A computing system provides normal security services and high security services with a single operating system, comprising: a thread ordering service, scheduling resources of the computing system to normal threads (NT1 to NTn) and a pseudo normal thread (any of PNT1-PNTm), wherein the normal threads and the pseudo normal thread are limited to a normal security environment that the computing system builds for the normal security services while a secure thread (selected from ST1-STm) is limited to a high security environment that the computing system builds for the high security services, the normal threads are used in executing the normal security services, the security thread relates to one of the high security services, and the pseudo normal thread is a temporary of the secure thread to compete with the normal threads in the thread ordering service to gain access to the resources through scheduling, and when any of the pseudo normal thread gains access to the resources a switch to the secure state is performed to execute the corresponding secure thread; a secure application programming interface (secure API, 112) for the high security services, called by the pseudo normal thread that gains access to the resources according to the thread ordering service;; a driver layer invoked by the secure API, calling a world switch instruction (SMC 504 of Figs. 5 and 6); and a monitor, activated by the world switch instruction from the driver layer to save context of the pseudo normal thread, change the computing system to the high security environment, obtain the secure thread corresponding to the pseudo normal thread, create or restore context of the secure thread, and dispatch the secure thread to be executed by the computing system that has been changed to the high security environment. a thread ordering service, scheduling resources of the computing system to normal threads (NT1 to NTn) and a pseudo normal thread (any of PNT1-PNTm), wherein the normal threads and the pseudo normal thread are limited to a normal security environment that the computing system builds for the normal security services while a secure thread (selected from ST1-STm) is limited to a high security environment that the computing system builds for the high security services, the normal threads are used in executing the normal security services, the security thread relates to one of the high security services, and the pseudo normal thread is a temporary of the secure thread to compete with the normal threads in the thread ordering service to gain access to the resources through scheduling, and when any of the pseudo normal thread gains access to the resources a switch to the secure state is performed to execute the corresponding secure thread; a secure application programming interface (secure API, 112) for the high security services, called by the pseudo normal thread that gains access to the resources according to the thread ordering service;; a driver layer invoked by the secure API, calling a world switch instruction (SMC 504 of Figs. 5 and 6); and a monitor, activated by the world switch instruction from the driver layer to save context of the pseudo normal thread, change the computing system to the high security environment, obtain the secure thread corresponding to the pseudo normal thread, create or restore context of the secure thread, and dispatch the secure thread to be executed by the computing system that has been changed to the high security environment.