Abstract: A memory unit (23,24) is proposed for a computer system having a processing unit and a data bus for transferring data between the processing unit and the memory unit. The memory unit (23,24) stores data at a plurality of locations (“data items”) in a logical memory space (32), such that each data item has an address given by at least one index variable. In addition to read and write commands, the memory unit is operative to receive a shift command in a predefined format and including shift data which indicates a source address in the logical space. Upon receiving the command, the memory unit is operative to recognise it as a shift command and accordingly perform a predefined shift function comprising (i) using the source address to identify a portion of data in the memory space and (ii) writing that portion of data to a different location in the memory space. Thus, the portion of data can be shifted within the memory space without a need to transfer the portion of data along the bus.