Need definition of FPGA terms!!

Q. How does Xilinx measure density?
Xilinx measures density in terms of system gates, using the same basic measurement established with the Virtex family. It is a combination of logic, memory, and custom circuit resources that would be utilized in a typical design. The system gate estimate is found in typical designs using a portion of the resources available on the device. This does not count a sum total of all the logic, memory, and custom circuit resources available on each device. Of course, each design uses a different amount of logic and memory, so the density measurement will vary. If a design uses only logic portion of the resources on the devices, the achieved density will be far less than if the design were to use both the logic and a good portion of the memory.

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