I'm trying to grasp the functionality of tmr0 interrupt.

I have written a basic tmr0 interrupt based c code. (I used hi-tech picc
compiler in my testings)

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <htc.h>

__CONFIG(XT & WDTDIS);

void interrupt tmr0_isr (void) {
    RB0 ^= 1;
    T0IF = 0;
    TMR0 = 255;
}

void main (void) {

    PSA = 1;            // prescalar assigned to the WDT
    T0CS = 0;            // select internal clock
    T0IE = 1;            // enable timer interrupt
    GIE = 1;            // enable global interrupts

    TRISB0 = 0;
    RB0 = 0;

    TMR0 = 255;       // Load the tmr0 for the minimum interrupt rate

    while(1);

}

// All the calculations are based on 4 MHZ XT.
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Above i would expecting the toggle rate of RB0 is about 1 us  + 2us (synch
cycles) + a few more for interrupt latency which is totally about 5 us. But
when i tested the rate with MPLAB stopwatch i get an interrupt at each 18us.
So the tmr0_init_value = 256 - (delay_time * clock_freq/4) dont work with my
tmr0 calculations. (assumed no prescaler).

So where am i misunderstanding, is this the problem of c compiler or me?
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