METHODS AND APPARATUS FOR PROVIDING A USER ORIENTED MICROPROCESSOR TEST INTERFACE FOR A COMPLEX, SINGLE CHIP, GENERAL PURPOSE CENTRAL PROCESSING UNIT

Methods and apparatus are disclosed that facilitate the testing and development of computer systems that include at least one single chip microprocessor. In particular, a parallel test interface is described that allows an external test unit to (1) directly load instructions into the microprocessor under test utilizing the existing bus structure of the computer system; (2) step the processor through preselected test instruction sequences; (3) monitor processor states in both the processor's test and normal execution modes; and (4) halt and resume normal instruction processing. According to the invention, the microprocessor test interface comprises a plurality of dedicated CPU status output pins and a plurality of dedicated CPU control input pins, used by the test unit in combination with the existing bus structure of the computer system to provide the desired test facility for the single chip microprocessor. The preferred embodiment of the invention is realized in a RISC environment where the instruction lengths are fixed and the instruction processor has a single cycle execution time. Such an embodiment facilitates the direct insertion of instructions by the tester into the processor for decoding, without having to queue instructions or pass through complicated intervening hardware or test logic.