Decode Unit Bypasses Scheduler for Load Constant Micro-operations
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Solution Overview
Problem
Microprocessor design faces challenges in reducing execution latency and execution bandwidth consumption, which negatively impact performance and power consumption, especially due to dependencies between instructions and the need for efficient handling of load instructions.
Innovation Solution
The microprocessor employs a decode unit to map architectural instructions into micro-operations, including a load constant micro-operation (LCM) that specifies a constant and a register, allowing direct writing to the register file without dispatching to execution units, thereby achieving zero execution latency and zero bandwidth consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If load instructions are executed through the normal execution pipeline, then the processor can handle various instruction types uniformly, but the execution latency increases and performance decreases
Solution Approach 1:
The decode unit performs preliminary action by directly writing constant values to the register file during the decode stage, before the instruction reaches the execution units. This eliminates the need for load instructions to traverse the entire execution pipeline, reducing execution latency from multiple clock cycles to essentially zero cycles for the actual data availability.
Solution Approach 2:
The patent extracts the constant loading function from the normal execution pipeline and implements it directly in the decode unit. By separating this specific function from the general execution path, the patent creates a dedicated shortcut that bypasses the scheduler and execution units, thereby reducing execution latency while maintaining uniform handling of other instruction types.
2Productivity
If load constant operations are dispatched to execution units, then the execution units can handle all operations uniformly, but execution bandwidth is consumed and power consumption increases
Solution Approach 1:
The patent extracts the constant loading operation from the general execution path and handles it separately in the decode unit. This separation eliminates the need for execution units and schedulers to process these simple operations, reducing their workload and power consumption while maintaining overall processor throughput.
Solution Approach 2:
The decode unit performs the constant loading operation itself without requiring service from the scheduler or execution units. By making the decode unit self-sufficient for this specific operation, the patent reduces the involvement of other power-consuming components, thereby reducing overall power consumption while maintaining productivity.
3Loss of time
If the pipeline of execution units is shortened, then load-to-use latency is reduced, but the execution units become less versatile and cannot handle complex operations
Solution Approach 1:
The patent segments the instruction handling into two distinct paths: simple constant loading operations are handled by the decode unit through direct register file writing, while complex operations continue to be handled by the execution units. This segmentation allows the execution units to maintain their full functionality and versatility while eliminating unnecessary pipeline stages for simple operations, thereby reducing load-to-use latency without sacrificing adaptability.
Data Source
AI summary
A microprocessor includes a decode unit that maps architectural instructions into micro-operations and dispatches them to a scheduler that issues them to execution units that execute them by reading source operands from a register file and writing execution results to the register file. An architectural instruction instructs the microprocessor to load a constant into an architectural destination register. The decode unit maps the architectural instruction into a load constant micro-operation (LCM) and writes the LCM constant directly to a register of the register file without dispatching the LCM to the scheduler, such that the LCM is not issued to the execution units. In the same clock cycle, the decode unit indicates the LCM constant is available for consumption, such that the LCM imposes zero execution latency on dependent micro-operations and dispatches to the scheduler micro-operations other than the LCM. The register file may include a decode unit-dedicated write port.