when i use 89C51 i calculate every line's time of the source code of any pgrograme by addong all the duty cycle of every line and after adding i multiply all of them by 1.08 Us(i was using 11.0 crystal)
i want that value or could i use the same 1.08 for PIC controller with 20 Mhz crystal ?
Basically yes, but remember that the '877 executes one instruction per 4 clock cycles. A few instructions may require two machine cycles (8 clock cycles). The data sheet will tell you how many cycles are needed and under which circumstances the extra machine cycle is needed.