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Architecture and micro-architecture

Architecture does not tell you how a processor is built and actually works. The build and design of a processor is referred to as micro-architecture. Micro-architecture tells you how a particular processor works.

Micro-architecture includes things like:

  • Pipeline length and layout.
  • Number and sizes of caches.
  • Cycle counts for individual instructions.
  • Which optional features are implemented.

For example, Cortex-A53 and Cortex-A72 are both implementations of the Armv8-A architecture. This means that they have the same architecture, but they have very different micro-architectures, as shown in the following image:

  A diagram to show Cortex-A53. A diagram to show Cortex-A72.

Target

 Optimized for power efficiency Optimized for performance
Pipeline 8 stages
In-order 		15+ stages
Out-of-order
Caches L1 I cache: 8KB - 64KB
L1 D cache: 8KB - 64KB
L2 cache: optional, up to 2MB 		L1 I cache: 48KB fixed
L1 D cache: 48KB fixed
L2 cache: mandatory, up to 2MB

Software that is architecturally-compliant can run on either the Cortex-A53 or Cortex-A72 without modification, because they both implement the same architecture.

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