Xilinx Virtex-6 Manual page 4

Chapter 1: Introduction
Design Entry Methods
For each design element in this guide, Xilinx evaluates four options for using the design
element, and recommends what we believe is the best solution for you. The four options
are:
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Instantiation - This component can be instantiated directly into the design. This
method is useful if you want to control the exact placement of the individual blocks.
Inference - This component can be inferred by most supported synthesis tools. You
should use this method if you want to have complete flexibility and portability of the
code to multiple architectures. Inference also gives the tools the ability to optimize
for performance, area, or power, as specified by the user to the synthesis tool.
Coregen & Wizards - This component can be used through CORE Generator or
other Wizards. You should use this method if you want to build large blocks of any
FPGA primitive that cannot be inferred. When using this flow, you will have to
re-generate your cores for each architecture that you are targeting.
Macro Support - This component has a UniMacro that can be used. These
components are in the UniMacro library in the Xilinx tool, and are used to instantiate
primitives that are too complex to instantiate by just using the primitives. The
synthesis tools will automatically expand UniMacros to their underlying primitives.
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Virtex-6 Libraries Guide for HDL Designs
UG623 (v 14.5) March 20, 2013