An apparatus, circuit arrangement and method for emulating a hardware design by time division multiplexing data communicated between an emulator and a runtime assist unit (RTAU), such as a behavior card. Data from the emulator may be received directly at the general purpose registers of the RTAU. A programmable delay may be used in conjunction with a step generator to initiate concurrent cycle processes. Code executed by the RTAU may be coded in assembly, and external interrupts that might otherwise affect the determined processing time of the RTAU task are disabled. The time multiplexing reduces card port, cabling and processing cycle requirements.