Processor Architecture Recycling Instruction Slack Time
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Solution Overview
Problem
In synchronous computer architectures, the variation in instruction and operand delays leads to significant 'slack' time at the end of a clock cycle, reducing processor performance and energy efficiency, which existing methods struggle to effectively utilize or manage.
Innovation Solution
The introduction of 'transfer gate transparency' between clock cycles allows for early data transmission between execution units, enabling the recycling of slack time by switching transmission gates to a second mode on clock cycle boundaries, and using scheduling circuits to determine completion instants and allocate instructions for optimal execution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the clock cycle length is set conservatively to accommodate PVT delay and instruction/operand delay variations, then reliability is improved, but productivity deteriorates due to slack time reducing effective processor performance
Solution Approach 1:
The patent applies preliminary action by allowing the succeeding instruction to begin execution during the slack time of the current instruction's clock cycle. The execution unit starts processing the next instruction before the clock cycle boundary, effectively utilizing the idle time that would otherwise be wasted. This prepares the processor for the next operation in advance, improving productivity without compromising reliability.
2Reliability
If transfer gates enforce synchronous data transfer only at clock cycle boundaries, then reliability is improved by eliminating race conditions, but loss of time increases due to inability to utilize slack time
Solution Approach 1:
The patent applies dynamics by making the transfer gate timing flexible rather than strictly synchronous. The transfer gate remains closed during the critical synchronous transfer at clock boundaries to prevent race conditions, but is opened during the slack time period to allow data transfer for the next instruction. This dynamic timing control maintains reliability while eliminating time loss.
3Productivity
If clock speed is adjusted based on PVT measurements to reduce slack time, then productivity is improved, but use of energy increases due to voltage and temperature adjustments
Solution Approach 1:
The patent applies self-service by allowing the processor to utilize its own slack time for executing the next instruction without requiring external adjustments to voltage or temperature. The execution unit autonomously begins processing the succeeding instruction during the available slack time, improving productivity through internal resource utilization rather than energy-consuming external adjustments.
4Productivity
If special circuitry is added to accelerate time-consuming instructions, then productivity is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by making the existing execution unit perform multiple functions: it executes instructions synchronously at clock boundaries and also executes succeeding instructions during slack time. The same execution unit hardware is utilized for both purposes without requiring special accelerated circuitry, thereby improving productivity while avoiding increased device complexity.
Data Source
AI summary
A computer architecture suitable for out-of-order processors manages the problem of timing slack, in which an instruction completes before its clock cycle, by recycling that slack to allow the next succeeding instruction allowing that instruction to begin execution earlier. This recycling mechanism is enabled through the use of a transparent gating between execution units which allows data transfer before clock cycle boundaries and, in some cases, by aggressively issuing children instructions contemporaneously with their parent instruction after a grandparent instruction is issued.