Exceptions in low-level architectures are implemented as synchronous interrupts: upon the execution of a faulty instruction the processor jumps to a piece of code that handles the error. Previous work has shown that assembly programs can be written, verified and run using higher-order separation logic. However, execution of faulty instructions is then specified as either being undefined or terminating with an error. In this paper, we study synchronous interrupts and show their usefulness by implementing a memory allocator. This shows that it is indeed possible to write positive specifications of programs that fault. All of our results are mechanised in the interactive proof assistant Coq
