说明：  改进型CLOCK 页面置换算法实现在进程运行过程中，若其要访问的页面不在内存而需要调入内存，但分配该进程的内存 空间已经用完时，为了保证该进程能继续执行，需要从内存中把该进程的一些页调入外存中 的对换区，但调出那个页面，可以采用不同的算法。
(Improved CLOCK page replacement algorithm in the process of running the course, if its trying to access pages of memory need not be transferred to memory, but the distribution of the process had run out of memory space, in order to ensure that the process can continue execution from memory in the process of redeployment of a number of pages for Out-of-Core of the area, but out of that page, you can use different algorithms.)