Synthesizable-from-C Embedded Processor Based on MIPS-ISA and OISC

We describe a lightweight open-source MIPS-ISA processor, wherein performance and area can be flexibly traded-off with one another. The processor contains an ultra-low-cost co-processor capable of executing programs comprised of SUBLEQ instructions (subtract and branch if the difference is ≤ 0), which recent work has shown to be sufficient for any computation. Area/performance trade-offs are realized by implementing a user-selectable subset of MIPS instructions with functionally equivalent SUBLEQ sub-routines that run on the coprocessor. Silicon area is reduced as more MIPS instructions are implemented with the co-processor, rather than "natively" using functional units within the host MIPS. The processor is described in the C language and synthesized to an FPGA hardware implementation with high-level synthesis (HLS). Since it is specified at a high level of abstraction, it is straightforward to tailor to any application. As such, the processor can be viewed as a family of processors with different area/performance/power characteristics. In an experimental study, we compare a variety of processor variants, wherein different subsets of MIPS instructions are handled by the co-processor. We also compare the proposed synthesizable processor with a hand-designed 5-pipeline-stage MIPS implementation, and achieve area reductions ranging from 2.5 - 4×.