Hardware Support for Safety-Critical Java Scope Checks

Memory management in Safety-Critical Java (SCJ) is based on time bounded, non garbage collected scoped memory regions used to store temporary objects. Scoped memory regions may have different life times during the execution of a program and hence, to avoid leaving dangling pointers, it is necessary to check that reference assignments are performed only from objects in shorter lived scopes to objects in longer lived scopes (or between objects in the same scoped memory area). SCJ offers, compared to the RTSJ, a simplified memory model where only the immortal and mission memory scoped areas are shared between threads and any other scoped region is thread private. In this paper we present how, due to this simplified model, a single scope nesting level can be used to check the legality of every reference assignment. We also show that with simple hardware extensions a processor can see some improvement in terms of execution time for applications where cross-scope references are frequent. Our proposal was implemented and tested on the Java Optimized Processor (JOP).