DCM are used for clock multiplication and division in FPGA.
If multiple clocks are there in ur design use syncronizers to handle
data exchanges.
BUFG is a global clock buffer in FPGA . The clock pin u r assigning in
ur fpga will be driving through this BUFG primitive from which it is
routed to the whole FPGA.
Thanks,
Venkateswara rao.
LAKSHMIKANTHA BM
VLSI TRAINEE,SEAS,BANGALORE.