Prefix Code System for 16-bit Instruction Space Extension
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Solution Overview
Problem
16-bit fixed-length instruction sets face challenges in extending instruction code space, leading to increased instruction frequency and power consumption, particularly in embedded processors, due to limited operand assignment and complexity in decoding variable-length instructions.
Innovation Solution
The implementation of a prefix code system that modifies existing instructions to extend the instruction code space without increasing its size, allowing for efficient superscalar instruction issuance by replacing fixed registers, changing operand sizes, and extending displacement fields, thereby reducing transfer instruction frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a 16-bit fixed-length instruction set is used, then code efficiency is improved and program size is decreased, but the number of instructions increases and performance deteriorates
Solution Approach 1:
The instruction set is segmented into base 16-bit fixed-length instructions and optional prefix codes. The prefix codes provide additional functionality without requiring full 32-bit instructions, thereby maintaining code efficiency while reducing the total number of instructions needed.
Solution Approach 2:
The instruction encoding moves from a single 16-bit dimension to a two-dimensional structure: a base 16-bit instruction code plus an optional 16-bit prefix. This allows extension of the effective instruction code space from 2^16 to potentially 2^32 combinations without requiring all instructions to be 32 bits long.
2Quantity of substance
If the number of instructions is increased, then code efficiency is improved, but instruction transfer frequency increases and power consumption increases
Solution Approach 1:
The instruction encoding is made dynamic through optional prefix codes. Frequently used instructions remain as simple 16-bit operations, while less common operations can use prefixes to extend functionality. This dynamic structure allows the processor to adapt the effective instruction length based on usage patterns, reducing average transfer frequency and power consumption.
3Adaptability or versatility
If prefix codes are added to extend instruction code space, then instruction functionality is improved, but decoding complexity increases and execution cycles increase
Solution Approach 1:
The prefix code mechanism is prepared in advance with clearly defined formats and meanings. The decoder is pre-configured to recognize prefix patterns and automatically apply the appropriate interpretation rules, eliminating the need for complex runtime analysis and reducing actual decoding complexity during execution.
Solution Approach 2:
The prefix code acts as an intermediary layer between the base instruction set and extended functionality. Rather than directly complicating the base instruction decoding, the prefix provides a standardized interface that mediates the addition of complex operations, keeping the core decoding logic simple while enabling extended functionality.
4Ease of operation
If fixed register assignment is used, then operand assignment is simplified, but flexibility of register allocation decreases and performance is limited
Solution Approach 1:
The prefix code mechanism provides multi-functionality to the register assignment system. The same base instruction can work with fixed register assignment for simple cases, while prefix codes enable flexible register allocation for complex cases. This universal approach allows the instruction set to adapt to different register allocation strategies based on the specific operation being performed.
Data Source
AI summary
For efficient issue of a superscalar instruction a circuit is employed which retrieves an instruction of each instruction code type other than a prefix based on a determination result of decoders for determining instruction code type, adds the immediately preceding instruction to the retrieved instruction, and outputs the resultant. When an instruction of a target code type is detected in a plurality of instruction units to be searched, the circuit outputs the detected instruction code and the immediately preceding instruction other than the target code type as prefix code candidates. When an instruction of a target code type cannot be detected at the rear end of the instruction units, the circuit outputs the instruction at the rear end as a prefix code candidate. When an instruction of a target code type is detected at the head in the instruction code search, the circuit outputs the instruction code at the head.