源代码版本openjdk-7u40-fcs-src-b43-26_aug_2013
标记字（mark word）的定义在markOop.hpp中，内容如下所示
// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
32位平台上，标记字中字段的长度、偏移如下所示(gist)，注意是按照大端序存储：
offset length description
lock 0 2
biased_lock 2 1
age 3 4
cms 7 0
hash 7 25
epoch 7 2
其中，lock字段共两位，其含义如下所示：
00 当前对象已被加轻量级锁，markOop指针指向线程栈中的对实际象头
01 当前对象未被加锁，当前markOop指向不同的对象头
10 当前对象已被加重量级锁，对象头已被换出
11 垃圾回收标记，用于标记当前对象已经无用