某计算机系统中有一个CPU、一台输入设备和一台输出设备，假设系统中有四个作业T1、T2、T3和T4，系统采用优先级调度，且T1的优先级＞T2的优先级＞T3的优先级＞T4的优先级。每个作业具有三个程序段：输入Ii、计算Ci和输出Pi（i=1,2,3,4），其执行顺序为Ii→Ci→Pi。这四个作业各程序段并发执行的前驱图如下所示。图中④、⑤、⑥分别为（ ）。
- A.C2、C4、P4
- B.I2、I3、C4
- C.I3、P3、P4
- D.C4、P3、P4
正确答案及解析
正确答案
解析
前趋图是一个有向无循环图，由节点和有向边组成，节点代表各程序段的操作，而节点间的有向边表示两个程序段操作之间存在的前趋关系(“→”)。程序段Pi和Pj的前趋关系可表示成Pi→Pj，其中Pi是Pj的前趋，Pj是Pi的后继，其含义是Pi执行结束后Pj才能执行。本题完整的前趋图如下图所示，具体分析如下。
根据题意，I1执行结束后C1才能执行，C1执行结束后P1才能执行，因此I1是C1、P1的前趋，C1是P1的前趋。可见，图中③应为C1。又因为计算机系统中只有一台输入设备，所以I1执行结束后I2和I3才能执行，故I1是I2和I3的前趋，I2是I3的前趋。可见，图中①、②分别为I2、I3。根据题意，I4、C3执行结束后C4才能执行，即I4、C3是C4的前趋，所以④应为C4。又因为计算机系统中只有一个CPU和一台输出设备，所以C3、P2执行结束后P3才能执行，C3、P2是P3的前趋；同理C4、P3执行结束后P4才能执行，C4、P3是P4的前趋。经分析可知图中⑤、⑥分别为P3、P4。计算机系统中只有一个CPU，而且系统采用优先级调度，所以C1是C2的前趋，C2是C3的前趋。可见，图中④应为C4。
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