Abstract: Digital processor apparatus having an instruction set architecture (ISA) with instruction words of varying length. In the exemplary embodiment, the processor comprises an extended user-configurable RISC processor with four-stage pipeline (fetch, decode, and writeback) and associated logic that is adapted to decode and process both 32-execute, bit and 16-bit instruction words present in a single program, thereby increasing the flexibility of the instruction set, and allowing for greater code compression and reduced memory overhead. Free-form use of the different length instructions is provided with no required mode shift. An improved instruction aligner and code compression architecture is also disclosed.