Field device for use in a process control system having a communications infrastructure, said field device comprising: a processor; and a nonvolatile memory configured to store embedded firmware and to further store application specific code downloaded thereto via the communications infrastructure, said memory configured to store the application specific code separately from the embedded firmware, said embedded firmware including computer-executable instructions that, when executed by the processor: perform at least one of a fixed behavior and a measurement to implement a predetermined function of the field device; and call a runtime interpreter, said runtime interpreter executing the application specific code to implement a custom function of the field device; wherein the processor is configured to repeatedly execute a processing cycle, the processing cycle including a standard portion during which the processor executes operations to perform the at least one fixed behavior or measurement, the processing cycle further including an interpreter portion following the standard portion of the processing cycle during which the processor calls the runtime interpreter to execute the application specific code without affecting the standard portion of the processing cycle, the interpreter portion of the processing cycle having a predefined maximum duration during the processing cycle; wherein the processor is configured to complete processing to perform the at least one fixed behavior or measurement within the standard portion of each processing cycle before calling the runtime interpreter to execute the application specific code within the interpreter portion of the processing cycle; wherein the runtime interpreter is configured to begin executing the application specific code during the interpreter portion of the processing cycle after the processor completes processing during the standard portion of the processing cycle to perform the at least one fixed behavior or measurement; wherein the processor is configured to pause the runtime interpreter from executing the application specific code when the predefined maximum duration of the interpreter portion of the processing cycle has lapsed before completing execution of the downloaded application specific code; and wherein the processor is configured to complete processing to perform the at least one fixed behavior or measurement within the standard portion of a subsequent processing cycle after the runtime interpreter was paused and further configured to resume the runtime interpreter for continued execution of the downloaded application specific code during the interpreter portion of the subsequent processing cycle after the runtime interpreter was paused. a processor; and a nonvolatile memory configured to store embedded firmware and to further store application specific code downloaded thereto via the communications infrastructure, said memory configured to store the application specific code separately from the embedded firmware, said embedded firmware including computer-executable instructions that, when executed by the processor: perform at least one of a fixed behavior and a measurement to implement a predetermined function of the field device; and call a runtime interpreter, said runtime interpreter executing the application specific code to implement a custom function of the field device; perform at least one of a fixed behavior and a measurement to implement a predetermined function of the field device; and call a runtime interpreter, said runtime interpreter executing the application specific code to implement a custom function of the field device; wherein the processor is configured to repeatedly execute a processing cycle, the processing cycle including a standard portion during which the processor executes operations to perform the at least one fixed behavior or measurement, the processing cycle further including an interpreter portion following the standard portion of the processing cycle during which the processor calls the runtime interpreter to execute the application specific code without affecting the standard portion of the processing cycle, the interpreter portion of the processing cycle having a predefined maximum duration during the processing cycle; wherein the processor is configured to complete processing to perform the at least one fixed behavior or measurement within the standard portion of each processing cycle before calling the runtime interpreter to execute the application specific code within the interpreter portion of the processing cycle; wherein the runtime interpreter is configured to begin executing the application specific code during the interpreter portion of the processing cycle after the processor completes processing during the standard portion of the processing cycle to perform the at least one fixed behavior or measurement; wherein the processor is configured to pause the runtime interpreter from executing the application specific code when the predefined maximum duration of the interpreter portion of the processing cycle has lapsed before completing execution of the downloaded application specific code; and wherein the processor is configured to complete processing to perform the at least one fixed behavior or measurement within the standard portion of a subsequent processing cycle after the runtime interpreter was paused and further configured to resume the runtime interpreter for continued execution of the downloaded application specific code during the interpreter portion of the subsequent processing cycle after the runtime interpreter was paused.