I add a new interrupt handler for timer 3 to my program: void Timer3_ISR(void) interrupt 14 { } I compile for C8051F020, using C51 v7.50 and LX51 v3.65b After compilation, the linker display: linking... *** WARNING L30: MEMORY SPACE OVERLAP FROM: C:001073H TO: C:001075H *** WARNING L30: MEMORY SPACE OVERLAP FROM: C:001075H TO: C:00107AH Remarks: I don't use interrupt 14 twice.