If a predetermined field (FIG. 3/27) within a source instruction indexes and accesses a body of control information from memory (FIG. 2/5), and if control information (FIG. 4) designates the field-to-field (register-to-register) mapping (FIG. 6), then a skeleton target instruction (FIG. 3/29; FIG. 4) can be filled in by either selectively copying the fields of the source instruction or otherwise computing same. If the mapping is executed by an interposed independent processor then overlapping of such conversion enhances throughput, the independent processor converting multifield instructions for a CPU of a first kind to multifield instructions for a CPU of a second kind without disrupting the logical flow or execution of either source or target instruction streams.