A data mover for moving blocks of data stored in a first location of the working store of a data processing system to a second location in the working store. The data mover is provided with the necessary registers, switches, counters and control circuits to issue read and write commands to the working store, to receive and store in registers data read out of the working store as the result of its having issued a read command, and to write data read out of working store and stored in its registers in response to a read command issued by the data mover into another location in the working store. These steps are repeated until a block, measured in thousands, of data words has been moved from the first to the second location. The address preparation circuits of the high speed multiplexer of the data processing system through which the data mover communicates with the working store of the system is used to provide a substitute memory command for one of the two types of memory commands issued by the data mover. Between the time the data mover issues a read command, and receives back from memory the data read out of memory in response to such a command, the data mover can issue a write command and transfer to the high speed multiplexer data received in response to the previous read command.