1. A method for implementing a register renaming scheme for a digital data processor using a plurality of physical register files for supporting out-of-order execution of a plurality of instructions, the method comprising: partitioning the plurality of physical register files into a first set of register files having a fixed one-to-one mapping with a first set of logical registers in an architected state and into a second set of register files being mapped to a second set having multiple copies of logical registers in a non-architected state by a register-renaming unit; assigning an instruction tag to each of the plurality of instructions for tracking instruction dependencies; using a DEF table to store the instruction dependencies between the plurality of instructions using the instruction tags, the DEF table being indexed by a logical register name and including one entry per logical register; using a rename USE table indexed by the instruction tags to store logical-to-physical register mapping information shared by multiple sets of different types of non-architected copies of logical registers used by multiple threads; using a last USE table to transfer data of the multiple sets of different types of non-architected copies of logical registers into a first set of architected registered files, the last USE table being indexed by a physical register name in a second set of rename registered files; and performing a register-renaming scheme at an instruction dispatch or wake-up/issue time in order to reduce a number of a plurality of physical register files; wherein the register-renaming unit is a mapper; wherein the mapper maps a first set of logical registers to a plurality of physical registers in a rename register file and maintains a copy of previous instruction dependency information for a flush/recovery function; wherein the plurality of physical registers used by in-flight instructions are mapped to either the first set of logical registers in architected state or to a second set of logical registers in a non-architected state.