Design of a Self-Checking Microprogram Control

In designing a self-checking processor, it is essential to recognize the types of failures that are most probable. Matching the checking techniques with the type of faults that are expected to occur should yield the best result with the least amount of hardware. The microprogram control will consist of integrated circuits: large-scale integration (LSI) for the memory and small-scale integration (SSI) for the associated control logic. Because of the density of chips on a plug-in package and the physical proximity of the devices on an integrated circuit, multiple faults within a single circuit are highly probable. The types of faults within a circuit have been analyzed and found to be of the type which would tend to affect the bits in a unidirectional manner. Also the failed bits would probably be adjacent rather than randomly dispersed throughout-the microprogram store word. The checking technique implemented takes advantage of the error characteristics mentioned above. This led to the choice of using m-out-of-n codes for the control fields of the microprogram instruction words. This code permits the detection of all multiple unidirectional errors. However, for the address field, it is desirable to maintain the data in binary form. Consequently, a check code which is systematic is essential to give this flexibility. By recognizing that the multiple bit faults tend to affect adjacent bits, we can take advantage of this fact by interleaving the binary address field with the m-out-of-n codes in the microprogram store. Any multiple adjacent bit faults would then affect both the binary address field and the m-out-of-n code. Therefore, a single parity check bit is adequate to detect single bit faults in the binary field with multiple adjacent bit fault detected by the m-out-of-n check.