Pathlength reduction features in the PA-RISC architecture

The authors describe representative pathlength reduction features of PA-RISC (reduced instruction set computer) instructions in memory accessing, functional operations, and instruction sequencing. To illustrate the multi-op instructions in PA-RISC, comparison is made with the MIPS instruction set, rather than with some hypothetical single-op RISC instructions. It is noted that, while other RISC architectures strive to enable short cycle times and single cycle instruction execution, PA-RISC also supports pathlength reduction, without impacting either the cycle time or the CPI. Frequent operations are combined into a single multi-op instruction. Subword data are also operated on in parallel, making full use of the datapath width. Such instruction level parallelism gives PA-RISC some of the advantages of a very simple VLIW (very long instruction word) architecture (with short 32-b instructions), in addition to the inherent advantages of a streamlined RISC architecture.