Field programmable gate arrays (FPGA's) may be structured in accordance with the disclosure to have a register-intensive architecture that provides, for each of plural function-spawning LookUp Tables (e.g. a 4-input, base LUT's) within a logic block, a plurality of in-block accessible registers. A register-feeding multiplexer means may be provided for allowing each of the plural registers to equivalently capture and store a result signal output by the corresponding, base LUT of the plural registers. Registerable, primary and secondary feedthroughs may be provided for each base LUT so that locally-acquired input signals of the LUT may be fed-through to the corresponding, in-block registers for register-recovery purposes without fully consuming (wasting) the lookup resources of the associated, base LUT. A multi-stage, input switch matrix (ISM) may be further provided for acquiring and routing input signals from adjacent, block-interconnect lines (AIL's) and/or block-intra-connect lines (e.g., FB's) to the base LUT's and/or their respective, registerable feedthroughs. Techniques are disclosed for utilizing the many in-block registers and/or the registerable feedthroughs and/or the multi-stage ISM's for efficiently implementing various circuit designs by appropriately configuring such register-intensive FPGA's.