https://arjunsreedharan.org/post/82710718100/kernels-101-lets-write-a-kernel
神文檔，可操作
寫一下這個文檔的理解過程

**How does an x86 machine boot**
Most registers of the x86 CPU have well defined values after power-on. The Instruction Pointer (EIP) register holds the memory address for the instruction being executed by the processor. EIP is hardcoded to the value **0xFFFFFFF0**. Thus, the x86 CPU is hardwired to begin execution at the physical address 0xFFFFFFF0\. It is in fact, the last 16 bytes of the 32-bit address space. This memory address is called reset vector.

商店以后，特別特殊的(E)IP寄存器，它上電以后固定是一堆（7個）F跟一個0，
上電以后IP從這里開始，那也就是只給留了16Byte的32位空間（每個Byte32位）呀，
這也就夠一個跳轉(zhuǎn)的，也就是reset跳轉(zhuǎn)，這個七個f一個0就叫reset vector
Now, the chipset’s memory map makes sure that 0xFFFFFFF0 is mapped to a certain part of the BIOS, not to the RAM. Meanwhile, the BIOS copies itself to the RAM for faster access. This is called shadowing. The address 0xFFFFFFF0 will contain just a jump instruction to the address in memory where BIOS has copied itself.
既然這樣，主板廠商在做BIOS的時候第一件事兒就是把它的代碼復制到RAM上，這個過程叫Shadowing。
那七F一0地址就特別重要，基本一定BIOS復制過來的代碼里，這個位置肯定是一個跳轉(zhuǎn)唄。
Thus, the BIOS code starts its execution. BIOS first searches for a bootable device in the configured boot device order. It checks for a certain magic number to determine if the device is bootable or not. (whether bytes 511 and 512 of first sector are 0xAA55)
從七F一0跳轉(zhuǎn)一下，跳到BIOS代碼開始的地方，BIOS就這么跑起來了，
這時候BIOS就去找引導設備了，挨個存儲設備看引導扇區(qū)，也就是它們的0扇區(qū)
Once the BIOS has found a bootable device, it copies the contents of the device’s first sector into RAM starting from physical address 0x7c00; and then jumps into the address and executes the code just loaded. This code is called the bootloader.
在哪個設備上看到0xAA55了，那就牛逼了，可以把這個扇區(qū)復制到RAM內(nèi)存的0x7C00地址傻瓜，寫成32位的話，其實是0x00007c00。
之后跳到這個7c00開始執(zhí)行代碼，這種占用一個扇區(qū)大小的代碼叫bootloader
The bootloader then loads the kernel at the physical address 0x100000. The address 0x100000 is used as the start-address for all big kernels on x86 machines.
這個一扇區(qū)的代碼，會把內(nèi)核kernel加載到物理地址0x10 0000，這個地址不就是高位2低位0咩
據(jù)說在所有的大內(nèi)核系統(tǒng)里面，都喜歡用這個地址來作為開始地址，可能也是IBM規(guī)定的
All x86 processors begin in a simplistic 16-bit mode called real mode. The GRUB bootloader makes the switch to 32-bit protected mode by setting the lowest bit of CR0 register to 1. Thus the kernel loads in 32-bit protected mode.
上電以后，都是16比特的實模式，可以通過grub來配置成保護模式，
做法就是把CR0的值配置成1，這樣一整加載內(nèi)核的時候就是按照32位加載了
Do note that in case of linux kernel, GRUB detects linux boot protocol and loads linux kernel in real mode. Linux kernel itself [makes the switch] to protected mode.
Linux的做法是，grub檢測到啟動協(xié)議，之后用16位的實模式來加載內(nèi)核，
之后內(nèi)核運行起來了，由Linux內(nèi)核來切換到保護模式

話說這樣有啥好處呀