Master boot record
- First sector of a physical hard disk.
- This sector is not a part of any partition (OS or data).
- Contains two parts:
- MBR bootstrap program to locates the boot sector of other partitions.
- Partition table contains a master list of all available partitions on all available disks.
- Master boot record is detected by boot signature (magic number).
MBR Partition table
- Apply to a hard disk drive, use to manage partitions in disk
- 16 bytes, total’s 64 bytes. Limited to 4 entries. To create more, use an extended partition.
- Maximum size of a single MBR partition is 2TB.
- Boot indicator: inactive - active/bootable partition
- Bits to define Starting head, sector and Cylinder
- System ID: define filesystem of the partition (ext2, fat32, NTFS, etc.)
- Relative sector
- Total sectors in the partition
- A solution to add more than 4 partitions to the partition table.
- Partition can have one and only one entry with SystemID = 0x5 or 0xF to define as an extended partition.
- Extended partition is a physical partition that’s subpartitioned into “logical” partitions.
- partition table entry describing it has a StartLBA and NumSectors that describe the space inside which any number of logical partitions may sit.
- At the start of an extended partition is an extended partition table. Format of extended partition table is same with MBR partition table.
Master Boot Record sequence
- After BIOS finish their job, they load the first sector (512 bytes) of devices (Hardisk, disc, USB, etc.) to check boot signature.
- The boot signature is called a magic number. If a device has a boot signature, it’s can bootable.
- Boot signature is in a boot sector number 0.
- Contains byte sequence: 0x55, 0xAA at byte offets 510 (0x1FE) and 511 (0x1FF).
- 0x1FE and 0x1FF is 2 last bytes of 512 bytes of the first sector.
- When BIOS recognize a boot device, master boot record is loaded into memory at 0x0000:0x7c00 (sector 0, address 0x7c00) from this device. It contains executable code.
- Execution is transferred to the loaded boot record bootstrap program.
Master boot record bootstrap program:
- Currently, the DL register is set to “drive number” that MBR was loaded.
- Master boot record process relocates itself away from 0x7c00: memory copy and far jump.
- Determine which partition/device to boot OS. If the user selects inactive partition, then set the partition chosen entry to active and clear active state from others.
- Rewrite MBR if the partition table entries were modified.
- Load Volume Boot Record (boot sector of bootloader) from selected partition.
- Set DS:SI pointing to the selected partition table entry.
- Jump to 0x7c00 (with CS set to 0, and DL set to the “drive number”).
Volume boot record
- First sector of the first partition containing the OS.
- Contains boot loader code and some file system info.
- A program is in the boot sector (master boot record or Volume boot record).
- Use to loads OS kernel.