SEVEN VIEWS OF COMPUTER SYSTEMS 25
technologies used and includes the kind of logic and how it is packaged and interconnected. Realization includes all the details associated with the physical aspects of the machine.
Modern architectures (ISPs) usually have multiple (RT) implementations. For example, the LSI-l1, PDP-l1/40, and PDP-l 1/60 are different implementations of the same basic PDP 11 instruction set. Sometimes, although rarely, a particular implementation has more than one realization. For example, the IBM 7090 has the same architecture and implementation (i.e., the same ISP and RT structure) as the IBM 709. The difference lies in realization: the 709 used vacuum tubes, the 7090 transistors. For a more recent example, two models of the PDP-11 architecture that share the same implementation are the DEC PDP-11/34 and Norden' s 1 l/34M. The realization differs, however, as the latter uses militarized semiconductor components and component mountings, and a different packaging and cooling system. Table 6 attempts to clarify the distinguishing characteristics of architecture, implementation, and realization.
Table 6. Characteristics of Design Areas [Blaauw and Brooks, in preparation: Chapter 1]
|
Architecture |
Implementation |
Realization |
|
|
Purpose |
Function |
Cost and performance |
Buildable and maintainable |
|
Product |
Principles of operation |
Logic design |
Release to manufacturing |
|
Language |
Written algorithms |
Block diagram, expressions |
Lists and diagrams |
|
Quality measure |
Consistency |
Broad scope |
Reliability |
|
Meanings(used herein) |
ISP Machine ISP |
RT level machine; microprogrammed sequential machine(at logic level) |
Physical realization; physical implementation |
This book concentrates on the realization and implementation columns in Table 6. Instruction set architecture is discussed only in-so far as it interacts with the other two characteristics. There are also some differences between the views of Blaauw and Brooks [in preparation] and those expressed in this book. It is important to try to reconcile these differences, because everyone engaged in computer engineering uses the words "architecture," "implementation," and "realization" - quite often to mean different things. This book will not limit the definition of architecture to just a machine as seen by a machine language programmer. Instead, it will use architecture to mean the ISP associated with any of the ma chine levels described in View 2, Levels-of-Interpreters. Therefore, architecture standing alone will mean the machine language, the ISP. This book will also use architecture of the micro programmed machine as seen by a micro programmed machine's microprogrammer, architecture of the operating system as the combined machine of operating system and ma chine language, and architecture of a language