Architecture
Microarchitecture
The hardware implementation of an ISA, defining how instructions are executed internally.
Detailed Explanation
While the ISA defines what instructions do, microarchitecture defines how. The same ISA can have vastly different microarchitectures—simple in-order pipelines to complex out-of-order superscalar designs. Microarchitecture determines performance, power, and area.
Microarchitectural features include pipeline depth, execution units, branch prediction, cache hierarchy, and memory system design. These are invisible to software but dramatically affect performance.