"A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturing – hence "field-programmable". The FPGA configuration is generally specified using a hardware description language (HDL), similar to that used for an application-specific integrated circuit (ASIC)." per Wikipedia
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FPGAs code is usually written using the HDLs Verilog or VHDL. FPGAs often include pre-designed logic such as memories and DSP/math functions. FPGAs may include serial transceivers and embedded microprocessors.
There are fours major manufacturers of FPGAs: