GDT finally works

kernel/hal/x86_64/serial.c

@@ -0,0 +1,50 @@
+#include <stdint.h>
+#include <stdbool.h>
+
+#define SERIAL_PORT 0x3f8
+
+extern uint8_t serial_inb(uint16_t port);
+extern void serial_outb(uint16_t port, uint8_t value);
+
+static int serial_received(void) {
+  return serial_inb(SERIAL_PORT + 5) & 1;
+}
+
+static uint8_t serial_read(void) {
+  while (serial_received() == 0);
+  return serial_inb(SERIAL_PORT);
+}
+
+static int serial_trans_empty(void) {
+  return serial_inb(SERIAL_PORT + 5) & 0x20;
+}
+
+static void serial_write(uint8_t value) {
+  while (!serial_trans_empty());
+  serial_outb(SERIAL_PORT, value);
+}
+
+// REFERENCE: https://wiki.osdev.org/Serial_Ports
+bool serial_init(void) {
+  serial_outb(SERIAL_PORT + 1, 0x00); 
+  serial_outb(SERIAL_PORT + 3, 0x80); 
+  serial_outb(SERIAL_PORT + 0, 0x03);
+  serial_outb(SERIAL_PORT + 1, 0x00);
+  serial_outb(SERIAL_PORT + 3, 0x03);
+  serial_outb(SERIAL_PORT + 2, 0xc7);
+  serial_outb(SERIAL_PORT + 4, 0x0b);
+  serial_outb(SERIAL_PORT + 4, 0x1e);
+  serial_outb(SERIAL_PORT + 0, 0xae);
+
+  if (serial_inb(SERIAL_PORT + 0) != 0xae) {
+    return false;
+  }
+
+  serial_outb(SERIAL_PORT + 4, 0x0f);
+  return true;
+}
+
+// For printf library
+void putchar_(char c) {
+  serial_write(c);
+}