Mode Switching in GIA - An Ada based Real-Time Framework

This article is based on an Ada framework for multi-moded real-time applications called GIA (Giotto in Ada). As the name indicates it is based on the Giotto programming model, developed at the University of Berkeley. Within the GIA project original Giotto has been hosted in Ada, strictly speaking SPARK a safe subset of Ada, and has subsequently been extended. One absence of Giotto is its protocol for mode switching (transition from a source/old mode to a target/new mode). The original protocol is consistent in terms of shared resources, ensures periodicity of tasks during the transition and can be statically analysed. Its drawbacks are poor promptness and over-constrained semantics. This paper presents improvements to increase promptness of mode switching by keeping consistency and schedulability, without increasing run-time costs. Therefore the main starting points are the alleviation of constraints for active tasks during mode switches as well as the introduction of deadlines in the tasking model, which are not implemented in Giotto. Furthermore Ada’s features for asynchronous transfer of control are used to increase promptness without breaking suppositions for static analyses.