timer.cpp

// If INTER_MODULE_OPT macro is defined,
// this file is included into cpu.cpp
// to make inter module optimizations possible
#ifndef INTER_MODULE_OPT

#include "timer.h"

#include "cpu.h"
#include "interrupt.h"

static unsigned int prev_time;
static unsigned int elapsed;
static unsigned int ticks;

static unsigned char tac;
static unsigned int started;
static unsigned int speed;
static unsigned int counter;
static unsigned int divider;
static unsigned int modulo;

void timer_set_div(unsigned char v) {
  (void)v;
  divider = 0;
}

unsigned char timer_get_div(void) { return divider; }

void timer_set_counter(unsigned char v) { counter = v; }

unsigned char timer_get_counter(void) { return counter; }

void timer_set_modulo(unsigned char v) { modulo = v; }

unsigned char timer_get_modulo(void) { return modulo; }

void timer_set_tac(unsigned char v) {
  int speeds[] = {64, 1, 4, 16};
  tac = v;
  started = v & 4;
  speed = speeds[v & 3];
}

unsigned char timer_get_tac(void) { return tac; }

static void timer_tick(void) {
  /* 1/262144Hz has elapsed */
  ticks++;

  /* Divider updates at 16384Hz */
  if (ticks == 16) {
    divider++;
    ticks = 0;
  }

  if (!started) return;

  if (ticks == speed) {
    counter++;
    ticks = 0;
  }

  if (counter == 0x100) {
    interrupt(INTR_TIMER);
    counter = modulo;
  }
}

void timer_cycle(unsigned int cycles) {
  /* The amount of ticks since we last ran */
  unsigned int delta = cycles - prev_time;
  prev_time = cycles;

  elapsed += delta * 4; /* 4 cycles to a timer tick */
  if (elapsed >= 16) {
    timer_tick();
    elapsed -= 16; /* keep track of the time overflow */
  }
}

#endif  // INTER_MODULE_OPT