster system level simulation for testing. A host system or processors (CPU) is simulated by the instruction level simulator and the simulation of an input/output subsystem is modeled by the logic level simulator. The two simulations work side by side communicating through an interprocess communication (IPC) device and both simulations can perform a read/write access. Hence, a DMA and a slave access can occur at the same time causing a deadlock situation where both simulators are waiting for data and acknowledgment from each other at the same time. An input/output subsystem SBus module resolves this deadlock by deferring the non-DMA transaction. Finally, the synchronization of the two simulations is handled by the invention allowing the two simulators to run as asynchronous peers.