A Dense and Efficient Instruction Set Architecture Encoding

Abstract

This paper introduces Scry, a novel and experimental Instruction Set Architecture (ISA) designed to maximize instruction density and encoding efficiency for modern processor implementations. Scry achieves instruction-feature parity with RISC-V's RV64IMC using only 2-byte instructions by incorporating forward-temporal referencing for data flow and internal data type tagging. This design drastically reduces the utilized encoding space to 28% and shows static instruction density comparable to or better than RV64IMC, particularly in larger functions.

Report

Key Highlights

• Novel ISA Introduction: The paper proposes Scry, an experimental Instruction Set Architecture focused on achieving extreme instruction density.
• Size Reduction: Scry instructions utilize only 2 bytes (16 bits), achieving feature parity with the 4-byte instructions used by the standard RISC-V RV64IMC base and extensions.
• Encoding Efficiency: Scry instructions occupy only 28% of the available 2-byte encoding space, contrasting sharply with RV64IMC, which utilizes 68% of the 4-byte space.
• Density Performance: Hand-compiled static instruction density for Scry is comparable to RV64IMC for small functions and improves significantly as function size increases.

Technical Details

• ISA Name: Scry.
• Core Design Philosophy: Revisiting first principles to maximize instruction density and encoding efficiency over factors like power or area (which are implementation-dependent).
• Data Flow Mechanism: Scry employs forward-temporal referencing, meaning instructions explicitly refer to the future instructions that will consume their outputs.
• Data Handling: Scry uses internal tagging, where the processor dynamically tracks data types internally, thereby reducing the need for explicit type instructions and increasing overall flexibility.
• Comparison Baseline: RISC-V RV64IMC (64-bit Integer, Atomic, and Compressed extensions).

Implications

• Challenging ISA Norms: Scry's successful demonstration of feature parity at half the instruction size (2 bytes vs. 4 bytes) challenges the current industry standard of 32-bit (4-byte) fixed-length instructions for general-purpose computing.
• Code Size Reduction: Superior instruction density translates directly to smaller binary size, leading to significant benefits in memory usage, faster I/O, and reduced cache pressure, which is crucial for embedded systems and modern large-scale data centers.
• Architectural Innovation: The adoption of forward-temporal referencing and internal tagging represents a major shift from traditional register-based ISAs, potentially opening new avenues for compiler design and processor pipeline optimization.
• Ecosystem Potential: While experimental, if proven viable, this approach could inform future iterations of highly dense ISA extensions (like RISC-V's 'C' extension) or spawn new, ultra-efficient architectures for specialized domain-specific computing.