Linux Kernel 2.6.37 启动过程笔记 ：MBR

http://blog.chinaunix.net/uid-1701789-id-125510.html

PC加电的过程就是将cs:ip指向 0xFFFF0，这里是BIOS的内容。BIOS接下来就接管系统启动，负责检查并初始化硬件设备，初始化实模式下中断向量表。然后i386系统就会将硬盘的第一个扇区(0扇区）加载到内存 0x7c00处---这就是MBR。

最简单的MBR的结构如下：

tructure of a master boot record

Address			Description		Size in bytes
Hex	Oct	Dec
0000	0000	0	code area		440 (max. 446)
01B8	0670	440	disk signature (optional)		4
01BC	0674	444	Usually nulls; 0x0000		2
01BE	0676	446	Table of primary partitions (Four 16-byte entries, IBM partition table scheme)		64
01FE	0776	510	55h	MBR signature; 0xAA55	2
01FF	0777	511	AAh
MBR, total size: 446 + 64 + 2 =					512

Layout of one 16-byte partition record

Offset	Field length (bytes)	Description
0x00	1	status (0x80 = bootable (active), 0x00 = non-bootable, other = invalid ) [a]
0x01	3	CHS address of first absolute sector in partition. [b] The format is described by 3 bytes, see the next 3 rows.
0x01	1	h7-0 head [c] x x x x x x x x
0x02	1	c9-8 s5–0 sector is in bits 5–0; bits 9–8 of cylinder are in bits 7–6 [c] x x x x x x x x
0x03	1	c7-0 bits 7–0 of cylinder [d] x x x x x x x x
0x04	1	partition type[5][6]
0x05	3	CHS address of last absolute sector in partition. [b] The format is described by 3 bytes, see the next 3 rows.
0x05	1	h7-0 head [c] x x x x x x x x
0x06	1	c9-8 s5–0 sector is in bits 5–0, bits 9–8 of cylinder are in bits 7–6 [c] x x x x x x x x
0x07	1	c7-0 bits 7–0 of cylinder [d] x x x x x x x x
0x08	4	LBA of first absolute sector in the partition [e]
0x0C	4	Number of sectors in partition, in little-endian format [e]
Notes: ^ a Formally, status values other than 0x00 and 0x80 are undefined. ^ b Starting Sector fields are limited to 1024 cylinders, 255 heads, and 63 sectors;[citation needed] Ending Sector fields have the same limitations. ^ c The range for sector is 1 through 63; the range for cylinder is 0 through 1023; the range for head is 0 through 254 inclusive.[citation needed] ^ d The 10-bit cylinder value is split into two bytes. The value can be reassembled using this formula: cylinder = ((byte[2] & 0xc0) << 2) | byte[3] ^ e Used by OSes only in certain circumstances; in such cases the CHS addresses are ignored.[7]

从上图中可以知道MBR已经知道硬盘的分区结构（最多四个主分区--16X4），并且知道哪个分区是活动分区。MBR找到活动分区并且把该活动分区的第一个扇区内容加载到 0x7c00的地方将自己覆盖掉，并将控制权交给0x7c00处的OS启动代码。在Linux里，这就是arch/boot/header.S

MBR实际就是Boot Loader,以上就是最简单的BootLoader的例子 ，现在的Bootloader一般都比这个复杂。一般来说一个BootLoader会占据不止一个扇区以支持多重启动。