1. A non-transitory computer-readable medium storing a program that causes a host central processing unit to execute a process for a native code simulation in a host operating system, the process comprising: in response to starting each of a plurality of tasks executed in a target operating system with multitasking, creating a stack region in the host central processing unit, a register preserving region in the target operating system, and a task state storing region in the target operating system, the native code simulation being a simulation of the target operating system and the plurality of tasks, the stack region storing function call information for each of the plurality of tasks with a last in first out structure in the order of functions to be called sequentially by each of the plurality of tasks, the register preserving region storing most updated register information for each of the plurality of tasks, the most updated register information being a register contents at the last interruption of execution of each of the plurality of tasks, the task state storing region storing a state information for each of the plurality of tasks, the state information showing a state of each of the plurality of tasks; in response to a transition of a state of any of the plurality of tasks from an execution state to a standby state or from the standby state to the execution state by occurrence of an interrupt or a directive for an interrupt, storing in the register preserving region the most updated register information which is a register contents of a time that a task in the execution state calls a function before the last time that the task calls a function, and also storing in the task state storing region a state information showing a state of the task in the execution state; and in response to the transition, switching from the task in the execution state to a task in the standby state of the plurality of tasks by storing in the stack region the most updated register information in the task in the standby state to be executed next, wherein the storing of the most updated register information stores in the register preserving region a value stored in a predetermined register in the host central processing unit as the most updated register information of the task in the execution state, and the predetermined register is provided for each of the plurality of tasks and wherein the switching switches the task in the execution state, by storing in the predetermined register the most updated register information of the task in the standby state.