Variable register and immediate field encoding in an instruction set architecture

A method and apparatus provide means for compressing instruction code size. An Instruction Set Architecture (ISA) encodes instructions compact, usual or extended bit lengths. Commonly used instructions are encoded having both compact and usual bit lengths, with compact or usual bit length instructions chosen based on power, performance or code size requirements. Instructions of the ISA can be used in both privileged and non-privileged operating modes of a microprocessor. The instruction encodings can be used interchangeably in software applications. Instructions from the ISA may be executed on any programmable device enabled for the ISA, including a single instruction set architecture processor or a multi-instruction set architecture processor.