Architecture
ISA (Instruction Set Architecture)
The abstract interface between hardware and software, defining instructions, registers, and programming model.
Detailed Explanation
The ISA specifies what instructions exist, their encoding, register file organization, memory addressing modes, and exception handling. It's the contract between hardware designers and software developers.
ISAs are classified as RISC (simple instructions, many registers, load-store) or CISC (complex instructions, memory operands). ARM and RISC-V are RISC; x86 is CISC. Modern CISC implementations often decode complex instructions into RISC-like micro-operations.