Abstract: Multiple applications upon an IC microprocessor are protected with bi-modal CPU operation, either application or system mode, using an operation flag determining the mode and dependent upon a mode change interrupt which clears all working memory unnecessary to operation in the next mode. Access authorization setting program and data memory boundaries according to the particular custom command in comparison registers is utilized in application initialization. From application mode, data files are accessed only through a system subroutine. 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 unnecessary to forward a status word indicating abnormal termination. Application completion forwards the result with a status word indicating successful completion.

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.