Heads and Tails Format SCALE Home Page正面和背面大规模网页格式.ppt

Heads and TailsA Variable-Length Instruction Format Supporting Parallel Fetch and Decode Heidi Pan and Krste Asanovi? MIT Laboratory for Computer Science CASES Conference, Nov. 2001 Motivation Tight space constraints Cost, power consumption, space constraints Program code size Variable-length instructions: more compact but less efficient to fetch and decode High performance Deep pipelines or superscalar issue Fixed-length instructions: easy to fetch and decode but less compact Heads and Tails (HAT) instruction format Easy to fetch and decode AND compact Related Work 16-bit version of existing RISC ISAs Compressed instructions in main memory Dictionary compression CISC 16-Bit Versions Examples MIPS16 (MIPS), Thumb (Arm) Feature(s) Dynamic switching between full-width half-width 16-Bit Versions, cont’d. Advantages Simple decompression of just mapping 16-bit to 32-bit instructions Static code size reduced by ~30-40% Disadvantages Can only encode limited subset of operations and operands; more dynamic instructions needed Shorter instructions can sometimes compensate for the increased number of instructions, but performance of systems with instruction cache reduced by ~20% Compression in Memory Examples CCRP, Kemp, Lekatsas, etc. Feature(s) Hold compressed instructions in memory then decompress when refilling cache Compression in Memory, cont’d. Advantages Processor unchanged (see regular instructions) Avoids latency energy consumption of decompression on cache hits Disadvantages Decrease effective capacity of cache increase energy used to fetch cached instructions Cache miss latencies increase Translate pc ; block decompressed sequentially Dictionary Compression Examples Araujo, Benini, Lefurgy, Liao, etc. Features Fixed-length code words in instruction stream point to a dictionary holding common instruction sequences Branch address modified to point in compressed instruction stream Dictionary Compression, cont’d. Advantage(s) Decompression is just fast table lo