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Update ch9 2device-driver-1.rst #237

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2 changes: 1 addition & 1 deletion source/chapter9/2device-driver-1.rst
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Expand Up @@ -9,7 +9,7 @@
串口驱动程序
------------------------------------

我们要管理是串口(UART)物理设备。我们在第一章其实就接触了串口，但当时是通过RustSBI来帮OS完成对串口的访问，即OS只需发出两种SBI调用请求就可以输出和获取字符了。但这种便捷性是有代价的。比如OS在调用获取字符的SBI调用请求后，RustSBI如果没收到串口字符，会返回 ``-1`` ，这样OS只能采用类似轮询的方式来继续查询。到第七章为止的串口驱动不支持中断是导致在多进程情况下，系统效率低下的主要原因之一。大家也不要遗憾，我们的第一阶段的目标是 **Just do it** ，先把OS做出来。在本节，我们需要逐步改进优化对串口的操作了。
我们要管理的是串口(UART)物理设备。我们在第一章其实就接触了串口，但当时是通过RustSBI来帮OS完成对串口的访问，即OS只需发出两种SBI调用请求就可以输出和获取字符了。但这种便捷性是有代价的。比如OS在调用获取字符的SBI调用请求后，RustSBI如果没收到串口字符，会返回 ``-1`` ，这样OS只能采用类似轮询的方式来继续查询。到第七章为止的串口驱动不支持中断是导致在多进程情况下，系统效率低下的主要原因之一。大家也不要遗憾，我们的第一阶段的目标是 **Just do it** ，先把OS做出来。在本节，我们需要逐步改进优化对串口的操作了。

.. image:: uart-drive.png
:align: center
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