In order to simplify the application program,the Real Time Operating System(RTOS) is utilized in the embedded system development.This paper proposes the method of porting open source RTOS Toppers/asp onto ARM7(S3C44B0),which is a popular microcontroller dedicated to embedded system.It solves the key problems in the startup and loading of system,and presents the most important and difficult problems in the course of porting Toppers/asp.Multitask running result shows that the system is steady and reliable.