1. A data transfer apparatus which transfers data from a first address space to a second address space, the first and second address spaces being different in data structure, the apparatus comprising: address translation table storage means for storing an address translation table which is organized by a plurality of translation descriptor domains to support a plurality of translation step sizes; base address storage means for storing base addresses of a plurality of portions of the second address space which are corresponding to the plurality of translation descriptor domains; boundary location storage means for storing boundary locations of the plurality of translation descriptor domains; translation step size storage means for storing translation step sizes that are respectively supported in the plurality of translation descriptor domains; translation descriptor domain selecting means for selecting one of the translation descriptor domains that is suitable for mapping a data segment in the first address space; and data segment associating means for associating the data segment in the first address space with another data segment in the second address space, according to the translation step size supported in the translation descriptor domain selected by said translation descriptor domain selecting means.