1 files changed, 32 insertions, 0 deletions
diff --git a/timer.c b/timer.c
new file mode 100644
index 0000000..ba4e6c4
--- /dev/null
+++ b/timer.c
@@ -0,0 +1,32 @@
+// Intel 8253/8254/82C54 Programmable Interval Timer (PIT).
+// Only used on uniprocessors;
+// SMP machines use the local APIC timer.
+
+#include "types.h"
+#include "defs.h"
+#include "traps.h"
+#include "x86.h"
+
+#define IO_TIMER1       0x040           // 8253 Timer #1
+
+// Frequency of all three count-down timers;
+// (TIMER_FREQ/freq) is the appropriate count
+// to generate a frequency of freq Hz.
+
+#define TIMER_FREQ      1193182
+#define TIMER_DIV(x)    ((TIMER_FREQ+(x)/2)/(x))
+
+#define TIMER_MODE      (IO_TIMER1 + 3) // timer mode port
+#define TIMER_SEL0      0x00    // select counter 0
+#define TIMER_RATEGEN   0x04    // mode 2, rate generator
+#define TIMER_16BIT     0x30    // r/w counter 16 bits, LSB first
+
+void
+timer_init(void)
+{
+  // Interrupt 100 times/sec.
+  outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
+  outb(IO_TIMER1, TIMER_DIV(100) % 256);
+  outb(IO_TIMER1, TIMER_DIV(100) / 256);
+  irq_enable(IRQ_TIMER);
+}