Look at my codes (not all), the stepper must move one step at each interrupt.

///////////////////////////////////////////////////////////
uint8 t = 0;

main()
{
Initialization();

Timer0Init();

while(1)
{
if(t > 1) // of course I freeze the Timer before reading
{
// we missed (t-1) steps, wrong!
}
else if (t == 1)
{
t = 0;
//move one step;
}

// RS232 processing

// other stuff..
}
//should not be here!
}

Timer0ISR
{
t++; // <- Very simple ISR, isn't it?
}

////////////////////////////////////////////////////////
sometimes I got missed one step error! That's why I want to check the CPU usage.

RS232 is not critical, the master can repeat the last mssage if it does not get a response within a preset time period.

I know where I should set the pin 1 and clear it. Thank you Russ.