Superscalar Length Decode Unit for Variable Instruction Boundary Identification
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Solution Overview
Problem
Existing processor architectures require significant resources to store and verify endbits for variable length instructions, leading to increased die area and cost, as well as complex decode mechanisms to handle corrupted endbits.
Innovation Solution
A superscalar decode mechanism that includes a length decode unit to estimate instruction lengths and store pointers for variable length instructions within a scan window, allowing for efficient decoding and boundary identification without the need for extensive circuitry to repair corrupted endbits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If endbits are stored in the instruction cache and L2 cache to identify instruction boundaries, then the instruction boundaries can be correctly identified, but the cache size and die area increase significantly
Solution Approach 1:
The patent extracts the essential function of boundary identification from the endbits data structure and implements it through a length decode unit that calculates boundaries on-the-fly. Instead of storing endbits in cache, the system stores only the instruction bytes and computes boundaries when needed, removing the redundant data storage requirement while maintaining accurate boundary identification.
Solution Approach 2:
The patent creates a simplified copy of the boundary identification mechanism by implementing a length decode unit that replicates the boundary calculation logic. Rather than storing pre-computed endbits, the system uses the length decode unit to generate boundary information dynamically from the instruction bytes themselves, reducing cache requirements while preserving identification accuracy.
2Reliability
If circuitry is added to confirm and repair corrupted endbits, then the decode mechanism can handle errors, but the device complexity increases
Solution Approach 1:
The length decode unit performs self-service by automatically verifying instruction boundaries through its own length decoding logic. When a potential boundary is identified, the unit independently verifies it by attempting to decode the instruction and checking if the length is valid. This self-verification mechanism eliminates the need for separate confirmation and repair circuitry, maintaining reliability while reducing complexity.
Solution Approach 2:
The system implements feedback through the length decode unit's verification process. When an endbit is suspected to be corrupted, the unit uses feedback from the length decoding operation itself to detect the error and trigger recalibration. This feedback mechanism provides reliable error handling without requiring complex additional circuitry, as the verification is integrated into the normal decoding flow.
3Measurement precision
If endbits are stored and verified for each instruction, then accurate boundary identification is achieved, but the processing time increases due to verification and potential repair operations
Solution Approach 1:
The length decode unit performs preliminary action by pre-calculating instruction lengths and boundaries as instructions are fetched and stored in the instruction cache. This preliminary length decoding prepares the boundary information in advance, so when verification is needed, the work is already done. This eliminates the need for time-consuming verification and repair operations during the critical decoding path, maintaining both accuracy and speed.
Data Source
AI summary
A mechanism for superscalar decode of variable length instructions. The decode mechanism may be included within a processing unit, and may comprise a length decode unit. The length decode unit may obtain a plurality of instruction bytes. The instruction bytes may be associated with a plurality of variable length instructions, which are to be executed by the processing unit. The length decode unit may perform a length decode operation for each of the plurality of instruction bytes. For each instruction byte, the length decode unit may estimate the instruction length of a current variable length instruction associated with a current instruction byte. Furthermore, during the length decode operation, for each instruction byte, the length decode unit may estimate the start of a next variable length instruction based on the estimated instruction length of the current variable length instruction, and store a first pointer to the estimated start of the next variable length instruction.