Abstract: Hybrid computing systems, consisting of a CPU server coupled with a Field-Programmable Gate Array (FPGA) for application acceleration, are today a common facility in datacenters and clouds. FPGAs can deliver tremendous improvements in performance and energy efficiency for a range or workloads, but development and deployment of FPGA-based applications remains cumbersome, leading to recent work which replicates subsets of the traditional OS execution environment (virtual memory, processes, etc.) on the FPGA. In this paper we ask a different question: to what extent do traditional OS abstractions make sense in the context of an FPGA as part of a hybrid system, particularly when taken as a , as they would be in an OS? To answer this, we built and evaluated Coyote, an open source, portable, configurable "shell"' for FPGAs which provides a full suite of OS abstractions, working with the host OS. Coyote supports secure spatial and temporal multiplexing of the FPGA between tenants, virtual memory, communication, and memory management inside a uniform execution environment. The overhead of Coyote is small and the performance benefit is significant, but more importantly it allows us to reflect on whether importing OS abstractions wholesale to FPGAs is the best way forward.