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# 9.8 UART读取键盘输入

在UART的另一侧，会有类似的事情发生，有时Shell会调用read从键盘中读取字符。 在read系统调用的底层，会调用fileread函数。在这个函数中，如果读取的文件类型是设备，会调用相应设备的read函数。

![](/files/-MNrubhKUjW6DnLvxjQ6)

在我们的例子中，read函数就是console.c文件中的consoleread函数。

![](/files/-MNrv41RJWmQNiCKb4Ve)

这里与UART类似，也有一个buffer，包含了128个字符。其他的基本一样，也有producer和consumser。但是在这个场景下Shell变成了consumser，因为Shell是从buffer中读取数据。而键盘是producer，它将数据写入到buffer中。

![](/files/-MNrvQJe25cNXU0LuIaw)

从consoleread函数中可以看出，当读指针和写指针一样时，说明buffer为空，进程会sleep。所以Shell在打印完“$ ”之后，如果键盘没有输入，Shell进程会sleep，直到键盘有一个字符输入。所以在某个时间点，假设用户通过键盘输入了“l”，这会导致“l”被发送到主板上的UART芯片，产生中断之后再被PLIC路由到某个CPU核，之后会触发devintr函数，devintr可以发现这是一个UART中断，然后通过uartgetc函数获取到相应的字符，之后再将字符传递给consoleintr函数。

![](/files/-MNs0P8PukOn1-st2X5I)

默认情况下，字符会通过consputc，输出到console上给用户查看。之后，字符被存放在buffer中。在遇到换行符的时候，唤醒之前sleep的进程，也就是Shell，再从buffer中将数据读出。

所以这里也是通过buffer将consumer和producer之间解耦，这样它们才能按照自己的速度，独立的并行运行。如果某一个运行的过快了，那么buffer要么是满的要么是空的，consumer和producer其中一个会sleep并等待另一个追上来。


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