A Case for Hybrid Instruction Encoding for Reducing Code Size in Embedded System-on-Chips based on RISC Processor Cores

Embedded computing differs from general purpose computing in several aspects. In most embedded systems, size, cost and power consumption are more important than performance. In embedded System-onChips (SoC), memory is a scarce resource and it poses constraints on chip space, cost and power consumption. Whereas fixed instruction length feature of RISC architecture simplifies instruction decoding and pipeline implementation, its undesirable side effect is code size increase caused by large number of unused bits. Code size reduction minimizes memory size, chip space and power consumption all of which are significant for low power portable embedded systems. Though code size reduction has drawn the attention of architects and developers, the solutions currently used are more of cure than of prevention. Considering the huge number of embedded applications, there is a need for a dedicated processor optimized for low power and portable embedded systems. In the study, we propose a variation of Hybrid Instruction Encoding (HIE) for the embedded processors. Our scheme uses fixed number of multiple instruction lengths with provision for hybrid sizes for the offset and the immediate fields thereby reducing the number of unused bits. We simulated the HIE for the MIPS32 processors and measured code sizes of various embedded applications of MiBench and MediaBench benchmarks using an offline tool developed newly. We noticed up to 27% code reduction for large and medium sized embedded applications respectively. This results in reduction of on-chip memory capacity up to 1 mega bytes that is very significant for SoC based embedded applications. Considering the large market share of embedded systems, it is worth investing in a new architecture and development of dedicated HIE-RISC processor cores for portable embedded systems based on SoCs.