鍵盤敲入 A 字母時，期間發生了什麼....

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/fc/fc5de2393d263dfcbe6aa84e930ce54d.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"前言"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"鍵盤可以說是我們最常使用的輸入硬件設備了，但身爲程序員的你，你知道「"},{"type":"text","marks":[{"type":"strong"}],"text":"鍵盤敲入A 字母時，操作系統期間發生了什麼嗎"},{"type":"text","text":"」？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那要想知道這個發生的過程，我們得先了解了解「操作系統是如何管理多種多樣的的輸入輸出設備」的，等了解完這個後，我們再來看看這個問題，你就會發現問題已經被迎刃而解了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/3a/3a57816b2c68f4b07e72e6433e2c01b3.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"正文"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"設備控制器"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們的電腦設備可以接非常多的輸入輸出設備，比如鍵盤、鼠標、顯示器、網卡、硬盤、打印機、音響等等，每個設備的用法和功能都不同，那操作系統是如何把這些輸入輸出設備統一管理的呢?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了屏蔽設備之間的差異，每個設備都有一個叫*"},{"type":"text","marks":[{"type":"italic"}],"text":"設備控制器（"},{"type":"text","text":"Device 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進行通信，比如："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通過寫入這些寄存器，操作系統可以命令設備發送數據、接收數據、開啓或關閉，或者執行某些其他操作。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通過讀取這些寄存器，操作系統可以瞭解設備的狀態，是否準備好接收一個新的命令等。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"實際上，控制器是有三類寄存器，它們分別是*"},{"type":"text","marks":[{"type":"italic"}],"text":"狀態寄存器（"},{"type":"text","text":"Status Register*）"},{"type":"text","marks":[{"type":"strong"}],"text":"、 "},{"type":"text","text":"命令寄存器（"},{"type":"text","marks":[{"type":"italic"}],"text":"Command 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設備。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"命令寄存器"},{"type":"text","text":"，CPU 發送一個命令，告訴 I/O 設備，要進行輸入/輸出操作，於是就會交給 I/O 設備去工作，任務完成後，會把狀態寄存器裏面的狀態標記爲完成。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"狀態寄存器"},{"type":"text","text":"，目的是告訴 CPU ，現在已經在工作或工作已經完成，如果已經在工作狀態，CPU 再發送數據或者命令過來，都是沒有用的，直到前面的工作已經完成，狀態寄存標記成已完成，CPU 才能發送下一個字符和命令。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CPU 通過讀寫設備控制器中的寄存器控制設備，這可比 CPU 直接控制輸入輸出設備，要方便和標準很多。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外， 輸入輸出設備可分爲兩大類 ：*"},{"type":"text","marks":[{"type":"italic"}],"text":"塊設備（"},{"type":"text","text":"Block Device*）"},{"type":"text","marks":[{"type":"strong"}],"text":"和"},{"type":"text","text":"字符設備（"},{"type":"text","marks":[{"type":"italic"}],"text":"Character Device"},{"type":"text","text":"）**。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"塊設備"},{"type":"text","text":"，把數據存儲在固定大小的塊中，每個塊有自己的地址，硬盤、USB 是常見的塊設備。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"字符設備"},{"type":"text","text":"，以字符爲單位發送或接收一個字符流，字符設備是不可尋址的，也沒有任何尋道操作，鼠標是常見的字符設備。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"塊設備通常傳輸的數據量會非常大，於是控制器設立了一個可讀寫的"},{"type":"text","marks":[{"type":"strong"}],"text":"數據緩衝區"},{"type":"text","text":"。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CPU 寫入數據到控制器的緩衝區時，當緩衝區的數據囤夠了一部分，纔會發給設備。