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下面我们来讨论进程挂起 

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在前面我们讨论

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进程状态模型的时候呢 

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它主要讨论的是

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跟CPU相关的这些状态 

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但实际上我们在

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进程当中的状态里头呢

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还有一类是在跟存储相关 

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也就是说进程一部分存储是

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放到外存里头的 

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和虚拟存储相关联起来 

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这是呢就是我们这里的

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挂起进程模型 

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它是指什么意思呢 

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它是指处于挂起状态的进程是

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放在磁盘上的 

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放到磁盘上的目的呢

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是为了减少内存的占用量 

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那减少它的内存占用量

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就可以提供更多的内存

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给其它的进程来使用 

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有了进程挂起之后 

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我们的进程状态模型图呢

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也会因此而发生变化 

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这就是我们这里的挂起进程模型 

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和我们前面讲的三状态进程模型

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相比呢在这里多了两种状态 

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就绪挂起和等待挂起

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这两种新加的状态呢 

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实际上就是为了描述

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在外存当中的进程的状态 

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那其它的状态它是不会在外存里的 

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第一个呢是等待挂起 

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那这时候描述的是说进程在

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外存并且等待事件的出现 

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那就相当于在等待的基础上

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加了一个关于进程的位置的信息 

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而接下来一个呢是就绪挂起 

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那就绪挂起呢实际上就相当于

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它处于就绪的状态 

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但这时候呢进程在外存当中 

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那它只要进入到内存里头呢 

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它就可以运行了 

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那这时候它进不到内存里的原因呢 

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是内存的空间不够 

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或者说它的优先级不够高 

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好 有了这两个状态之后 

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我们就会在状态之间呢

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多加了一组变迁 

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这组变迁呢

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我们可以把它分成这样几类 

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一类是挂起 

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也就是说把进程从内存转到外存 

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那具体说起来呢

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有三个与此相关的变迁 

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第一个是从等待到等待挂起 

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那出现这种变化的原因呢

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是没有进程处于就绪状态 

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或者说就绪进程

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需要更多的内存空间 

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然后第二个呢是说 

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就绪到就绪挂起 

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那在这里头呢

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有高优先级的进程等待 

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而这时候呢低优先级的进程呢

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在内存当中处于就绪状态 

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那为了让这个高优先级等待进程

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进来之后有足够的空间 

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那这时候它把它 

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把这个低优先级的就绪进程呢

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对换到外存当中 

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从而呢成为挂起就绪 

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好 还有一个变迁呢是

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直接从运行状态到了挂起就绪 

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那在这里头呢

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实际上就是对于分时系统

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如果有抢先的话

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在极特殊的一些情况下

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会出现这种情况 

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比如说有高优先级等待的进程

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因事件的出现而进入就绪 

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而这时候呢没有足够的内存空间 

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它就会把当前

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正在运行的这个进程呢

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抢先并且把它变成挂起就绪的状态 

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好 这是三个从内存到外存的变迁 

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分别涉及到就绪运行和等待 

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也就相当于三状态进程模型里 

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在内存里三个状态呢 

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都会有可能会 放到外存里头去 

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从而产生相应的挂起 

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然后在外存当中

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可能出现的状态呢就是 

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我们在这里内存当中的等待到就绪 

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这地方有挂起 

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等待挂起到就绪挂起 

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好 那这个地方呢

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是等待的事件出现

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并且相关的进程呢处于等待挂起上 

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那这时候它就变成是就绪挂起 

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好 反过身来这几个

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在外存当中的进程呢 

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它可能会被倒回到内存当中来 

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这时候的变迁呢称之为叫激活 

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那具体说起来的激活有两种情况 

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一种是从就绪挂起到就绪 

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那这种情况是说

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我在内存当中没有就绪进程 

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或者说挂起就绪这个进程呢

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它的优先级比较高 

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这个时候它会把它倒回来 

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还有一种呢就是从等待挂起到等待 

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这种状态呢通常情况下是

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前面的进程释放了大量的空间 

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而且这个等待挂起的

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这个进程呢优先级比较高 

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好 这是呢我们在这里说到的

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挂起进程模型里头 

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它所涉及到进程模型的变化 

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那我们看到由于状态的不同 

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我们会涉及到很多的队列 

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把进程控制块连成一体 

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好 从这个角度来讲呢 

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我们甚至可以用状态队列

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来对进程的状态呢

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有某种形式的描述 

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也就是说我们通常情况下

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在代码当中去考察进程状态时候 

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就是它的标志性操作 

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就是把这个PCB

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挂到哪个队列当中 

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好 我们在这里呢

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维护了一组队列 

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表示进程的当前状态 

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然后每一个不同的队列呢 

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它表示不同的状态 

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你比如说各种各样的等待 

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就绪队列 

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好 这些队列的状态的变化呢 

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导致了相应的进程在

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不同队列之间的切换 

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或者说移动 

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那这时候呢

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我们可以把这种状态的变化呢 

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以一种队列的形式来体现出来 

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我们这儿有就绪队列若干个 

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然后这个地方有等待队列若干个 

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这是处理机 

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实际上这时候的状态变迁呢 

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变成是队列之间的进程的搬迁 

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好 到这儿呢

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我们就把进程的状态呢说明白了 

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