边界标识法：系统将所有的空闲块链接在一个双重循环链表结构的可利用空间表中，无须从头到尾扫描链表也能确定一个释放块的相邻块是否为空闲。

-Boundary marking method: the system will link all of the free block list in a double cycle structure of the table to the use of space, there is no need to scan list from start to finish can also set a block adjacent to block release for free.