Provided is a method of performing an internal processing operation of a memory device in a system including a host device and the memory device. The memory device includes a memory cell array and a processor-in-memory (PIM) performing an internal processing operation. In an internal processing mode, by the PIM, the memory device performs the internal processing operation based on internal processing information stored in the memory cell array. When the internal processing information is an internal processing operation command indicating a type of the internal processing operation, the memory device outputs the internal processing operation command including an internal processing read command and an internal processing write command to the host device. The host device issues to the memory device a priority command determined from among a data transaction command and the internal processing operation command.