從根上理解用戶態與內核態

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"歡迎來到操作系統系列，採用圖解 + 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APIs)來達到訪問硬件和內存，在這種保護模式下，即時程序發生崩潰也是可以恢復的，在電腦上大部分程序都是在，用戶模式下運行的","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"link","attrs":{"href":"#用戶態與內核態","title":null,"type":null}},{"type":"text","text":"用戶態與內核態","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通關了C P U 指令集權限，現在再說用戶態與內核態就十分簡單了，用戶態與內核態的概念就是C P U 指令集權限的區別，進程中要讀寫 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"I O","attrs":{}}],"attrs":{}},{"type":"text","text":"，必然會用到 ring 0 級別的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"C P U 指令集","attrs":{}}],"attrs":{}},{"type":"text","text":"，而此時 C P U 的指令集操作權限只有 ring 3，爲了可以操作ring 0 級別的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"C P U 指令集","attrs":{}}],"attrs":{}},{"type":"text","text":"， C P U 切換指令集操作權限級別爲 ring 0，C P U再執行相應的ring 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指令集","attrs":{}}],"attrs":{}},{"type":"text","text":"（內核代碼），執行的內核代碼會使用當前進程的內核棧。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"PS：每個進程都有兩個棧，分別是用戶棧與內核棧，對應用戶態與內核態的使用","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"link","attrs":{"href":"#用戶態與內核態的空間","title":null,"type":null}},{"type":"text","text":"用戶態與內核態的空間","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在內存資源上的使用，操作系統對用戶態與內核態也做了限制，每個進程創建都會分配「虛擬空間地址」（不懂可以參考我的另一篇文章“15分鐘！一文幫小白搞懂操作系統之內存”），以Linux32位操作系統爲例，它的尋址空間範圍是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"4G","attrs":{}}],"attrs":{}},{"type":"text","text":"（2的32次方），而操作系統會把虛擬控制地址劃分爲兩部分，一部分爲內核空間，另一部分爲用戶空間，高位的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"1G","attrs":{}}],"attrs":{}},{"type":"text","text":"（從虛擬地址 0xC0000000 到 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高位 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"1G","attrs":{}}],"attrs":{}},{"type":"text","text":" 的內核空間是被所有進程共享的！","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最後做個小結，我們通過指令集權限區分用戶態和內核態，還限制了內存資源的使用，操作系統爲用戶態與內核態劃分了兩塊內存空間，給它們對應的指令集使用","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"link","attrs":{"href":"#用戶態與內核態的切換","title":null,"type":null}},{"type":"text","text":"用戶態與內核態的切換","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"相信大家都聽過這樣的話「用戶態和內核態切換的開銷大」，但是它的開銷大在那裏呢？簡單點來說有下面幾點","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"保留用戶態現場（上下文、寄存器、用戶棧等）","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"複製用戶態參數，用戶棧切到內核棧，進入內核態","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"額外的檢查（因爲內核代碼對用戶不信任）","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"執行內核態代碼","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"複製內核態代碼執行結果，回到用戶態","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"恢復用戶態現場（上下文、寄存器、用戶棧等）","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"實際上操作系統會比上述的更復雜，這裏只是個大概，我們可以發現一次切換經歷了「用戶態 -> 內核態 -> 用戶態」。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶態要主動切換到內核態，那必須要有入口才行，實際上內核態是提供了統一的入口，下面是Linux整體架構圖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5d/5d8f8867129703f74530fa3e1767cfda.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"從上圖我們可以看出來通過系統調用將Linux整個體系分爲用戶態和內核態，爲了使應用程序訪問到內核的資源，如CPU、內存、I/O，內核必須提供一組通用的訪問接口，這些接口就叫系統調用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"庫函數就是屏蔽這些複雜的底層實現細節，減輕程序員的負擔，從而更加關注上層的邏輯實現，它對系統調用進行封裝，提供簡單的基本接口給程序員。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Shell顧名思義，就是外殼的意思，就好像把內核包裹起來的外殼，它是一種特殊的應用程序，俗稱命令行。Shell也是可編程的，它有標準的Shell 語法，符合其語法的文本叫Shell腳本，很多人都會用Shell腳本實現一些常用的功能，可以提高工作效率。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最後來說說，什麼情況會導致用戶態到內核態切換","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"系統調用：用戶態進程主動切換到內核態的方式，用戶態進程通過系統調用向操作系統申請資源完成工作，例如 fork（）就是一個創建新進程的系統調用，系統調用的機制核心使用了操作系統爲用戶特別開放的一箇中斷來實現，如Linux 的 int 80h 中斷，也可以稱爲軟中斷","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"異常：當 C P U 在執行用戶態的進程時，發生了一些沒有預知的異常，這時當前運行進程會切換到處理此異常的內核相關進程中，也就是切換到了內核態，如缺頁異常","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"中斷：當 C P U 在執行用戶態的進程時，外圍設備完成用戶請求的操作後，會向 C P U 發出相應的中斷信號，這時 C P U 會暫停執行下一條即將要執行的指令，轉到與中斷信號對應的處理程序去執行，也就是切換到了內核態。如硬盤讀寫操作完成，系統會切換到硬盤讀寫的中斷處理程序中執行後邊的操作等。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"link","attrs":{"href":"#關聯好文章推薦","title":null,"type":null}},{"type":"text","text":"關聯好文章推薦","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/J2XA86nKyodeTyTU2Q6W0g","title":null,"type":null},"content":[{"type":"text","text":"15分鐘！一文幫小白搞懂操作系統之內存","attrs":{}}]}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/jhOSjVyRA6rNKqVT2pKMIQ","title":null,"type":null},"content":[{"type":"text","text":"進程、線程與協程傻傻分不清？一文帶你喫透！","attrs":{}}]}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"link","attrs":{"href":"#關於我","title":null,"type":null}},{"type":"text","text":"關於我","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Hi這裏是阿星，一個熱愛技術的93年Java程序猿，在公衆號 ","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"「程序猿阿星」","attrs":{}},{"type":"text","text":" 裏將會定期分享操作系統、計算機網絡、Java、分佈式、數據庫等精品原創文章，2021，與您在 Be Better 的路上共同成長！。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"非常感謝各位人才能 看到這裏，創作不易，文章有幫助可以「點個贊」或「分享與評論」，都是支持（莫要白嫖）！","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"願你我都能奔赴在各自想去的路上，我們下篇文章見！","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/cd/cd93a342bb6bb4cd5478beb9f35689e0.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}