PAD Instructions for Memory Address Alignment in VLIW
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Solution Overview
Problem
In Very-Long Instruction Word (VLIW) architectures, unaligned instruction packets lead to increased clock cycles for execution due to inefficient memory access, resulting in performance degradation and code density issues, as existing solutions like NOP instructions are inflexible and wasteful.
Innovation Solution
The introduction of PAD instructions, which are 16-bit instructions not assigned to functional units, allows for alignment of instruction packets by being multiplied to fill gaps, reducing the number of clock cycles required for execution without assigning them to functional units.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If unaligned instruction packets are processed using traditional alignment methods (NOP instructions), then memory access alignment is achieved, but execution time increases and code density decreases
Solution Approach 1:
The patent discards the traditional NOP instruction approach and recovers execution time by implementing a dual-fetch mechanism that simultaneously retrieves aligned instruction packets and padding data from memory, then combines them in the instruction buffer without requiring sequential padding operations
Solution Approach 2:
The system performs preliminary alignment by pre-fetching instruction packets and padding data together in a single memory access, preparing the complete aligned instruction set before execution begins, thereby eliminating runtime alignment delays
2Manufacturing precision
If unaligned instruction packets are processed using traditional alignment methods, then memory access alignment is achieved, but code density decreases
Solution Approach 1:
The patent merges the alignment data ( previously requiring separate NOP instructions) directly into the instruction packet structure by combining fetched instruction data with padding data in the instruction buffer, creating a unified aligned instruction stream that improves code density
3Manufacturing precision
If PAD instructions are multiplied to fill gaps for alignment, then instruction packet alignment is achieved, but the number of instructions increases
Solution Approach 1:
The patent extracts the alignment function from the instruction stream by separating padding data retrieval from the main instruction fetch, obtaining padding data through a dedicated padding fetch operation that does not increase the executed instruction count
4Productivity
If aligned instruction packets are used, then execution efficiency is maintained, but flexibility to handle unaligned addresses is reduced
Solution Approach 1:
The system dynamically adapts to both aligned and unaligned instruction packet addresses by implementing a conditional fetch mechanism that automatically determines whether padding is needed based on the fetched address alignment, maintaining execution efficiency while handling various address scenarios
Data Source
AI summary
An example apparatus includes example packet decode circuitry to decode an instruction packet for programmable circuitry into at least one instruction. Additionally, the example apparatus includes example instruction mapping circuitry to disregard a pad instruction included in the at least one instruction, the pad instruction having not been assigned to any functional unit of the programmable circuitry.