A combined-hardware-and-software secure-platform interface to which operating systems and customized control programs interface within a computer system. The combined-hardware-and-software secure-platform interface employs a hardware platform that provides at least four privilege levels, non-privileged instructions, non-privileged registers, privileged instructions, privileged registers, and firmware interfaces. The combined-hardware-and-software secure-platform interface conceals all privileged instructions, privileged registers, and firmware interfaces and privileged registers from direct access by operating systems and custom control programs, providing to the operating systems and custom control programs the non-privileged instructions and non-privileged registers provided by the hardware platform as well as a set of callable software services. The callable services provide a set of secure-platform management services for operational control of hardware resources that neither exposes privileged instructions, privileged registers, nor firmware interfaces of the hardware nor simulates privileged instructions and privileged registers. The callable services also provide a set of security-management services that employ internally generated secret data, each compartmentalized security-management service managing internal secret data without exposing the internal secret data to computational entities other than the security-management service itself.