ISA (instruction set architecture)
Instruction Set Architecture — the "contract" between hardware and software: which instructions a processor understands, which registers it has, how memory is addressed. It's the abstraction that lets the same program run on any CPU with the same ISA, regardless of internal design.
Role: the ISA defines everything a programmer/compiler needs to know — the instructions (add, load, jump), the registers, the data types, the memory model — but says nothing about HOW it's implemented (that's the microarchitecture). Major ISAs: x86-64 (Intel/AMD, the CISC heritage, dominates desktop/server), ARM (RISC, dominates mobile and growing in server/laptop via Apple Silicon), and RISC-V (an open, license-free ISA on the rise). The value of a stable ISA: binary backward compatibility (old software runs on new CPUs) and that hardware makers can freely improve the implementation without breaking programs. The ISA is also a strategic/business asset (x86 licenses, ARM licensing, RISC-V's openness). One of the most fundamental concepts in computer architecture. Related to microarchitecture and RISC vs CISC.