Importance

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Abstract

Multiple applications upon an IC microprocessor are protected with bi-modal CPU operation, either in application mode or system mode, using an operation flag determining the mode and a functional interrupt with each mode change. Direct subroutine calling is replaced by a software interrupt which clears all working memory and registers except those holding parameters including the return address placed in stack. Access authorization utilizing a comparison register containing application and system memory boundaries according to the particular custom command utilized is associated with a mode change interrupt in application initialization. System subroutine running involves two mode change interrupts, from application to system and back, and includes access authorization. Request of an address beyond the territory assigned to the custom command utilized results in a hardware interrupt which clears all working memory and registers in the system mode and except that necessary to forward a status word indicating abnormal termination. Application completion forwards the result with a status word indicating successful completion. Hard-wired bi-modal CPU operation utilizing a mode change interrupt which saves parameters in stack but clears all other registers and working memory and utilizes a comparison register to authorize the system address prior to return to application mode with the return address held in stack ensures that operation in the application authorized by the custom command utilized will result either in successful completion or abnormal termination without possible access of any other application contained upon the IC card.

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First Claim