In Reconfigurable Systems-On-Chip (RSoCs), operating systems can primarily (1) manage the sharing of limited reconfigurable resources, and (2) support communication between reconfigurable accelerators and user applications. It has been shown in previous work that the operating system can dramatically simplify the interface to reconfigurable coprocessors and isolate the programmer from all details of the hardware. A further potential of the operating system is developed here: the operating system can observe accelerators at runtime and dynamically take actions which improve their execution. The strength of involving the operating system consists in achieving better performance without any information from the end user and without changes either in the coprocessor hardware design or in the software application. Specifically, this paper presents an operating system module that monitors reconfigurable coprocessors, predicts their future memory accesses, and performs memory prefetching accordingly; the goal is to hide completely memory-to-memory communication latency. The module uses a lightweight hardware support to detect coprocessors memory access patterns. The effectiveness of the technique is demonstrated for two applications on an embedded RSoC board running the Linux operating system. Significant speedup is achieved compared to the nonprefetching version, and the improvement is obtained in a manner completely transparent to the application programmer.