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来源：百度知道 编辑：UC知道 时间：2024/09/22 02:00:33
考虑一个这样的任务，从键盘读入一个输入字符然后在显示屏幕上产生一个输出字符。完成这个I/O任务的一种简单方法是使用众所周知的程序控制I/O方法。从键盘到计算机的实际传递速度是受用户打字速度所限制的。它不会超过每秒几个字符。而从计算机到显示器的输出传送速度是非常高的。这个速率根据字符传输中经过的计算机和显示设备之间的连接情况而决定，一般是每秒几千个字符。但是，它还是比处理器的速度慢很多，处理器每秒能执行几百万条指令。这种处理器和I/O之间速度的差异产生了在机制上要对它们之间的数据传送进行同步的要求。
解决这一问题的方法如下：在输出时，处理器发送第一个字符，然后就等待来自于显示器的表示已接收到这个字符的信号。然后再发送第二个字符，依次类推。来自键盘的输入使用类似的方法；处理器等待着一个标志信号，这个信号表示键盘上的一个字符键被敲击，并且它的编码已在一些与键盘相关联的缓冲寄存器中保存着，然后处理器去读这个代码。
The basic input / output operations
Consider one of the task, read from a keyboard input characters on the screen and then create a show that the output of characters. Completion of this I / O tasks of a simple method is to use the well-known program control I / O methods. From the computer keyboard to the actual transfer rate is subject to the users typing speed limit. It would not be more than a few characters per second. From the computer to display the output transmission speed is very high. This transmission rate in accordance with the character of computers and the display device connected between the circumstances of decision is generally a few thousand characters per second. However, it is slower than many processors, processors per second 1 million to implement several directions. This processor and the I / O speed of the difference between produced in the mechanism to transfer data between them for synchronization requirements.
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