It is unrealistic to assume when designing microprocessor systems that programs and interface connections can be developed on paper and will then always perform as intended. Almost invariably there will be initial errors which can be discovered only by running the system and monitoring its behaviour. This is particularly the case with programs for high-volume products which are held in mask ROM. The mask charge may be several thousand pounds, and the slightest error will require a new mask and another charge for designing and making it.