From 5f8a0701b0162c2ed5676e765e336b7eebac2bc9 Mon Sep 17 00:00:00 2001
From: hailin cai <hailin.cai.neulion@gmail.com>
Date: Fri, 11 Mar 2016 08:17:49 -0800
Subject: [PATCH] Update Booting/linux-bootstrap-3.md

---
 Booting/linux-bootstrap-3.md | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/Booting/linux-bootstrap-3.md b/Booting/linux-bootstrap-3.md
index ca43674..e97bedc 100644
--- a/Booting/linux-bootstrap-3.md
+++ b/Booting/linux-bootstrap-3.md
@@ -288,13 +288,32 @@ static int vga_set_mode(struct mode_info *mode)
 在切换到保护模式之前的最后的准备工作
 --------------------------------------------------------------------------------
 
-We can see the last function call - `go_to_protected_mode` - in [main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L184). As the comment says: `Do the last things and invoke protected mode`, so let's see these last things and switch into protected mode.
+在进入保护模式之前的最后一个函数调用发生在 [main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c#L184) 中的 `go_to_protected_mode` 函数，就像这个函数的注释说的，这个函数将进行最后的准备工作然后进入保护模式，线面就让我们来具体看看这个最后的准备工作是什么，以及如何切换如保护模式的。
 
-`go_to_protected_mode` is defined in [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pm.c#L104). It contains some functions which make the last preparations before we can jump into protected mode, so let's look at it and try to understand what they do and how it works.
+`go_to_protected_mode` 函数本身定义在 [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pm.c#L104)。 这个函数调用了一些其他的函数进行最后的准备工作，下面就让我们来看看这些函数是如何工作的。
 
-First is the call to the `realmode_switch_hook` function in `go_to_protected_mode`. This function invokes the real mode switch hook if it is present and disables [NMI](http://en.wikipedia.org/wiki/Non-maskable_interrupt). Hooks are used if the bootloader runs in a hostile environment. You can read more about hooks in the [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) (see **ADVANCED BOOT LOADER HOOKS**).
+`go_to_protected_mode` 函数首先调用的 `realmode_switch_hook` 函数，后者如果发现 `realmode_swtch` hook， 那么将调用它并禁止 [NMI](http://en.wikipedia.org/wiki/Non-maskable_interrupt) 中断，反之将直接禁止 NMI 中断。只有当 bootloader 运行在特殊的环境下（比如 DOS ）才会被使用。你可以在 [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) (see **ADVANCED BOOT LOADER HOOKS**) 中详细了解 hook 函数的信息。
 
-The `realmode_switch` hook presents a pointer to the 16-bit real mode far subroutine which disables non-maskable interrupts. After `realmode_switch` hook (it isn't present for me) is checked, disabling of Non-Maskable Interrupts(NMI) occurs:
+```c
+/*
+ * Invoke the realmode switch hook if present; otherwise
+ * disable all interrupts.
+ */
+static void realmode_switch_hook(void)
+{
+	if (boot_params.hdr.realmode_swtch) {
+		asm volatile("lcallw *%0"
+			     : : "m" (boot_params.hdr.realmode_swtch)
+			     : "eax", "ebx", "ecx", "edx");
+	} else {
+		asm volatile("cli");
+		outb(0x80, 0x70); /* Disable NMI */
+		io_delay();
+	}
+}
+```
+
+`realmode_switch` 指向了一个16 位实模式代码地址（远跳转指针），这个16位代码将禁止 NMI 中断。所以在上述代码中，如果 `realmode_swtch` hook 存在，代码是用了 `lcallw` 指令进行远函数调用。在我的环境中，因为不存在这个 hook ，所以代码是直接进入 `else` 部分进行了 NMI 的禁止：
 
 ```assembly
 asm volatile("cli");
@@ -302,7 +321,7 @@ outb(0x80, 0x70);	/* Disable NMI */
 io_delay();
 ```
 
-At first there is an inline assembly instruction with a `cli` instruction which clears the interrupt flag (`IF`). After this, external interrupts are disabled. The next line disables NMI (non-maskable interrupt).
+上面的代码首先调用 `cli` 汇编指令清楚了中断标志 `IF`，这条指令执行之后，外部中断就被禁止了，紧接着的下一行代码就禁止了 NMI 中断。
 
 An interrupt is a signal to the CPU which is emitted by hardware or software. After getting the signal, the CPU suspends the current instruction sequence, saves its state and transfers control to the interrupt handler. After the interrupt handler has finished it's work, it transfers control to the interrupted instruction. Non-maskable interrupts (NMI) are interrupts which are always processed, independently of permission. It cannot be ignored and is typically used to signal for non-recoverable hardware errors. We will not dive into details of interrupts now, but will discuss it in the next posts.