Enable the PIC and add a timer.

This causes a GP fault after the timer runs for some time.

zion/CMakeLists.txt

@@ -5,6 +5,7 @@ add_executable(zion
   debug/debug.cpp
   interrupt/interrupt.cpp
   interrupt/interrupt_enter.s
+  interrupt/timer.cpp
   loader/elf_loader.cpp
   loader/init_loader.cpp
   memory/kernel_heap.cpp

zion/interrupt/interrupt.cpp

@@ -2,6 +2,7 @@
 
 #include <stdint.h>
 
+#include "common/port.h"
 #include "debug/debug.h"
 
 #define IDT_INTERRUPT_GATE 0x8E
@@ -95,13 +96,37 @@ extern "C" void interrupt_page_fault(InterruptFrame* frame) {
   panic("PF");
 }
 
+#define PIC1_COMMAND 0x20
+#define PIC1_DATA 0x21
+#define PIC_EOI 0x20
+
+uint64_t cnt = 0;
+extern "C" void isr_timer();
+extern "C" void interrupt_timer(InterruptFrame*) {
+  cnt++;
+  if (cnt % 1000 == 0) {
+    dbgln("timer: %u", cnt);
+  }
+  outb(PIC1_COMMAND, PIC_EOI);
+}
+
+void EnablePic() {
+  outb(PIC1_COMMAND, 0x11);
+  outb(PIC1_DATA, 0x20);  // PIC1 offset.
+  outb(PIC1_DATA, 0x4);
+  outb(PIC1_DATA, 0x1);
+}
+
 void InitIdt() {
   gIdt[0] = CreateDescriptor(isr_divide_by_zero);
   gIdt[13] = CreateDescriptor(isr_protection_fault);
   gIdt[14] = CreateDescriptor(isr_page_fault);
+  gIdt[32] = CreateDescriptor(isr_timer);
   InterruptDescriptorTablePointer idtp{
       .size = sizeof(gIdt),
       .base = reinterpret_cast<uint64_t>(gIdt),
   };
   asm volatile("lidt %0" ::"m"(idtp));
+
+  EnablePic();
 }

zion/interrupt/interrupt_enter.s

@@ -55,3 +55,5 @@ isr_\name:
 isr_handler divide_by_zero
 isr_handler protection_fault,1
 isr_handler page_fault,1
+
+isr_handler timer

zion/interrupt/timer.cpp

@@ -0,0 +1,41 @@
+#include "interrupt/timer.h"
+
+#include "common/port.h"
+
+namespace {
+
+// IO Ports
+constexpr uint8_t kPit0Ctl = 0x40;
+constexpr uint8_t kPit1Ctl = 0x41;
+constexpr uint8_t kPit2Ctl = 0x42;
+constexpr uint8_t kPitCmd = 0x43;
+
+// Command Register
+// Mode
+constexpr uint8_t kPitMode0 = 0x00;
+constexpr uint8_t kPitMode1 = 0x02;
+constexpr uint8_t kPitMode2 = 0x04;
+constexpr uint8_t kPitMode3 = 0x06;
+constexpr uint8_t kPitMode4 = 0x08;
+constexpr uint8_t kPitMode5 = 0x0A;
+// R/W
+constexpr uint8_t kCmdLatch = 0x00;
+constexpr uint8_t kCmdRwLow = 0x10;
+constexpr uint8_t kCmdRwHi = 0x20;
+constexpr uint8_t kCmdRwBoth = 0x30;
+// PIT Select
+constexpr uint8_t kSelect0 = 0x00;
+constexpr uint8_t kSelect1 = 0x40;
+constexpr uint8_t kSelect2 = 0x80;
+constexpr uint8_t kReadback = 0xC0;
+
+constexpr uint32_t kPitFrequency = 1193182;
+
+}  // namespace
+
+void SetFrequency(uint64_t hertz) {
+  uint16_t reload = kPitFrequency / hertz;
+  outb(kPitCmd, kPitMode3 | kCmdRwBoth | kSelect0);
+  outb(kPit0Ctl, reload & 0xFF);
+  outb(kPit0Ctl, reload >> 8);
+}

zion/interrupt/timer.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include <stdint.h>
+
+void SetFrequency(uint64_t hertz);

zion/zion.cpp

@@ -3,6 +3,7 @@
 #include "common/gdt.h"
 #include "debug/debug.h"
 #include "interrupt/interrupt.h"
+#include "interrupt/timer.h"
 #include "loader/init_loader.h"
 #include "memory/kernel_heap.h"
 #include "memory/paging_util.h"
@@ -21,6 +22,7 @@ extern "C" void zion() {
 
   InitSyscall();
 
+  SetFrequency(/* hertz= */ 2000);
   sched::InitScheduler();
   sched::EnableScheduler();