This invention relates to a single chip microcomputer comprising a program memory which stores a program consisting of a succession of instructions and to a method of debugging a program stored in such a single chip microcomputer by combining the microcomputer with an external device.
In general, a recent remarkable development of a semiconductor integration technique has enabled the advent of the single chip microcomputer. The single chip microcomputer has a single semiconductor chip comprising a central processing unit (CPU), a program memory storing a program, a random access memory (RAM), and the like and has been widely used in various fields. The program memory is formed by a read-only memory (ROM) in the single chip microcomputer.
Inasmuch as the program is stored in the read-only memory in the single chip microcomputer, it is difficult to voluntarily change the program stored in the program memory and to develop a new program by the use of the same single chip microcomputer. An evaluation chip which comprises no program memory, has therefore been used to develop a new program. Such an evaluation chip is operated in response to the new program supplied from an external device.
It is to be noted here that an extremely small number of evaluation chips are manufactured in comparison with single chip microcomputers operable in accordance with the new program developed by the use of the evaluation chips despite the fact that as much labor and time should be expended in manufacturing the evaluation chips as in manufacturing the single chip microcomputers. This is extremely inconvenient for a chip manufacturer.
Various requirements have often been offered to the single chip microcomputers by users or customers after completion of the evaluation chips. Under the circumstances, each evaluation chip itself should be modified in order to satisfy such requirements. Thus, it is difficult to easily respond to a technical development and users' requirements when an evaluation chip is used to develop a new program.
A specific one of the conventional single chip microcomputers is selectively operable in accordance with an internal program stored in the program memory and an external program supplied from an external device. For convenience of description, operation based on the internal and the external programs will be called a normal and a debug mode, respectively. With such a specific single chip microcomputer, it is unnecessary to individually develop an evaluation chip.
It is to be mentioned here that the specific single chip microcomputer inevitably comprises an external instruction terminal set for the external program and an indication terminal for indicating whether either the internal or the external program is used. The external instruction terminal set is used only in the debug mode and become unnecessary in the normal mode once the new program is completed and stored into the program memory. On the other hand, the number of terminals is subjected to a strict restriction in a single chip microcomputer. Therefore, the specific single chip microcomputer is insufficient in availability to terminals in the normal mode because of presence of extra terminals, such as the external instruction terminal set.