Hi I use mon51 debug code. mon51 stay in code 0x0000-0x1fff.my code stay in code 0x2000-0xbfff. stack pointer start is 0x80. And when i call some fucntion, the stack pointer is 0x87,when call memcpy, i found that content of 0x86 was changed, It mean that I cannot return the correct address when I finished call this function. Why wil this happend? Thanks. Ding