Linux Kernel 2.6.37 启动过程笔记 ：MBR

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

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。