When I attempt to infer dual-port RAM, XST reports a warning message similar to the following:
"WARNING:Xst:738 - 16384 flip-flops were inferred for signal <RAM>. You may be trying to describe a RAM in a way that is incompatible with block and distributed RAM resources available on Xilinx devices, or with a specific template that is not supported. Please review the Xilinx resources documentation and the XST user manual for coding guidelines. Taking advantage of RAM resources will lead to improved device usage and reduced synthesis time."
When you infer RAM components from an HDL description, be sure to use one of the supported templates documented in the XST User Guide.
The software manuals are available at:
This XST User Guide contains a number of methods for inferring RAM components for Xilinx Virtex devices.
Additional inference templates can be found in the ISE Language Templates.