Orthogonal Instruction Encoding for a 16-bit Embedded Processor with Dynamic Implied Addressing Mode

Although 32-bit architectures are becoming the norm for modern microprocessors, 16-bit ones are still employed by many low-end processors, for which small size and low power consumption are of high priority. However, 16-bit architectures have a critical disadvantage for embedded processors that they do not provide enough encoding space to add special instructions coined for certain applications. To overcome this, many existing architectures adopt non-orthogonal, irregular instruction sets to accommodate a variety of unusual addressing modes thru which more opcodes and operands are densely encoded within the narrow instruction word. In general, these non-orthogonal architectures are regarded compiler-unfriendly as they tend to requires extremely sophisticated compiler techniques for optimal code generation. To address this issue, we propose a compiler-friendly processor with a new addressing mode, called the dynamic implied addressing mode (DIAM). In this paper, we will demonstrate that the DIAM provides more encoding space for our 16-bit processor so that we are able to support more instructions specially customized for our applications. And yet, the processor maintains a RISC-style orthogonal architecture, thereby allowing us to use traditional code generation algorithms. In our experiment, the architecture augmented with DIAMs shows 6.2% code size reduction and 3.5% performance increase on average, as compared to the basic architecture without DIAMs.