Added timer ISR

src/x86/isr.c

@@ -1,5 +1,6 @@
 #include "isr.h"
 #include "stdio.h"
+#include "apic.h"
 
 __attribute__ ((interrupt))
 void isr_division_by_zero(void* frame)
@@ -25,6 +26,13 @@ void isr_double_fault(void* frame, unsigned int error)
     asm("hlt");
 }
 
+__attribute__ ((interrupt))
+void isr_timer(void* frame)
+{
+    printf("Timer tick.\n");
+    apic_eoi();
+}
+
 __attribute__ ((naked))
 void isr_ap_start(void* frame)
 {

src/x86/isr.h

@@ -12,6 +12,9 @@ void isr_page_fault(void* frame, unsigned int error);
 __attribute__ ((interrupt))
 void isr_double_fault(void* frame, unsigned int error);
 
+__attribute__ ((interrupt))
+void isr_timer(void* frame);
+
 __attribute__ ((naked))
 void isr_ap_start(void* frame);