Instruction Set Architecture(ISA) is important from user’s/ compilers perspective. Instruction Set Architecture is a ‘functional appearance to its immediate user/programmer’. ISA involves details like addressing modes, opcodes, registers, etc. Microarchitecture is important from the perspective of processor designer. Microarchitecture defines the logical structure or organisation that implements ISA. It involves details of pipelining stages, caches, physical registers, functional units, etc.Realization of hardware is important from the perspective of chip designer. Hardware is the actual “physical structure that embodies the implementation.” It involves micro-details of gates, transistors, wires, etc.Processors having identical ISA may be very different in organization. Processors with identical ISA and nearly identical organization are still not nearly identical.