Branch Instruction Bonding for Dispatch Bandwidth
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Solution Overview
Problem
Superscalar processors face challenges in achieving higher throughput without compromising frequency due to the need for wider machines, which increases hardware costs and complexity, particularly in RISC architectures where instruction bloat and power consumption become significant issues.
Innovation Solution
The technique of branch instruction bonding, where a branch instruction is combined with its architectural delay slot to form a single bonded instruction, is loaded into the instruction buffer, effectively increasing dispatch bandwidth at minimal cost and complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dispatch width is increased to improve throughput, then instruction processing capacity increases, but hardware complexity and area increase quadratically
Solution Approach 1:
The patent merges a branch instruction with its architectural delay slot into a single bonded instruction entity. This combining allows the processor to treat two instructions as one, effectively increasing dispatch bandwidth without requiring proportionally wider hardware structures. The bonded instruction is loaded as a single unit into the instruction buffer, reducing the number of separate instruction entries needed and lowering register renamer complexity while maintaining improved throughput.
2Productivity
If processor width is increased to process more instructions per cycle, then throughput improves, but frequency decreases proportionally
Solution Approach 1:
By bonding a branch instruction with its delay slot into a single instruction entity, the patent enables the processor to dispatch more effective work per cycle without proportionally increasing the physical width of the processor. This allows wider effective dispatch bandwidth while maintaining the same clock frequency, as the bonded instruction count reduces the actual number of separate instruction entries that need to be managed in the pipeline.
3Productivity
If RISC processor width is increased to compete with CISC processors, then instruction processing capability improves, but power consumption increases intolerably
Solution Approach 1:
The patent applies instruction bonding to combine branch instructions with their architectural delay slots, effectively increasing the processing capability of RISC processors without requiring proportionally wider hardware. This reduces the need for additional processing channels that would consume excessive power, as the bonded instructions are handled as single entities through the existing processor infrastructure.
4Productivity
If instruction buffer size is increased to handle wider dispatch, then more instructions can be dispatched, but area and cost increase
Solution Approach 1:
By loading bonded instructions as single entities into the instruction buffer, the patent effectively increases dispatch bandwidth without proportionally increasing buffer area. The bonding reduces the number of separate instruction entries needed, as each bonded instruction represents two original instructions packaged as one, thereby reducing the area required in the instruction buffer while maintaining enhanced dispatch capability.
Data Source
AI summary
A processor is configured to identify a branch instruction immediately followed by an architectural delay slot. A single bonded instruction comprising the branch instruction immediately followed by the architectural delay slot is created. The single bonded instruction is loaded into an instruction buffer.