Abstract: The invention relates to a hardware/software co-simulation system comprising a simulator and a hardware block coupled thereto with an I/O manager and at least one DUT (“Device under Test”), the co-simulation system comprising a first operating mode (Mode 1), wherein the I/O manager can forward a request from the simulator to the DUT on each clock cycle and a method for hardware/software co-simulation of a simulator with a synchronous electronic system. The aim of the invention is to improve the above such that the simulation times are significantly reduced. Said aim is achieved, whereby the co-simulation system comprises a second operating mode (Mode 2), wherein the I/O manager only forwards the clock signal (DUTC1k) to the DUT without forwarding further requests to the DUT and a seamless switching between both operating modes (Mode 1, Mode 2) is possible during a simulation.