1. In computer operation, a method for selecting data entities from a memory and writing the data entities to a register file, comprising steps of: (a) consulting a first map of entities to copy relative to a first address; (b) selecting and reading those entities indicated by the map; (c) consulting a second map of positions to write the entities copied from the memory, relative to a first register; and (d) writing the entities to the register file according to the second map wherein the first and second maps are implemented as bitstrings, wherein the position of bits in the string indicate the positions for data entities to be selected from memory, and the registers to which data entities are to be written and wherein bit string maps are expressed as submasks, and submasks are linkable in different combinations to provide combined masks.