enable uart output

Makefile

@@ -145,11 +145,13 @@ run: $(IMAGE)
 	@qemu-system-i386		\
 	-boot order=a			\
 	-drive file=$<,format=raw	\
+	-serial stdio \
 
 gdb: $(IMAGE)
 	@qemu-system-i386		\
 	-boot order=a			\
 	-drive file=$<,format=raw	\
+	-serial stdio \
 	-s -S				\
 
 gdb-no-graphic: $(IMAGE)
@@ -157,6 +159,7 @@ gdb-no-graphic: $(IMAGE)
 	-nographic			\
 	-boot order=a			\
 	-drive file=$<,format=raw	\
+	-serial stdio \
 	-s -S				\
 
 # 调试的内核代码elf

include/proto.h

@@ -12,6 +12,7 @@ int vga_get_disppos();
 void vga_write_char(const char ch, u8 color);
 void vga_write_str(const char * str);
 void vga_write_str_color(const char * str, u8 color);
+int uart_kprintf(const char *fmt, ...);
 
 //added by zcr
 void	disable_irq(int irq);