Block RAMs and FIFOs can be inferredif implemented correctly in your HDL code.The
XST User Guide (UG627) discusses in detail how you need to code in order to infer a block RAM or FIFO in your design:
http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_2/xst.pdf Specifically, the "RAMs and ROMs Hardware Description Language (HDL) Coding Techniques" section provides details on how to infer RAMs and ROMs built using block RAM in your design.
In addition, the
HDL Coding Practices to Accelerate Design Performance White Paper (WP231)provides additional information on coding techniques that can be used to optimize performance of Block RAM in your design.Refer tothe "Maximize Block RAM performance" section:
http://www.xilinx.com/support/documentation/white_papers/wp231.pdf