When a signal/wire is driven by more than one source, XST issues the following warning:
"WARNING:Xst:644 - Signal <signal_name> has a multisource."
The above error occurs when a signal/wire is updated in more than one process/always block. All VHDL processes and all Verilog always blocks are considered to execute in parallel (i.e., always/process blocks are not sequential statements); in this case, the signal is being assigned in more than one always/process block. This causes the multisource warning in XST.
To avoid this warning message, search the entire design for the signal and ensure that it is being updated in only one process/always block.