The tremendous expansion of data volume has driven the transition from monolithic architectures towards systems integrated with discrete and distributed subcomponents in modern scalable high performance computing (HPC) systems. As such, multi-layered software infrastructures have become essential to bridge the gap between heterogeneous commodity devices. However, operations across synthesized components with divergent interfaces inevitably lead to redundant software footprints and undesired latency. Therefore, a scalable and unified computing platform, capable of supporting efficient interactions between individual components, is desirable for largescale data-intensive applications. In this work, we introduce the Extended Base Global Address Space, or xBGAS, microarchitecture extension to the RISC-V instruction set architecture (ISA) for scalable high performance computing. The xBGAS extension provides native ISA-level support for direct accesses to remote shared memory by mapping remote data objects into a system's extended address space. We perform both software and hardware evaluations of the xBGAS design. The results show that xBGAS reduces instruction count generated by interprocess communication by 69.26% on average. Overall, xBGAS achieves an average performance gain of 21.96% (up to 37.29%) across the tested workloads.