The physical computer is itself complex enough to have its own internal hierarchy, designed and described at multiple levels of abstraction. A significant aspect of this hierarchy is the distinction of the physical computer, as a symbol processing computational artefact, from the physical components obeying the laws of physics realizing this artefact: the computer’s architecture. ‘The discipline of computer architecture’ describes the areas of computer science concerned with design, descriptions, analysis, and study of the logical organization, behaviour, and functional elements of a physical computer. Outer and inner architectures are explained along with the control unit, microprogramming, and parallel computing. Computer architecture is an experimental, heuristic science of the artificial.