A Dynamic Memory Management Unit for Real Time Systems

Time predictability is a first class requirement in safety critical system design. Techniques exist for the timing analysis of programs designed in memory managed languages, but these require detailed knowledge of memory allocation. Moreover, enforcing hard real-time guarantees for systems designed in such garbage collected languages is difficult, because of the so called collection pause - however incremental. This paper proposes a novel solution whereby a separate reference counting unit replaces the garbage collector. We present the underlying concepts of the Reference Counting Memory Management Unit (RCMMU) and its use for garbage collection. In addition, we present an implementation that includes a specialized memory arrangement that allows for object management transactions to occur concurrently with program execution, with no resultant impact upon program performance. The RCMMU has been targeted for use with Java but can be adopted to be used with other memory managed languages. The hardware-implemented RCMMU removes all overhead associated with garbage collection operations when compared against a software-only collector. Additionally, it simplifies the static worst-case execution time analysis of programs.