自己动手写操作系统源代码编译，链接错误，求解。

Question

自己动手写操作系统源码(D:\Tinix\chapter5\h),编译结果如下：
$ MAKE
ld -s -Ttext 0x30400 -o kernel.bin kernel/start.o lib/string.o lib/klib.o kernel/kernel.o
kernel/start.o:start.c:(.text+0xe): undefined reference to disp_str'
kernel/start.o:start.c:(.text+0x37): undefined reference tomemcpy'
kernel/start.o:start.c:(.text+0x63): undefined reference to disp_str'
kernel/kernel.o: In functionstart':
kernel/kernel.asm:(.text+0x8): undefined reference to gdt_ptr'
kernel/kernel.asm:(.text+0xd): undefined reference tocstart'
kernel/kernel.asm:(.text+0x14): undefined reference to `gdt_ptr'
Makefile:82: recipe for target 'kernel.bin' failed
MAKE: *** [kernel.bin] Error 1
环境：windows8 + cygwin