The macrocell approach to the design of VLSI integrated circuits needs a tool for the solution of the switchbox problem, where terminals lie on all the edges of a generic rectilinear region. In the paper, such a problem is thoroughly analysed and a two-layer symbolic router is proposed, which is able to successfully route very dense examples of switchboxes. The program applies a suitable algorithm which tries to connect nets like an expert designer. Connections are placed without affecting the wireability of further connections, when this is possible. In crowded situations, blocking nets are identified and other solutions are tried by a rip-up and rerouting step. In this way, it is possible to notably extend the space in which to search for a solution.
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