A cpu implemented in a FieldProgrammableGateArray (FPGA). An FPGA is a logic chip which can be programmed or reprogrammed with a Configuration Bit Stream. (See Xilinx, Altera, Actel, Lucent and Cypress for examples.)

An example of a hardware and software FPGA development kit is the Nallatech Xilinx Xtreme DSP kit (http://www.nallatech.com/solutions/products/kits/XtremeDSP-II/).  
The Vertex 2 FPGA and the excellent Nallatech hardware and software make this an effective learning and development kit.

A full website and and email list is available at http://www.fpgacpu.org.
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There are also many MinimalInstructionSetComputer designs targeted at FPGAs.
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The following wiki may be a better location than C2 for discussing CPUs-in-FPGAs:

"Computer Architecture Lab" :
a hands-on introduction into computer architecture.
The main target is to build a simple, pipelined microprocessor and run it in an FPGA.
http://en.wikiversity.org/wiki/Computer_Architecture_Lab

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See
AlternativeMicroprocessorDesign

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CategoryHardware
CategoryComputerArchitecture