Hard vs. soft: the central question of pre-fabricated silicon

Abstract

It is possible to foresee the day when prefabricated, programmable devices such as Field-Programmable Gate Arrays (FPGAs) are used as the dominant silicon implementation medium. This paper will explore the forces that already drive in that direction and the architecture, CAD and circuit enhancement opportunities that may also help to make it happen. We will focus on the central question in FPGA architecture: what hard, dedicated circuit structures should be included on the FPGA? These structures are contrasted with the regular soft fabric, which can always be used to implement logic functions, but with less efficiency and performance. We will discuss the trade-offs involved, and the requirements for CAD tools and algorithms needed to support these hard structures. An interesting specific case that will be addressed is whether processors should be implemented in hard or soft form. Finally we will look an alternative: enhancing the capability of the soft fabric itself.