Master Boot Record (MBR)
这是古老且长久的分区表格式。
MBR位于磁盘的第0个LBA（logical block addressing），现在linux系统已经没有CHS （Cylinder-Head-Sector）的概念了。
MBR还包括boot code，分区表。分区表只是MBR的一部分。
LBA通常是512字节。
MBR中有4个分区表记录，每个是16字节，共64个字节。偏移从446开始，到510结束。LBA的最后2个字节是boot signature
Partition numberOffset
Partition 10x01BE (446)
Partition 20x01CE (462)
Partition 30x01DE (478)
Partition 40x01EE (494)
16个字节的分区内容是：
| Element (offset) | Size | Description |
|---|---|---|
| 0 | byte | Boot indicator bit flag: 0 = no, 0x80 = bootable (or "active") |
| 1 | byte | Starting head |
| 2 | 6 bits | Starting sector (Bits 6-7 are the upper two bits for the Starting Cylinder field.) |
| 3 | 10 bits | Starting Cylinder |
| 4 | byte | System ID |
| 5 | byte | Ending Head |
| 6 | 6 bits | Ending Sector (Bits 6-7 are the upper two bits for the ending cylinder field) |
| 7 | 10 bits | Ending Cylinder |
| 8 | uint32_t | Relative Sector (to start of partition -- also equals the partition's starting LBA value) |
| 12 | uint32_t | Total Sectors in partition |
Relative sector是分区的起始扇区。
Total sectors是分区的扇区数。
这两个字段是小端存储的。
一个32M的loop device设备，创建两个分区，分别为4M和8M。sfdisk的显示：
# sfdisk -l /dev/loop17
Disk /dev/loop17: 32 MiB, 33554432 bytes, 65536 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x0cfdb114
Device Boot Start End Sectors Size Id Type
/dev/loop17p1 2048 10239 8192 4M 83 Linux
/dev/loop17p2 10240 26623 16384 8M 83 Linux
打印出MBR内容如下：
1 2 3 4 5 6 7 8 9 | # hexdump -C -n 512 /dev/loop1700000000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|*000001b0 ff ff ff ff ff ff ff ff 14 b1 fd 0c 00 00 00 20 |............... |000001c0 21 00 83 a2 22 00 00 08 00 00 00 20 00 00 00 a2 |!..."...... ....|000001d0 23 00 83 a7 26 01 00 28 00 00 00 40 00 00 00 00 |#...&..(...@....|000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|00000200 |
第一个分区从0x1be开始，内容就是：00 20 21 00 83 a2 22 00 00 08 00 00 00 20 00 00
第二个分区，紧跟第一个分区，内容：00 a2 23 00 83 a7 26 01 00 28 00 00 00 40 00 00
可以看到第一个分区的起始扇区是：0x00000800，也就是2048。大小是：0x00002000，也就是8192.
和sfdisk是完全吻合的。
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