gistfile1.c

// include the XC and PIC18 libraries
#include <xc.h>
#include <pic18.h>

#pragma config FOSC=HS1

// number of milliseconds that the device has been on
unsigned long int milliseconds;

//  main loop of the program
void main() {
    // enable Timer 0, part of the T0CON register (datasheet page 211)
    TMR0ON = 1;
    
    // enable Timer 0's interrupt
    TMR0IE = 1;
    
    // enable general interrupts
    GIE = 1;
    
    // main loop
    while (1) { }
}

// interrupt service routine to handle all interrupts
void interrupt main_isr(void) {
    // if Timer 0's interrupt flag has been raised
    if (TMR0IF) {
        // increment the millisecond counter
        milliseconds++;
        // lower the interrupt flag
        TMR0IF = 0;
    }
}