进程P_A不断地向管道写数据，进程P_B从管道中读数据并加工处理，如下图所示。如果采用PV操作来实现进程P_A和进程P_B间的管道通信，并且保证这两个进程并发执行的正确性，则至少需要______。

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An Intranet is(71)the application of Internet technology within an internal or closed user group．Internets are company(72)and do not have a(73)connection to the Internet．Used properly an Internet is highly effective corporate tool，capable of regularly(74)information to empower the workforce with the information needed to perform. their roles．Used in this way，Internet represent a step towards the(75)office．

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在UML提供的图中，可以采用(30)对逻辑数据厍的建模；(31)用于接口、类和协作的行为建模，并强调对象行为的事件顺序；(32)用于系统的功能建模，并强调对象之间的控制流。

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