Need definition of FPGA terms!!

[nand_gates]

xl23_2.pdf

Hi,
I need defination/exact meaning of following FPGA terms
1. System Gates
2. Gate Density
3. Gate count

 

[tut]

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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Also see,
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http://neil.franklin.ch/Usenet/comp.arch.fpga/20000806_Help_Virtex_system_gate_count
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tut..