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Exception handling is a well known mechanism to react on errors or exceptional events during the runtime of a program. In this paper we discuss the reliability and efficiency demands that real-time systems make upon exception handling. The reliability requirement means that no runtime error should occur due to the wrong use of the exception handling commands in a program. We use static type checking to guarantee the correct typing of the exception handling constructs. Data flow analysis is required to check whether an appropriate handler exists for each exception. The efficiency requirement means that the execution of all statements being involved in exception handling should produce as little runtime overhead as possible. We describe how the two most time-critical steps in exception handling, the determination of the associated handler and the scheduling, can be implemented efficiently by using specific runtime data structures and by making use of data flow analysis.

The sequence motif 5′ TYTTCACATGY 3′ functions as an upstream activation site common to both yeast fatty acid synthase genes, FAS1 and FAS2. In addition, this UASFAS element is shared by all so far characterized genes of yeast phospholipid biosynthesis. We have investigated the influence of a functional INO4 gene previously described as a regulator of inositol biosynthesis on the expression of FAS1 and FAS2. In a δINO4 null allele strain, both genes are expressed at only 50% of wild type level. Using individual UASFAS sequence motifs inserted into a heterologous test system, a drastic decrease of reporter gene expression to 2 -10% of the wild type reference was observed in the AINO4 mutant. In gel retardation assays, the protein-DNA complex involving the previously described FAS binding factor 1, Fbf 1, was absent when using a protein extract from δINO4 mutant. On the other hand, this signal was enhanced with an extract from cells grown under conditions of inositol/choline derepression. Subsequent experiments demonstrated that INO4 expression is itself affected by phospholipid precursors, mediated by an UASFAS element in the INO4 upstream region. Thus, in addition of being an activator of phospholipid biosynthetic genes, INO4 is also subject to a positive autoregulatory loop in its own biosynthesis.