From 6cedec8d7a3f586b08ce6f51ad853d9bd19ceda9 Mon Sep 17 00:00:00 2001
From: Nathan Wagner <nw@hydaspes.if.org>
Date: Mon, 24 Oct 2016 20:32:34 -0500
Subject: [PATCH] timer interrupt handing

---
 timer.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 timer.h |  3 +++
 2 files changed, 78 insertions(+)
 create mode 100644 timer.c
 create mode 100644 timer.h

diff --git a/timer.c b/timer.c
new file mode 100644
index 0000000..e344e91
--- /dev/null
+++ b/timer.c
@@ -0,0 +1,75 @@
+#include <stdint.h>
+
+#include "interrupt.h"
+#include "kernel.h"
+#include "timer.h"
+
+#include "ioport.h"
+#include "cpu.h"
+
+#include <time.h>
+
+static struct interrupt_handler timer;
+
+/* This will keep track of how many ticks that the system
+*  has been running for */
+volatile uint64_t timer_ticks = 0;
+static int timer_hz = 18;
+
+static uint64_t timer_ns; /* nanoseconds per time tick */
+
+struct timespec system_time; /* global system time */
+
+#define ONE_BILLION 1000000000ULL
+
+/* See http://wiki.osdev.org/Programmable_Interval_Timer
+ * for a possibly better algorithm
+ */
+static void setfreq(int hz) {
+	int enable;
+	unsigned int divisor;
+
+	if (hz < 18) hz = 18;
+	divisor = 1193182/hz;
+
+	timer_hz = 1193182 / divisor;
+
+#if 0
+	printk("timer hz %llu\n", (uint64_t)hz);
+	printk("timer divisor %u\n", divisor);
+	printk("timer hz actual %llu\n", (uint64_t)timer_hz);
+#endif
+
+	enable = interrupts_enabled();
+	if (enable) disable_interrupts();
+	outport8(0x43, 0x34);
+	outport8(0x40, (uint8_t)(divisor & 0xFF));   /* Set low byte of divisor */
+	outport8(0x40, (uint8_t)(divisor >> 8 & 0xFF));     /* Set high byte of divisor */
+
+	timer_ns = (uint64_t)1000000000ULL / timer_hz;
+	/* now, let's disable the pit and use the RTC */
+	if (enable) enable_interrupts();
+}
+
+static void timer_handler(struct interrupt_context *c, void *unused) {
+	timer_ticks++;
+	system_time.tv_nsec += timer_ns;
+	while (system_time.tv_nsec >= ONE_BILLION) {
+		system_time.tv_sec++;
+		system_time.tv_nsec -= ONE_BILLION;
+	}
+}
+
+void init_timer(int hz) {
+	int enable;
+	timer.handler = timer_handler;
+	timer.context = 0;
+	system_time.tv_sec = 0;
+	system_time.tv_nsec = 0;
+
+	enable = interrupts_enabled();
+	if (enable) disable_interrupts();
+	interrupt_add_handler(IRQ0, &timer);
+	setfreq(hz);
+	if (enable) enable_interrupts();
+}
diff --git a/timer.h b/timer.h
new file mode 100644
index 0000000..7e3b215
--- /dev/null
+++ b/timer.h
@@ -0,0 +1,3 @@
+void init_timer(int hz);
+extern volatile uint64_t timer_ticks;
+extern struct timespec system_time;
-- 
2.40.0