Out-of-Order Access to Shared Microcode Sequencer
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Solution Overview
Problem
Current processor architectures face performance bottlenecks due to the restrictive arbitration scheme for access to a shared microcode sequencer, which limits decode clusters from operating in parallel and results in stalling, reducing decode bandwidth and overall performance.
Innovation Solution
Implementing an arbitration circuit that allows out-of-order access to the shared microcode sequencer by decode clusters, enabling younger decode clusters to access the sequencer if the instruction requires fewer than a threshold number of clock cycles or micro-operations, thereby reducing stalling and enhancing parallel operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If in-order access arbitration is used for the shared microcode sequencer, then access order is maintained, but decode bandwidth is reduced and performance is limited
Solution Approach 1:
The patent implements dynamic arbitration that allows younger decode clusters to access the microcode sequencer out-of-order when instructions require fewer than a threshold number of clock cycles or micro-operations. This dynamic access control enables parallel operation of multiple decode clusters, increasing decode bandwidth while maintaining manageable complexity through cycle-count-based eligibility criteria.
2Productivity
If decode clusters operate in parallel, then productivity increases, but access to shared microcode sequencer becomes conflicted
Solution Approach 1:
The patent applies preliminary action by having decode clusters pre-determine their eligibility to access the microcode sequencer based on the cycle count or micro-operation count of their instructions before actually accessing it. This pre-check mechanism ensures that only clusters with instructions requiring fewer than the threshold number of cycles or micro-operations can access the sequencer simultaneously, preventing conflicts while enabling parallel operation.
3Loss of time
If strict in-order access is enforced, then sequencing is maintained, but stalling increases and time is lost
Solution Approach 1:
The patent changes the arbitration parameter from strict in-order sequencing to a cycle-count-based eligibility system. This parameter change allows younger decode clusters to access the microcode sequencer when their instructions require fewer than a threshold number of clock cycles or micro-operations, reducing stalling time while maintaining sequencing integrity through the threshold-based control mechanism.
Data Source
AI summary
Systems, methods, and apparatuses relating to circuitry to implement out-of-order access to a shared microcode sequencer by a clustered decode pipeline are described. In one embodiment, a hardware processor core includes a first decode cluster comprising a plurality of decoder circuits, a second decode cluster comprising a plurality of decoder circuits, a fetch circuit to fetch a first block of instructions and send the first block of instructions to the first decode cluster for decoding, and fetch a second block of instructions younger in program order than the first block of instructions and send the second block of instructions to the second decode cluster for decoding, a microcode sequencer comprising a memory that stores a plurality of micro-operations, and an arbitration circuit to arbitrate access by the first decode cluster and the second decode cluster to a shared read port of the memory, wherein the arbitration circuit is to allow the second decode cluster decoding the second block of instructions access to the shared read port of the memory instead of the first decode cluster decoding the first block of instructions when an instruction of the second block of instructions has a number of corresponding micro-operations in the microcode sequencer below an arbitration threshold.