A processing device used in a bus, for executing a programming language function of a central processing unit (CPU), comprises a receiving circuit, for receiving a joint command from the CPU, to assist the CPU to execute the programming language function, wherein the joint command comprises an extended read command and an extended write command; a transmitting circuit, coupled to the receiving circuit, for transmitting the extended read command to a slave device, to receive a first response message via the receiving circuit in response to the extended read command from the slave device, wherein the first response message comprises at least one data read by the slave device from a memory block; and a writing circuit, coupled to the receiving circuit and transmitting circuit, for writing the at least one data into a destination address corresponding to the programming language function according to the extended write command.