Pre-Decoded Instruction Cache for Variable-Length RISC Decoding
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Solution Overview
Problem
Processors with variable-length instructions face challenges in decoding due to the difficulty in determining the start of each instruction, leading to increased latency and inefficiency in decoding processes.
Innovation Solution
Implementing a pre-decoded instruction cache that stores information derived from a pre-decoding process, including branching and instruction length information, to facilitate efficient decoding by reducing the number of clock cycles required for decoding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If variable-length instructions are used to achieve compact encoding and enhanced code density, then code density is improved, but decoding complexity increases due to the difficulty in determining instruction boundaries
Solution Approach 1:
The patent applies preliminary action by performing pre-decoding of instruction boundaries and characteristics before the main decoding stage. The pre-decoder circuit analyzes the fetched instruction block to identify instruction starts, lengths, and types in advance, storing this information in a pre-decoded instruction cache. This preliminary analysis eliminates the need for complex chaining operations during the main decode stage, thereby maintaining high code density with variable-length instructions while reducing decoding complexity.
2Measurement precision
If chaining operations are performed to determine instruction boundaries in variable-length instruction blocks, then correct instruction identification is achieved, but decoding latency increases
Solution Approach 1:
The pre-decoder circuit performs the chaining operation and instruction boundary identification in advance, before the main decode stage. By pre-processing the instruction block to determine all instruction boundaries, lengths, and types, the system stores this information in the pre-decoded instruction cache. During the main decode stage, the processor simply retrieves pre-computed boundary information, reducing decoding latency from multiple chaining operations to a single cache lookup while maintaining accurate instruction boundary identification.
Solution Approach 2:
The patent segments the decoding process into two distinct stages: a pre-decoding stage that performs chaining and boundary identification, and a main decoding stage that executes instructions using pre-computed information. This segmentation allows the complex chaining operation to be performed once on the entire instruction block, with results cached for efficient retrieval during execution, thereby reducing overall decoding latency while maintaining precision.
3Productivity
If parallel decoding is attempted to improve processing speed, then throughput is improved, but instruction boundary determination becomes more difficult with variable-length instructions
Solution Approach 1:
The pre-decoder circuit performs preliminary analysis of the entire instruction block in parallel to identify all instruction boundaries, lengths, and types before the main decode stage. By pre-computing this information and storing it in the pre-decoded instruction cache, the system enables the main decode stage to process multiple instructions in parallel using the pre-established boundary information, thereby achieving high throughput without the complexity of real-time boundary determination during parallel decoding.
Data Source
AI summary
Systems and methods related to instruction caching schemes are disclosed herein. A set of leading groups of bits in a set of instructions from a cache may be evaluated, in parallel and using a pre-decoder circuit, at a set of locations in the set of instructions. The set of locations may be spaced apart by a length of the smallest expected instruction and the lengths of the expected instructions may be multiples of the length of the smallest expected instruction. A set of instruction sizes associated with the set of locations may be determined from the set of leading groups of bits and stored in a set of entries in a pre-decoded instruction cache. The instructions may be decoded using a decoder circuit and the set of entries. The pre-decoded instruction cache and the pre-decoding processes may reduce the latency of decoding instructions.