blocks|key|172970|text|在用户领域，您可以通过对内核的系统调用来请求特权操作，内核会根据需要切换到内核模式。用户正在使用API，内核正在执行特权操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172971|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

In user land, you request privileged operations via system calls to the kernel, which performs the switch to kernel mode as necessary. The user is using an API, the kernel is performing the privileged operations.

blocks|key|2779575|text|通常有一组指令并不是真正地以一般方式切换到内核模式，而是请求系统服务。因此，这些切换到内核模式，但仅在调用某些功能的上下文中，这些功能是由操作系统设置的，目的是供用户代码调用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2779576|在大多数现代系统中，甚至这也被实现特定功能的API层所隐藏，该API层的一部分可能是进行如上所述的操作系统调用。|2779577|但一般来说，用户代码不能做等同于说“从现在开始，我想在内核模式下运行”的事情。|2779578|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

There are usually as set of instructions that are not really to switch to kernel mode in s general way, but to request system services. So these switch to kernel mode, but only in the context of calling some piece of functionality which was set up by the operation system for the purpose of being called by user code.

In most modern systems, even this is hidden by an API layer that implements a specific function part of which may be doing an operating system call as above.

But in general it is true that user code cannot do the equivalent of saying "from this point on, I want to be running in kernel mode".

blocks|key|175144|text|在用户模式下，你不能直接切换到内核模式。用户和内核之间的交互是通过系统调用完成的。每个系统调用都提供一个已定义的服务。用户发送服务名称(通常是一个数字)和所需的参数。这是一个真实的例子，如何做到这一点。它是x86+AT&T风格的汇编程序。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|175145|它将系统调用名移入EAX寄存器，将指向参数的指针移入CPU的EBX寄存器，然后发出软件中断号42。中断处理将切换到内核模式。在中断描述符表(IDT)中查找中断号，并调用在那里注册的函数，即syscall处理程序。此处理程序在内核模式下执行。在返回到用户模式时，代码将把EAX的内容移到变量ret中。|175146|pok_ret_t+pok_do_syscall+(pok_syscall_id_t+syscall_id,+pok_syscall_args_t*+args)
{
++pok_ret_t+ret;
++uint32_t++args_addr;
++uint32_t++id;

++args_addr+=+(uint32_t)+args;
++id++++++++=+(uint32_t)+syscall_id;

++asm+volatile+(+"movl+%251,%25%25eax++\n\t"
+++++++++++++++++"movl+%252,%25%25ebx++\n\t"
+++++++++++++++++"int+$42++++++++\n\t"
+++++++++++++++++"movl+%25%25eax,+%250+\n\t"
+++++++++++++++++:"=g"(ret)
+++++++++++++++++:"g"(id),+"g"(args_addr)
+++++++++++++++++:+"%25eax"+,+"%25ebx"
+++++++++++++++);
++return+ret;
}|code-block|syntax|javascript|175147|有关这方面的更多信息，OS+Dev+wiki是一个很好的切入点。|offset|length|175148|因此，您不仅可以切换到内核，还可以让内核为您做一些事情。然后内核会告诉你它是否完成了。|175149|entityMap|0|LINK|mutability|MUTABLE|url|http://wiki.osdev.org/System_Call^0|0|0|0|B|B|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@$K|10|L|11|1|12]]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

In user mode you cannot just switch to kernel mode. Interaction between user and kernel is done via system-calls. Each system-call is providing one defined service. The user sends the service name (usually a number) and the required parameters.
Here is a real world example how this is done. It's x86 AT&amp;T style assembler.

It moves the system-call name into the EAX register, the pointer to the parameters into the EBX register of the CPU and then emits the software interrupt number 42. The interrupt handling will do the switch to kernel mode. The interrupt number is looked up in the Interrupt Descriptor Table (IDT) and invokes the function that's registered there, the syscall handler. This handler executes in kernel mode. On return to the user mode the code will move the content of EAX into the variable ret. 

<pre><code>pok_ret_t pok_do_syscall (pok_syscall_id_t syscall_id, pok_syscall_args_t* args)
{
 pok_ret_t ret;
 uint32_t args_addr;
 uint32_t id;

 args_addr = (uint32_t) args;
 id = (uint32_t) syscall_id;

 asm volatile ( "movl %1,%%eax \n\t"
 "movl %2,%%ebx \n\t"
 "int $42 \n\t"
 "movl %%eax, %0 \n\t"
 :"=g"(ret)
 :"g"(id), "g"(args_addr)
 : "%eax" , "%ebx"
 );
 return ret;
}
</code></pre>

The <a href="http://wiki.osdev.org/System_Call" rel="noreferrer">OS Dev wiki</a> is a good point to read more about this. 

So you don't just switch to the kernel, but you can ask the kernel to do something for you. And then the kernel is telling you if it was done or not.

blocks|key|173009|text|这是在第8版中引入的一个打字错误，并保留在第9版中。在第七版的第19页上，它写道：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|173010|173011|“切换到用户模式的指令是特权指令的一个示例。”|blockquote|173012|173013|173014|这显然更有意义。|173015|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|M|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|N|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|O|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Q|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|K|$]]

That is a typo introduced in the 8th edition and kept in the 9th. In the seventh edition, page 19, it says instead:

<blockquote>
 "The instruction to switch to user mode is an example of a privileged
 instruction."
</blockquote>

Which clearly makes a lot more sense.

In my operating systems class, I'm asked whether switching from user to kernel mode is privileged. This is not OS-specific. At first I thought yes, but it seems like a big Catch 22. I referred to my textbook:

<blockquote>
 The hardware allows privileged instructions to be executed only in
 kernel mode. ...
 
 The instruction to switch to kernel mode is an example of a privileged
 instruction.
 
 Gagne, Greg; Abraham Silberschatz; Peter B. Galvin (2010-01-26).
 Operating System Concepts (p. 22). Wiley Higher Ed. Kindle Edition.
</blockquote>

So we start in user mode. To switch to kernel mode requires a privileged instruction. A privileged instruction must be done in kernel mode, therefore we must switch to kernel mode to enable switching to kernel mode.

I'm thinking the system does not allow a user to switch itself to kernel mode directly, but that it is done by the kernel when the user seeks to execute another privileged instruction. Is that correct?

Switching from user mode to kernel mode

在我的操作系统课上，有人问我从用户模式切换到内核模式是否具有特权。这不是特定于操作系统的。起初我认为是，但这似乎是一个大问题。我参考了我的教科书：硬件只允许特权指令在内核模式下执行。..。切换到内核模式的指令是特权指令的一个例子。加尼，格雷格；亚伯拉罕·西尔伯沙茨；彼得·B·加尔文(2010-01-26)。操作系统概念...

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