Study the traffic example bout RTX-51 and interrupt. I was so confused. Since I can not find any file called the function os_enable_isr(). And the serial file use the standard C51 interrupt function. and in the file, the serial prot initialization like: void serial_init (void) { SCON = 0x50; /* mode 1: 8-bit UART, enable receiver */ TMOD |= 0x20; /* timer 1 mode 2: 8-Bit reload */ TH1 = 0xf3; /* reload value 2400 baud */ TR1 = 1; /* timer 1 run */ ES = 1; /* enable serial port interrupt */ } it use ES=1 to enable interrupt!!!!!!!!!!!!! and according to the Keil RTX-51/251 User's Guide, on page 24, chapter 3 "Handling of the 8051/MCS 251 Interrupt Enable Register", it said that "The INTERRUPT ENABLE registers of the 8051/MCS 251 are managed by RTX-51/251 and must not be directly manipulated by user!". But if I use os_enable_isr(4) to enable serial port0 interrupt, it always return failure. What's the matter?