timer.c

#include "timer.h"
 
#include "../drivers/ports_io.h"
 
#include "interruption_tables.h"
#include "interruptions.h"
#include "task.h"
 
 
static u_dword tick = 0;
 
static void timer_callback(registers* regs) {
    tick++;
    switch_task();
}
 
void init_timer(u_dword frequency) {
    // Firstly, register our timer callback.
    set_interrupt_handler(IRQ0, &timer_callback);
 
    // The value we send to the PIT is the value to divide it's input clock
    // (1193180 Hz) by, to get our required frequency. Important to note is
    // that the divisor must be small enough to fit into 16-bits.
    u_dword divisor = 1193180 / frequency;
 
    // Send the command byte.
    port_byte_out(0x43, 0x36);
 
    // Divisor has to be sent byte-wise, so split here into upper/lower bytes.
    u_byte l = (u_byte)(divisor & 0xFF);
    u_byte h = (u_byte)( (divisor>>8) & 0xFF );
 
    // Send the frequency divisor.
    port_byte_out(0x40, l);
    port_byte_out(0x40, h);
}