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Extending the idea of preemptive multitasking to DPRS (Dynamic Partial Reconfiguration Systems) has farreaching implications as many mechanisms supporting the concept, such as context saving and restoring, have to be built practically from scratch. This paper addresses previously neglected issues, related to design of effective preemption mechanisms for Flip-Flop-based and RAM-based hardware tasks. Furthermore, a very efficient and complete solution to hardware task preemption for Virtex4-based DPRS is presented featuring in bitstream manipulation tool intended for PC and embedded system infrastructure with a DMA-based, instructiondriven reconfiguration/readback controller. Taking advantage of the developed lightweight bus, enhancing management of reconfigurable hardware modules, controller takes care of all essential hardware aspects related to context-switching thereby reducing CPU utilization to necessary minimum.

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Dynamic Partial Reconfiguration technology coupled with an Operating System for Reconfigurable Systems (OS4RS) allows for implementation of a hardware task concept, that is, an active computing object which can contend for reconfigurable computing resources and request OS services in a way software task does in a conventional OS. In this work, we show a complete model and implementation of a lightweight OS4RS supporting preemptable and clock-scalable hardware tasks. We also propose a novel, lightweight scheduling mechanism allowing for timely and priority-based reservation of reconfigurable resources, which aims at usage of preemption only at the time it brings benefits to the performance of a system. The architecture of the scheduler and the way it schedules allocations of the hardware tasks result in shorter latency of system calls, thereby reducing the overall OS overhead. Finally, we present a novel model and implementation of a channel-based intertask communication and synchronization suitable for software-hardware multitasking with preemptable and clock-scalable hardware tasks. It allows for optimizations of the communication on per task basis and utilizes point-to-point message passing rather than shared-memory communication, whenever it is possible. Extensive overhead tests of the OS4RS services as well as application speedup tests show efficiency of our approach.

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