why need preserve 1st sector (mbr) when booting from SDMMC -iMX51

Fdisk 一個 512M SD card，進入後： WARNING: DOS-compatible mode is deprecated. It's strongly recommended to switch off the mode (command 'c') and change display units to sectors (command 'u'). 用 p 顯示： Disk /dev/sdb: 507 MB, 507379712 bytes 16 heads, 61 sectors/track, 1015 cylinders Units = cylinders of 976 * 512 = 499712 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x6f20736b follow prompt，用 c command 關掉 DOS-compatible mode，就變成： Disk /dev/sdb: 507 MB, 507379712 bytes 16 heads, 61 sectors/track, 1015 cylinders, total 990976 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 Disk identifier: 0x6f20736b 可以看到，total unit 變成 total sectors。 bootable SD card 要preserve mbr 的原因是，ref xxx_Linux_UG.pdf: bootloader 需要看 mbr 內/後 的 partition table，好知道boot partition 的位置。 所以partition 好後，燒 bootloader, uImage 時，要把 mbr sector 保存起來，以免燒 bootloader 時被清掉。