How do I choose between Global Clocking and Regional Clocking?
Both regional clocking and global clocking with DCM can operate up to 1 Gbps in a Virtex-4 device. Regional
clocking requires fewer clock resources than global clocking with DCM; however, it places
an area restriction on the logic that uses the regional clock. See the "Constraining The Core" section of the SPI-4.2 User Guide.