While we strive to create robust language constructs and design patterns which prevent the introduction of faults during software development, an inevitable element of human error still remains. We must therefore endeavor to ease and accelerate the process of diagnosing and fixing software faults, commonly known as 'debugging'. Current support for debugging occam-pi programs is fairly limited. At best the developer is presented with a reference to the last known code line executed before their program abnormally terminated. This assumes the program does in fact terminate, and does not instead live-lock. In cases where this support is not sufficient, developers must instrument their own tracing support, ``printf style''. An exercise which typically enlightens one as to the true meaning of concurrency... In this paper we explore previous work in the field of debugging occam programs and introduce a new method for run-time monitoring of occam-pi applications, based on the Transterpreter virtual machine interpreter. By adding a set of extensions to the Transterpreter, we give occam-pi processes the ability to interact with their execution environment. Use of a virtual machine allows us to expose program execution state which would otherwise require non-portable or specialised hardware support. Using a model which bears similarities to that applied when debugging embedded systems with a JTAG connection, we describe debugging occam-pi by mediating the execution of one execution process from another.
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