os的基本操作：完成，若有不小于n的空闲块，则分配相应的存储块 否则显示NULL。 若分配后可利用空间表不空，则pav指向表中刚分配过的结点的后继结点-os basic operations: complete, if not less than n of the free block, then the distribution of the corresponding storage block or show NULL. If the distribution of the use of space after the table is not empty, then the pav point in the table just before the junction point of the distribution node successor