Bypass Directory Tracking for Processor Data Forwarding
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Solution Overview
Problem
Traditional pipelined processor data paths are power, area, and wiring intensive due to the need for extensive copying and tracking of results through multiple storage elements in bypass register files, which does not scale well with increasing pipeline length and number of functional units.
Innovation Solution
A bypass directory tracking system that sets and clears bits in a bypass directory to determine the location of the most recent result for an architectural register, using a rotating head pointer to simplify the control logic and reduce physical shifting of results, allowing for efficient forwarding of data between the architectural and bypass register files.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If traditional bypass register file implementation is used with extensive copying and tracking, then data forwarding functionality is achieved, but power consumption increases
Solution Approach 1:
The patent uses a directory structure that copies only metadata (tags and stage indicators) rather than actual data values. Each directory entry contains a tag identifying the architectural register and a stage indicator showing where the result resides, eliminating the need for complex control logic to track data through multiple bypass stages.
Solution Approach 2:
The bypass directory acts as an intermediary between the architectural register file and the bypass register file. Instead of directly comparing register specifiers across multiple storage elements, the directory provides a simplified lookup mechanism that directly indicates the location of the most recent result.
2Area of stationary object
If traditional bypass register file implementation is used with extensive copying and tracking, then data forwarding functionality is achieved, but area requirements increase
Solution Approach 1:
The patent segments the bypass tracking functionality into a separate directory structure independent of the bypass register file. The directory is organized into rows (one per architectural register) and columns (one per pipeline stage), allowing compact representation of data location information without requiring extensive control logic circuitry.
Solution Approach 2:
Instead of copying and tracking actual data values through multiple bypass stages, the patent copies only the essential location information (stage indicator) and identity information (tag) into the directory. This dramatically reduces the area required for control logic while maintaining full data forwarding functionality.
3Device complexity
If traditional bypass register file implementation is used with extensive copying and tracking, then data forwarding functionality is achieved, but wiring complexity increases
Solution Approach 1:
The bypass directory serves as an intermediary that simplifies the wiring between the architectural register file and bypass register file. By providing direct lookup of result locations through tags and stage indicators, the directory reduces the wiring complexity that would otherwise be required to implement complex control logic for data forwarding.
Solution Approach 2:
The patent copies location information (stage indicators) and identity information (tags) into a compact directory structure. This eliminates the need for extensive wiring to support complex control logic that would otherwise be required to track and forward data through multiple bypass stages.
4Adaptability or versatility
If traditional bypass register file implementation is used, then data forwarding is supported, but scalability deteriorates with increasing pipeline length
Solution Approach 1:
The patent segments the bypass tracking into a modular directory structure where each row corresponds to an architectural register and each column corresponds to a pipeline stage. This segmentation allows the directory to scale naturally with pipeline length by simply adding columns, without increasing control logic complexity.
Solution Approach 2:
The directory structure dynamically adapts to different pipeline configurations. By organizing the directory with columns representing pipeline stages and rows representing architectural registers, the system can accommodate varying pipeline lengths and functional unit counts without requiring redesign of the control logic.
Data Source
AI summary
A processing bypass directory system and method are disclosed. In one embodiment, a bypass directory tracking process includes setting bits in a bypass directory when a corresponding architectural register is written. The bits are selectively cleared in the bypass directory each cycle. The configuration of the bits is utilized to determine which stage of a bypass path processing information is at.