综述
为什么想起写这个类似于读书笔记的翻译呢，主要是MAC内核开发的资料基本都是en文的，中文的几乎没有（反正我没找到，你要有麻烦给一个，不甚感谢），而读en的资料对初学者来说又比较费时间，所以时间够的话我会把觉得重要的东西记下来，以飨读者。
一、Why You Should Avoid Programming in the Kernel
  一个内核bug可能导致整个系统Crash，所以应尽可能避免内核编程，而选择在用户空间编码。
二、The Early Boot Process
  和普通PC的BIOS类似，苹果机使用Boot ROM来引导和加载系统，大体流程如下：
Boot ROM
When the power to a Macintosh computer is turned on, the BootROM firmware is activated. BootROM (which is part of the computer’s hardware) has two primary responsibilities: it initializes system hardware and it selects an operating system to run. BootROM has two components to help it carry out these functions:
    •    POST (Power-On Self Test) initializes some hardware interfaces and verifies that sufficient memory is available and in a good state.
    •    EFI does basic hardware initialization and selects which operating system to use.
If multiple installations of Mac OS X are available, BootROM chooses the one that was last selected by the Startup Disk System Preference. The user can override this choice by holding down the Option key while the computer boots, which causes EFI to display a screen for choosing the boot volume.

The Boot Loader
Once BootROM is finished and a Mac OS X partition has been selected, control passes to the boot.efi boot loader. The principal job of this boot loader is to load the kernel environment. As it does this, the boot loader draws the “booting” image on the screen.
If full-disk encryption is enabled, the boot loader is responsible for drawing the login UI and prompting for the user’s password, which needed to access the encrypted disk to boot from it. (This UI is drawn by loginwindow otherwise.)
In the simplest case, the boot loader can be found in the /System/Library/CoreServices directory on the root partition, in a file named boot.efi .
Note:  Booting from a UFS volume is deprecated as of Mac OS X v10.5.
In order to speed up boot time, the boot loader uses several caches. The contents and location of these caches varies between versions of Mac OS X, but knowing some details about the caching may be helpful when debugging kernel extensions.
After you install or modify a kernel extension, touch the /System/Library/Extensions directory; the system rebuilds the caches automatically.
Important:  You should not depend on the implementation details of the kernel caches in your software.
In Mac OS X v10.7, the boot loader looks for the unified prelinked kernel. This cache contains all kernel extensions that may be needed to boot a Mac with any hardware configuration, with the extensions already linked against the kernel. It is located at /System/Library/Caches/com.apple.kext.caches/Startup/kernelcache .
In Mac OS X v10.6 and earlier, the boot loader first looks for the prelinked kernel (also called the kernel cache). This cache contains exactly the set of kernel extensions that were needed during the previous system startup, already linked against the kernel. If the prelinked kernel is missing or unusable (for example, because a hardware configuration has changed), the booter looks for the mkext cache, which contains all kernel extensions that may be needed to boot the system. Using the mkext cache is much slower because the linker must be run. On Mac OS X v10.5 and v10.6, these caches are located in /System/Library/Caches/com.apple.kext.caches/Startup/ ; on previous versions of Mac OS X, it was located at /System/Library/Caches/com.apple.kernelcaches/ .
Finally, if the caches cannot be used, the boot loader searches /System/Library/Extensions for drivers and other kernel extensions whose OSBundleRequired property is set to a value appropriate to the type of boot (for example, local or network boot). This process is very slow, because the Info.plist file of every kernel extension must be parsed, and then the linker must be run.
For more information on how drivers are loaded, see I/O Kit Fundamentals , the manual page for kextcache , and Kernel Extension Programming Topics .

Rooting
Once the kernel and all drivers necessary for booting are loaded, the boot loader starts the kernel’s initialization procedure. At this point, enough drivers are loaded for the kernel to find the root device.
The kernel initializes the Mach and BSD data structures and then initializes the I/O Kit . The I/O Kit links the loaded drivers into the kernel, using the device tree to determine which drivers to link. Once the kernel finds the root device, it roots(*) BSD off of it.
Note:  As a terminology aside, the term “boot” was historically reserved for loading a bootstrap loader and kernel off of a disk or partition. In more recent years, the usage has evolved to allow a second meaning: the entire process from initial bootstrap until the OS is generally usable by an end user. In this case, the term is used according to the former meaning.
As used here, the term “root” refers to mounting a partition as the root, or top-level, filesystem. Thus, while the OS boots off of the root partition, the kernel roots the OS off of the partition before executing startup scripts from it.
Boot≠Root is a technology that allows the system to boot from a partition other than the root partition. This is used to boot systems where the root partition is encrypted using full-disk encryption, or where the root partition is located on a device which requires additional drivers (such as a RAID array). Boot≠Root uses a helper partition to store the files needed to boot, such as the kernel cache. For more information on how to set up the property in a filter-scheme driver, see “Developing a Filter Scheme” in Mass Storage Device Driver Programming Guide .
三、参考资料
Information on the Web
Apple maintains several websites where developers can go for general and technical information on Mac OS X.
    •    Apple Developer Connection: Developer Documentation (http://developer.apple.com/documentation/ ). Features the same documentation that is installed on Mac OS X, except that often the documentation is more up-to-date. Also includes legacy documentation.
    •    Apple Developer Connection: Mac OS X (http://developer.apple.com/devcenter/macosx/ ). Offers SDKs, release notes, product notes and news, and other resources and information related to Mac OS X.
    •    AppleCare Tech Info Library (http://www.apple.com/support/ ). Contains technical articles, tutorials, FAQs, technical notes, and other information.
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原书地址：http://developer.apple.com/library/mac/#documentation/Darwin/Conceptual/KernelProgramming/About/About.htm