A method of a fast system call is provided. First, a logical operation to compute a kernel service routine is used. Then the logical operation result is compared with ciphertext from a key register. At least one input for the logical operation is from the relevant information of the required kernel service routine. For example, the start address of the kernel service routine or the content of the start address of the kernel service routine, or combinations thereof. If the logical operation result equals to the ciphertext of the key register, a switch from a user mode to a kernel mode to read the kernel service routine is allowed. Otherwise, the central processing system executes a corresponding exceptional handler routine. Then the operating system terminates the mode switch request and reports an error to the operating system.