Micro-Op RAM Main Memory for Lower Instruction Decoding Latency
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Solution Overview
Problem
Decoding circuitry in conventional microprocessors imposes latency issues due to real-time decoding of instructions according to an instruction set architecture (ISA), and existing optimizations are limited in flexibility and increase clock cycle analysis costs.
Innovation Solution
Employing micro-operations (micro-ops) random access memory (RAM) as main program memory, where control signal parameters are stored in row circuits, allowing direct communication to register files and execution circuits, thereby eliminating the need for a decoding stage and utilizing the instruction cache for micro-ops storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If real-time decoding of ISA instructions is performed in the decoding pipeline stage, then the processing unit can execute instructions according to the instruction set architecture, but processing latency increases due to the costly decoding operation
Solution Approach 1:
The patent pre-decodes ISA instructions into micro-ops and stores them in a micro-ops cache before execution. This preliminary action eliminates the need for real-time decoding during instruction execution, thereby reducing processing latency while maintaining the capability to execute ISA instructions.
Solution Approach 2:
The patent creates a copy of the decoding functionality by storing pre-decoded micro-ops in a cache memory. Instead of performing decoding operations each time an instruction is executed, the system retrieves pre-computed micro-ops from the cache, effectively copying the decoding result for repeated use.
2Loss of time
If the instruction cache is used to store pre-decoded micro-ops instead of ISA instructions, then decoding latency is reduced, but the instruction cache cannot serve its conventional function of storing ISA instructions
Solution Approach 1:
The patent transforms the instruction cache into a multi-functional structure that can store both pre-decoded micro-ops and original ISA instructions. This allows the cache to serve dual purposes: reducing decoding latency by providing quick access to micro-ops while maintaining the ability to store and retrieve ISA instructions when needed.
Solution Approach 2:
The patent implements a dynamic cache system that can adapt its content based on execution needs. The cache can switch between storing micro-ops for rapid execution and storing ISA instructions when micro-ops are not available or when the processor needs to handle new instruction types, providing flexible functionality.
3Speed
If look-up tables are used to retrieve pre-decoded instructions, then decoding speed is improved, but the complexity and cost of the retrieval mechanism increases
Solution Approach 1:
The patent extracts the decoding operation from the critical execution path and performs it separately before instruction execution. By pre-decoding instructions and storing the results in a cache, the system eliminates the need for complex look-up table mechanisms during instruction retrieval, simplifying the overall architecture while maintaining high decoding speed.
Data Source
AI summary
Aspects disclosed in the detailed description include a processing unit (PU) employing micro-operations (micro-ops) random access memory (RAM) as main program memory. The micro-ops RAM comprises row circuits each associated with a micro-op and configured to store control signal parameters and output ports configured to be coupled to a register file and one or more execution units. In contrast to fetching and decoding instructions of an ISA in a conventional PU, the processing unit loads a main program comprising micro-ops into the row circuits of the micro-ops RAM. When executing an individual micro-op of the main program, the processing unit activates a row circuit in the micro-ops RAM to cause its stored control signal parameters to be communicated through the output ports of the micro-ops RAM to the register file and/or the one or more execution units and avoids the need for a decoding stage circuit, advantageously decreasing processing latency.