Speculative DMA for architecturally visible storage in instruction set extensions

Instruction set extensions (ISEs) can accelerate embedded processor performance. Many algorithms for ISE generation have shown good potential; some of them have recently been expanded to include Architecturally Visible Storage (AVS) - compiler-controlled memories, similar to scratchpads, that are accessible only to ISEs. To achieve a speedup using AVS, Direct Memory Access (DMA) transfers are required to move data from the main memory to the AVS; unfortunately, this creates coherence problems between the AVS and the cache, which previous methods for ISEs with AVS failed to address; additionally, these methods need to leave many conservative DMA transfers in place, whose execution significantly limits the achievable speedup. This paper presents a memory coherence scheme for ISEs with AVS, which can ensure execution correctness and memory consistency with minimal area overhead. We also present a method that speculatively removes redundant DMA transfers. Cycle-accurate experimental results were obtained using an FPGA-emulation platform. These results show that the application-specific instruction-set extended processors with speculative DMA-enhanced AVS gain significantly over previous techniques, despite the overhead of the coherence mechanism.