Introduction

The first commercial microprocessor chip, Intel s 4004, appeared in November, 1971. Since then, most designers have used fixed-ISA (instruction-set architecture) processors in their system designs first as processor chips in board-level designs and later as processor cores in SOCs. In fact, many system designers cannot envision designing a custom processor for their projects because the use of fixed-ISA machines has become so thoroughly engrained in the conventional design methodology. A small cadre of designers created custom processors based on bit-slice technology in the 1980s for applications with very high performance requirements, but electronic system design has largely evolved into an exercise in adapting standardized processor and DSP architectures to target tasks, often with additional hardware acceleration to bridge the inevitable gap between a task s required computations and the fixed-ISA processor s abilities.

Designers working for conventional processor vendors have always had the ability to extend their own processor ISAs as needed. For example, two features that distinguish DSPs from general-purpose processors are hardware MACs (multiplier/accumulators) and dual load/store units for XY-memory addressing. No intrinsic hardware limitation prevents processor designers from incorporating such features in their general-purpose processor ISAs. However, adding such resources increases the processor s silicon area (and therefore its cost) which may not be used by many of the applications of that processor. Successful DSPs must have these features. Successful general-purpose processors don t. Similarly, many other task-specific features are not good candidates for universal inclusion in general-purpose processor designs.

However, vendors of customizable processor...