UCIe: Integrating Memory Semantics

• UCIe with Memory Semantics is a framework that combines formal and operational memory models to ensure precise, verifiable, and power-efficient shared memory across chiplet boundaries.
• Hardware integration leverages both logic die adapters and native DRAM attachments to achieve up to 10× bandwidth, 3× lower latency, and enhanced power efficiency.
• Advanced verification techniques, including simulation backends and formal proof assistants, validate memory ordering constraints and support safe compiler optimizations in complex chiplet architectures.

UCIe with Memory Semantics refers to the integration of formal and operational memory models into the Universal Chiplet Interconnect Express (UCIe) architecture to achieve rigorously defined, high-performance, power-efficient, and verifiable shared memory semantics in modern chiplet-based systems. This integration addresses both the physical design—enabling low-latency, high-bandwidth, and low-power memory attachment—and the formal correctness and compositionality of memory operations across chiplet boundaries. The unification of hardware-level memory protocol innovations and formal memory semantics is central to supporting emerging domains such as AI accelerators and heterogeneous computing, as well as ensuring safety and efficiency in C/C++ and managed language implementations.

1. Formal Operational Frameworks for Relaxed Memory Semantics

Foundational work (Boudol et al., 2012) establishes a formal operational model for relaxed memory architectures, using a temporary store (σ) to buffer memory operations, and a write grain W to control early visibility of writes. In UCIe systems, each chiplet or core is equipped with a local reorder buffer (the operational analog of σ), where memory operations are recorded in program order prior to global performance. The commutability predicate σ ⊔ (t, ξ) determines when these operations can become globally visible and observable by other chiplets. Writes are annotated as Write(ρ, v)^W,I, with W specifying the subset of chiplets allowed to access the value before the global commit.

This mechanism enables substantial relaxation of strict program order: operations that do not interfere as determined by the commutability relation can be executed ahead of time, enabling early execution and reduction of global memory serialization overhead. The precise definition and control of write grain are key to selectively allowing early visibility and supporting performance-sensitive, power-efficient use cases.

2. Hardware Integration of UCIe Memory Semantics

Advancements in packaging and memory interface design have enabled UCIe-based memory architectures to leverage both physical and protocol optimizations for improved power, performance, and cost characteristics (Sharma et al., 7 Oct 2025). By integrating DRAM using UCIe over either a logic die (which translates UCIe to LPDDR6 or HBM protocols) or direct DRAM die attachment with native UCIe PHY, memory semantics become tightly coupled with the point-to-point, serial protocol. This yields:

• Bandwidth density improvements: up to 10× over traditional HBM4/LPDDR, via serial, unidirectional, and high-density lane design.
• Latency reduction: up to 3× lower latency, as serialization/deserialization and clock-domain crossing penalties are minimized.
• Power reduction: up to 3× improved efficiency, achieved through rapid power gating and selective lane usage.
• Direct cost savings: reuse of existing DRAM technology without extensive redesign of the memory device.

Both approaches (logic die adapter and direct DRAM die interface with UCIe) enable fine-grained protocol mapping (including symmetric/asymmetric lane configuration for workload adaptation) and frequency synchronization, supporting high-throughput AI/HPC and low-cost mobile platforms.

3. Memory Model Validation and Verification Tools

To ensure the correctness and safety of UCIe memory semantics, operational models are equipped with simulator backends that exhaustively enumerate reachable relaxed configurations (Boudol et al., 2012). These simulators support litmus tests (e.g., IRIW, WRC) ported to the UCIe context, providing empirical validation that the implemented hardware-software system enforces the intended memory ordering constraints and properly implements fences/barriers. Techniques such as state merging and directed acyclic graph modeling enable exhaustive exploration of the state space.

This rigorous simulation is complemented by the formal mechanization of memory models in proof assistants such as Coq (Krebbers, 2015), establishing correctness of program transformations and foundational safety properties under the UCIe memory model. Separation logic with rich permission systems further enables modular reasoning about disjoint memory partitions, crucial for software verification on compositional UCIe systems.

