One embodiment of the present invention sets forth a technique to determine a bus address for an add-in card on a System Management bus (SMbus) that includes a hybrid microcontroller (hEC) and discrete graphics processing unit (dGPU). A graphics driver requests the System Basic Input/Output System (SBIOS) for a list of available slave addresses. The graphics driver receives the list and selects an available slave address to be assigned to the hEC. The graphics driver assigns the selected address to the hEC through an Inter-Integrated Circuit bus backdoor. The graphics driver then passes the selected address back to the SBIOS and the selected address is removed from the list of available addresses. Advantageously, this approach to dynamically assigning bus addresses provides compatibility with different types of hECs as well as with different motherboard configurations and other SMbus devices.