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CPU 從控制器的緩衝區讀取數據時，也需要緩衝區囤夠了一部分，才拷貝到內存。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣做是爲了，減少對設備的頻繁操作。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那 CPU 是如何與設備的控制寄存器和數據緩衝區進行通信的？存在兩個方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"端口 I/O"},{"type":"text","text":"，每個控制寄存器被分配一個 I/O 端口，可以通過特殊的彙編指令操作這些寄存器，比如 "},{"type":"codeinline","content":[{"type":"text","text":"in/out"}]},{"type":"text","text":" 類似的指令。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"內存映射 I/O"},{"type":"text","text":"，將所有控制寄存器映射到內存空間中，這樣就可以像讀寫內存一樣讀寫數據緩衝區。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"I/O 控制方式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在前面我知道，每種設備都有一個設備控制器，控制器相當於一個小 CPU，它可以自己處理一些事情，但有個問題是，當 CPU 給設備發送了一個指令，讓設備控制器去讀設備的數據，它讀完的時候，要怎麼通知 CPU 呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"控制器的寄存器一般會有狀態標記位，用來標識輸入或輸出操作是否完成。於是，我們想到第一種"},{"type":"text","marks":[{"type":"strong"}],"text":"輪詢等待"},{"type":"text","text":"的方法，讓 CPU 一直查寄存器的狀態，直到狀態標記爲完成，很明顯，這種方式非常的傻瓜，它會佔用 CPU 的全部時間。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那我們就想到第二種方法 —— "},{"type":"text","marks":[{"type":"strong"}],"text":"中斷"},{"type":"text","text":"，通知操作系統數據已經準備好了。我們一般會有一個硬件的"},{"type":"text","marks":[{"type":"strong"}],"text":"中斷控制器"},{"type":"text","text":"，當設備完成任務後觸發中斷到中斷控制器，中斷控制器就通知 CPU，一箇中斷產生了，CPU 需要停下當前手裏的事情來處理中斷。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外，中斷有兩種，一種"},{"type":"text","marks":[{"type":"strong"}],"text":"軟中斷"},{"type":"text","text":"，例如代碼調用 "},{"type":"codeinline","content":[{"type":"text","text":"INT"}]},{"type":"text","text":" 指令觸發，一種是"},{"type":"text","marks":[{"type":"strong"}],"text":"硬件中斷"},{"type":"text","text":"，就是硬件通過中斷控制器觸發的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但中斷的方式對於頻繁讀寫數據的磁盤，並不友好，這樣 CPU 容易經常被打斷，會佔用 CPU 大量的時間。對於這一類設備的問題的解決方法是使用 *"},{"type":"text","marks":[{"type":"italic"}],"text":"DMA（"},{"type":"text","text":"Direct Memory Access"},{"type":"text","marks":[{"type":"italic"}],"text":"）"},{"type":"text","text":"* 功能，它可以使得設備在 CPU 不參與的情況下，能夠自行完成把設備 I/O 數據放入到內存。那要實現 DMA 功能要有 「DMA 控制器」硬件的支持。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/cc/cc208f80356d4b85fade3d542247d528.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"DMA 的工作方式如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CPU 需對 DMA 控制器下發指令，告訴它想讀取多少數據，讀完的數據放在內存的某個地方就可以了；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接下來，DMA 控制器會向磁盤控制器發出指令，通知它從磁盤讀數據到其內部的緩衝區中，接着磁盤控制器將緩衝區的數據傳輸到內存；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當磁盤控制器把數據傳輸到內存的操作完成後，磁盤控制器在總線上發出一個確認成功的信號到 DMA 控制器；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"DMA 控制器收到信號後，DMA 控制器發中斷通知 CPU 指令完成，CPU 就可以直接用內存裏面現成的數據了；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到， CPU 當要讀取磁盤數據的時候，只需給 DMA 控制器發送指令，然後返回去做其他事情，當磁盤數據拷貝到內存後，DMA 控制機器通過中斷的方式，告訴 CPU 數據已經準備好了，可以從內存讀數據了。僅僅在傳送開始和結束時需要 CPU 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