PCI-Express: Evolution of a Ubiquitous Load-Store Interconnect Over Two Decades and the Path Forward for the Next Two Decades

The Peripheral Component Interconnect Express (PCI-Express or PCle) architecture has been the ubiquitous backbone interconnect in the evolving computing landscape for more than two decades. This paper delves into the multiple innovations driving the backward-compatible evolution of PCle for seven generations, doubling bandwidth every generation, while delivering power-efficient and cost-effective performance. Compute Express Link (CXL) overlays coherency and memory protocols on top of PCle for heterogeneous computing, addressing the memory wall, resource pooling and sharing across servers, and distributed computing using load-store based messaging. The die-todie industry-standard, Universal Chiplet Interconnect Express $^{TM}$ (UCle), offers orders of magnitude improvement in bandwidth density, power efficiency, and latency for PCle and CXL protocols on-package and pod-level connectivity with co-packaged optics. We foresee PCle will continue to evolve over the next few decades to serve future computing needs. It will do so by embracing alternate media while backward-compatible frequency scaling with copper will continue and protocol enhancements will enable non-tree fabric topologies.