20.1 INTRODUCTION

In this chapter, we survey work on space management, legalization, and detailed placement, the design flow steps normally falling between global placement and the start of routing. Over the past few years, the traditional physical design flow has evolved. Where there was once a sequence of discrete steps, one now sees a blurring of activities and a great deal of iterative improvement. The methods described here should not be viewed as standalone optimizations; rather, they should be considered as components in a more complex multifaceted approach.

In early design flows, the transition from global placement to detailed placement was relatively simple. The logic elements were aligned to cell rows, and then small local optimizations were performed. With changes to routing models, and the dominance of interconnect delay, space management has fundamentally changed the design flow, and has emerged as a key element of successful strategies.

Figure 20.1 illustrates a current approach; following global placement, a number of methods can be used to analyze a placement. Congestion estimation [1] can identify regions where routing demand will likely exceed the available resources; an effective technique is to insert additional white space between logic elements, spreading out the circuit and gaining more room for wiring. Similarly, timing analysis may find slow paths that can be improved through buffer insertion or gate sizing; again, spreading out of the circuit may be required to provide room for the new logic elements. Thermal hot spots are also...