4. Composition, Optimization, and Compiler Correctness

Memory semantics in UCIe must be compatible with a wide range of compiler optimizations and language-level transformations. Recent theoretical advances (Gopalakrishnan et al., 18 Sep 2024) define a methodology in which program optimizations are decomposed into elementary effects on execution traces (e.g., adjacent event reorderings), and safety is analyzed under both strict (SC) and relaxed (e.g., $SC_{RR}$, TSO) consistency models.

The compositional property (termed "Complete") ensures that if an optimization is safe under SC, it remains safe under the derived, weaker model if all elementary effects retain safety. This is critical for UCIe, where the hardware-level protocol may allow reorderings (e.g., via the temporary store and commutability relation) that the software must reliably reason about. The methodology:

• Characterizes pre-traces and candidate executions in terms of key relations (program order, reads-from, modification order).
• Relates compiler optimizations directly to permissible local reordering in the presence of hardware-level chiplet buffering.
• Ensures that verification and program transformation pipelines preserve correctness under the UCIe-consistent behaviors.

5. Conflict-Aware and Denotational Approaches for True Concurrency

Classic interleaving semantics are insufficient to handle the full spectrum of behaviors in modern UCIe systems, especially under weak or relaxed memory models. Conflict-aware true-concurrency frameworks (Narayanaswamy et al., 2016) and denotational approaches using pomsets (partially ordered multisets) (Kavanagh et al., 2018) more accurately capture the concurrency patterns that arise from chiplet-to-chiplet interactions, including data/control-flow branching and explicit conflicts.

Key features include:

• Representation of programs as event structures ($$\Gamma = \langle E, \mathcal{C}, \vdash, \lambda \rangle$$) or pomsets $(P, <, \Phi)$.
• Symbolic encoding of execution using event structures allows scalable assertion checking for software running on UCIe.
• Detection of assertion violations and data races in bounded and unbounded multi-threaded scenarios, with formal support for synchronization primitives (barriers, fences) as first-class semantic elements.

This formal grounding enables scalable verification and compositional reasoning on UCIe-based architectures where the standard sequential consistency assumption is too restrictive or not supported in hardware.

6. Liveness, Fairness, and System-Wide Verification

For UCIe architectures which employ relaxed memory consistency, verifying liveness properties under fairness constraints is essential. Recent work (Abdulla et al., 2023) provides a generic operational model for weak memory, introducing message-based propagation with per-variable, per-process channels and frontiers tracking the set of visible writes. Verification of omega-regular linear temporal (liveness) properties is reduced to safety problems by enforcing size-bounded and repeatedly plain execution conditions, which align with practical limitations on buffer sizes and hardware fairness.

This treatment ensures that demonic nondeterminism from scheduling, message propagation, or chiplet interleaving does not cause spurious violations of progress properties. The framework generalizes naturally to UCIe, where buffer propagation and inter-chiplet data visibility must be bounded and fair to guarantee system-wide correctness.

7. Practical Applications and Impact

UCIe with memory semantics delivers a unifying infrastructure for a broad spectrum of applications:

• AI/HPC accelerators—where high bandwidth density, reduced latency, and relaxed memory ordering enable scalable dataflow with correctness guarantees.
• Data center/server platforms—leveraging cost-effective on-package memory with formal guarantees on memory operation ordering and safety.
• Mobile and heterogeneous platforms—utilizing direct DRAM–UCIe connections for low power and cost, supported by verification of memory model correctness.
• Software-hardware codesign—enabling verified program transformations, compiler optimizations, and robust firmware that maintain correctness over hardware-induced reordering and early visibility semantics.

The convergence of formal memory models, scalable simulation and verification frameworks, and efficient hardware integration mechanisms under the UCIe umbrella provides a rigorous foundation for future chiplet-centric compute fabrics.