Ultrasmall: A Tiny Soft Processor Architecture with Multi-Bit Serial Datapaths for FPGAs


IEICE TRANSACTIONS on Information and Systems   Vol.E98-D   No.12   pp.2150-2158
Publication Date: 2015/12/01
Publicized: 2015/09/15
Online ISSN: 1745-1361
DOI: 10.1587/transinf.2015PAP0022
Type of Manuscript: Special Section PAPER (Special Section on Parallel and Distributed Computing and Networking)
Category: Architecture
soft processor,  processor architecture,  FPGA,  

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Soft processors are widely used in FPGA-based embedded computing systems. For such purposes, efficiency in resource utilization is as important as high performance. This paper proposes Ultrasmall, a new soft processor architecture for FPGAs. Ultrasmall supports a subset of the MIPS-I instruction set architecture and employs an area efficient microarchitecture to reduce the use of FPGA resources. While supporting the original 32-bit ISA, Ultrasmall uses a 2-bit serial ALU for all of its operations. This approach significantly reduces the resource utilization instead of increasing the performance overheads. In addition to these device-independent optimizations, we applied several device-dependent optimizations for Xilinx Spartan-3E FPGAs using 4-input lookup tables (LUTs). Optimizations using specific primitives aggressively reduce the number of occupied slices. Our evaluation result shows that Ultrasmall occupies only 84% of the previous small soft processor. In addition to the utilized resource reduction, Ultrasmall achieves 2.9 times higher performance than the previous approach.