An Instruction Set Architecture (ISA) defines, describes, and specifies how a particular computer processor core works. The ISA describes the registers and describes each machine-level instruction. The ISA tells exactly what each instruction does and how it is encoded into bits. The ISA forms the interface between hardware and software. Hardware engineers design digital circuits to implement a given ISA specification. Software engineers write code (operating systems, compilers, etc.) based on a given ISA specification.