A Master Boot Record (MBR) is the sector at cylinder 0, head 0, sector 1 of a hard disk. A MBR-like structure must be created in each of partitions by the FDISK program.
At the completion of your system's Power On Self Test (POST), INT 19H is called. Usually INT 19 tries to read a boot sector from the first floppy drive(12). If a boot sector is found on the floppy disk, that boot sector is read into memory at location 0000:7C00 and INT 19H jumps to memory location 0000:7C00. However, if no boot sector is found on the first floppy drive, INT 19H tries to read the MBR from the first hard drive. If an MBR is found it is read into memory at location 0000:7C00 and INT 19H jumps to memory location 0000:7C00. The small program in the MBR will attempt to locate an active (bootable) partition in its partition table(13). The small program in the boot sector must locate the first part of the operating system's kernel loader program (or perhaps the kernel itself or perhaps a boot manager program) and read that into memory.
INT 19H is also called when the CTRL-ALT-DEL keys are used. On most systems, CTRL-ALT-DEL causes an short version of the POST to be executed before INT 19H is called.
The stuff is:
However, the first 62 bytes of a boot sector are known as the BIOS Parameter Block (BPB), so GRUB cannot use these bytes for its own purpose.
If an active partition is found, that partition's boot record is read
into 0000:7C00 and the MBR code jumps to 0000:7C00 with SI
pointing to the partition table entry that describes the partition being
booted. The boot record program uses this data to determine the drive
being booted from and the location of the partition on the disk.
The first byte of an active partition table entry is 80. This byte is
loaded into the DL
register before INT 13H is called to read the
boot sector. When INT 13H is called, DL
is the BIOS device
number. Because of this, the boot sector read by this MBR program can
only be read from BIOS device number 80 (the first hard disk). This is
one of the reasons why it is usually not possible to boot from any other
hard disk.
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