The complexity of integrated circuits requires a hierarchical design methodology that allows the user to divide the problem into pieces, designeach piece independently, and assemble the pieces into the complete system. The design herarchy brings out composition problems, problems that are a property of the assembly as a whole, not of one single instance inthe hierarchy. Recent research has produced tools that automate part of the composition task - the logical connection of the pieces. However, these tools do notensure that signals driven over these connections will be driven sufficiently to give reasonable cycle speed of the resulting chips. It is easily possible to specify an assembly in which a small-sized gate is required to drive anenormous load. Parasitic capacitance of the wiring made automatically by the logical connection tool can be the dominant source of delay, so assemblytools can actually worsen the performance of the circuit and hide this factfrom the designer.When required to make large circuits, automated layout tools such as PLAgenerators can blindly make layouts that give abysmally poor performance.Here again, the delay is in a part of circuit that the designer did not specify,so it is hidden. Finding and correcting these problems is a difficult andtime-consuming task in integrated circuit design, and one that consumesvastly more people's time and computer time than the simple assembly ofthe chip.
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