Hi I am not enough familiar with the P89C51RD2 though as I know from other processors/OS the common way to use a HW interrupt is define the interrupt and set its address to the vector table. The connection of the interrupt table to the specific unit is a hardware dependent for example interrupt 1 is timer0 and so on. is trhis correct ? or there is a possible to define specificaly that timer0 will use for example, int 5 ? can you send a code example for timer0/1/2? and were it is written in the data sheet ? Regards Eyal