As a solution for dealing with the design complexity of multiprocessor SoC architectures, we present a joint Simulink-SystemC design flow that enables mixed hardware/software refinement and simulation in the early design process. First, we introduce the Simulink combined algorithm/architecture model (CAAM) unifying the algorithm and the abstract target architecture. From the Simulink CAAM, a hardware architecture generator produces architecture models at three different abstract levels, enabling a trade-off between simulation time and accuracy. A multithread code generator produces memory-efficient multithreaded programs to be executed on the architecture models. To show the applicability of the proposed design flow, we present experimental results on two real video applications.