By architecture I mean 'appearance to the user' - it is the functional specification of the system (its behavioural appearance). By implementation I … - Gerrit Blaauw

The architecture of a system can be defined as the functional appearance of the system to the user.

A hardware design language should be (i) sufficiently high level, (ii) conversational, (iii) general purpose, and (iv) structured:

There always is an architecture, whether it is defined in advance - as with modern computers - or found out after the fact - as with many older computers. For architecture is determined by behavior, not by words. Therefore, the term architecture, which rightly implies the notion of the arch, or prime structure, should not be understood as the vague overall idea. Rather, the product of the computer architecture, the principle of operations manual, should contain all detail which the user can know, and sooner or later is bound to know.

Blaauw and Brooks first develop a conceptual framework for understanding computer architecture. They then describe not only what present architectural practice is, but how it came to be so. A major theme is the early divergence and the later reconvergence of computer architectures. They examine both innovations that survived and became part of the standard computer, and the many ideas that were explored in real machines but did not survive. In describing the discards, they also address "why" these ideas did not make it