This paper presents a very lean DSP Extension Architecture (iDEA) soft processor for Field Programmable Gate Arrays (FPGAs). iDEA has been built to be as lightweight as possible, utilising the run-time flexibility of the DSP48E1 primitive in Xilinx FPGAs to serve as many processor functions as possible. We show how the primitive's flexibility can be leveraged within a general-purpose processor, what additional circuitry is needed, and present a full instruction-set architecture. The result is a very compact processor that can run at high speed, while executing a full gamut of general machine instructions. We provide results for a number of simple applications, and show how the processor's resource requirements and frequency compare to a Xilinx MicroBlaze soft core. Based on the DSP48E1, this processor can be deployed across next-generation Xilinx Artix-7, Kintex-7, and Virtex-7 families.