Designing Application-Specific Instruction-set Processors (ASIPs) usually requires designing a custom datapath, and modifying instruction-set, instruction decoder, and compiler. A new alternative to ASIPs is No-Instruction-Set-Computers (NISCs) that eliminate the instruction abstraction by compiling programs directly to a given datapath. The compiler analyzes the datapath and extracts possible operations and data flows. The NISC approach simplifies and accelerates the task of custom processor design. In this paper, we present a case-study of designing a custom datapath for a 2-D DCT algorithm. We applied several optimization techniques such as software transformations, operation chaining, datapath pipelining, controller pipelining, and functional unit customization to improve the quality of the design. Most of the techniques are general and can be applied to other applications. The result of synthesizing our final custom datapath on a Xilinx FPGA shows 7.14 times performance improvement, 1.64 times power reduction, 12.5 times energy savings, and more than 3 times area reduction compared to a softcore MIPS implementation. 1
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