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Implementation of a new instruction set architecture (ISA) is a non-trivial task which involves significant modifications to the system software, such as the compiler, the assembler, and the linker. This task also includes modifying and verifying functional and cycle accurate simulators to facilitate correct simulation and performance evaluation of programs under the new ISA. Isolating errors in these software components becomes extremely challenging and demands automated and semi-automated mechanisms since neither the compilation infrastructure nor the simulation infrastructure can be trusted as both parties have been heavily modified. Bootstrapping a new ISA is very common in embedded systems since there is a greater variety of embedded ISAs due to often not having a need to support backward compatibility of executables. In this paper, we present the tools and the verification mechanisms we have implemented to support the development of a number of related, but distinct ISAs. These ISAs are similar in complexity to the RISC-V ISA, and range from simple pipelined and superscalar processor ISAs, to a complete VLIW ISA. Our work in developing the system software and simulators for these ISAs demonstrate that a step-by-step semi-automated approach which relies on simple invariants can facilitate effective bootstrapping of the complete system software and the simulator infrastructure. 

While data filter caches (DFCs) have been shown to be effective at reducing data access energy, they have not been adopted in processors due to the associated performance penalty caused by high DFC miss rates. In this article, we present a design that both decreases the DFC miss rate and completely eliminates the DFC performance penalty even for a level-one data cache (L1 DC) with a single cycle access time. First, we show that a DFC that lazily fills each word in a DFC line from an L1 DC only when the word is referenced is more energy-efficient than eagerly filling the entire DFC line. For a 512B DFC, we are able to eliminate loads of words into the DFC that are never referenced before being evicted, which occurred for about 75% of the words in 32B lines. Second, we demonstrate that a lazily word filled DFC line can effectively share and pack data words from multiple L1 DC lines to lower the DFC miss rate. For a 512B DFC, we completely avoid accessing the L1 DC for loads about 23% of the time and avoid a fully associative L1 DC access for loads 50% of the time, where the DFC only requires about 2.5% of the size of the L1 DC. Finally, we present a method that completely eliminates the DFC performance penalty by speculatively performing DFC tag checks early and only accessing DFC data when a hit is guaranteed. For a 512B DFC, we improve data access energy usage for the DTLB and L1 DC by 33% with no performance degradation.