timer.c

#include "timer.h"

#define interval 10000000 // cycles; about 1 second in qemu.

void timer_init()
{
  // each CPU has a separate source of timer interrupts.
  int id = r_mhartid();

  // ask the CLINT for a timer interrupt.
  *(reg_t*)CLINT_MTIMECMP(id) = *(reg_t*)CLINT_MTIME + interval;

  // set the machine-mode trap handler.
  w_mtvec((reg_t)sys_timer);

  // enable machine-mode interrupts.
  w_mstatus(r_mstatus() | MSTATUS_MIE);

  // enable machine-mode timer interrupts.
  w_mie(r_mie() | MIE_MTIE);
}

static int timer_count = 0;

reg_t timer_handler(reg_t epc, reg_t cause)
{
  reg_t return_pc = epc;
  // disable machine-mode timer interrupts.
  w_mie(~((~r_mie()) | (1 << 7)));
  lib_printf("timer_handler: %d\n", ++timer_count);
  int id = r_mhartid();
  *(reg_t *)CLINT_MTIMECMP(id) = *(reg_t *)CLINT_MTIME + interval;
  // enable machine-mode timer interrupts.
  w_mie(r_mie() | MIE_MTIE);
  return return_pc;
}