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Modern FPGA devices, which include (multiple) processor core(s) as diffused IP on the silicon die, provide an excellent platform for developing custom multiprocessor systems-on-programmable chip (MPSoPC) architectures. As researchers are investigating new methods for migrating portions of applications into custom hardware circuits, it is also critical to develop new run-time service frameworks to support these capabilities. Hthreads (HybridThreads) is a multithreaded RTOS kernel for hybrid FPGA/CPU systems designed to meet this new growing need. A key capability of hthreads is the migration of thread management, synchronization primitives, and run-time scheduling services for both hardware and software threads into hardware. This paper describes the hthreads scheduler, a key component for controlling both software-resident threads (SW threads) and threads implemented in programmable logic (HW threads). Run-time analysis shows that the hthreads scheduler module helps in reducing unwanted system overhead and jitter when compared to historical software schedulers, while fielding scheduling requests from both hardware and software threads in parallel with application execution. Run time analysis shows the scheduler achieves constant time scheduling for up to 256 active threads with a total of 128 different priority levels, while using uniform APIs for threads requesting OS services from either side of the hardware/software boundary.

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In this paper, we present hthreads, a unifying programming model for specifying application threads running within a hybrid CPU/FPGA system. Threads are specified from a single pthreads multithreaded application program and compiled to run on the CPU or synthesized to run on the FPGA. The hthreads system, in general, is unique within the reconfigurable computing community as it abstracts the CPU/FPGA components into a unified custom threaded multiprocessor architecture platform. To support the abstraction of the CPU/FPGA component boundary, we have created the hardware thread interface (HWTI) component that frees the designer from having to specify and embed platform specific instructions to form customized hardware/software interactions. Instead, the hardware thread interface supports the generalized pthreads API semantics, and allows passing of abstract data types between hardware and software threads. Thus the hardware thread interface provides an abstract, platform independent compilation target that enables thread and instruction-level parallelism across the software/hardware boundary.