Microarchitecture and Circuits for a 200 MHz Out-of-Order Soft Processor Memory System

Abstract

Although FPGAs have grown in capacity, FPGA-based soft processors have grown very little because of the difficulty of achieving higher performance in exchange for area. Superscalar out-of-order processors promise large performance gains, and the memory subsystem is a key part of such a processor that must help supply increased performance. In this article, we describe and explore microarchitectural and circuit-level tradeoffs in the design of such a memory system. We show the significant instructions-per-cycle wins for providing various levels of out-of-order memory access and memory dependence speculation (1.32 × SPECint2000) and for the addition of a second-level cache (another 1.60 × ). With careful microarchitecture and circuit design, we also achieve a L1 translation lookaside buffers and cache lookup with 29% less logic delay than the simpler Nios II/f memory system.