A legacy standard for the layout of partition tables on storage devices, introduced in 1983 with PC DOS 2.0. It is located in the very first sector of a hard drive (Sector 0) and contains the information necessary to load the operating system and identify how the disk is divided.
Core Components
- Master Boot Code: A small piece of executable code (446 bytes) that the BIOS loads into memory to initiate the boot process. It scans the partition table to find the Active partition.
- Partition Table: A 64-byte data structure that describes the partitions on the disk. Because each entry is 16 bytes, MBR is hard-coded to support a maximum of 4 Primary Partitions.
- Boot Signature: A 2-byte magic number (0x55AA) at the end of the sector that identifies the disk as a valid bootable device to the BIOS.
Key Limitations
- 2.2 Terabyte Limit: MBR uses 32-bit values to represent the total number of sectors. This mathematically limits the addressable space to 2.2 TB; any capacity beyond this remains "unallocated" and unusable.
- Single Point of Failure: MBR data is stored in only one location. If the first sector of the disk is corrupted or develops a bad block, the entire disk becomes unreadable, and the OS will fail to boot.
- Partition Constraints: To bypass the 4-partition limit, users must create an "Extended Partition," which acts as a container for Logical Drives.
Key Terminology
- Active Partition: The specific primary partition marked as bootable. The MBR code looks for this flag to determine where the OS bootloader is located.
- Extended Partition: A workaround for the 4-partition limit. One primary slot is used to point to a chain of logical partitions.
- LBA (Logical Block Addressing): The method used to specify the location of blocks of data on the drive, which replaced the older Cylinder-Head-Sector (CHS) method.
- VBR (Volume Boot Record): The first sector of an individual partition. The MBR code hands off control to the VBR of the Active partition to continue the boot sequence.
Table: MBR vs. GPT
| Feature | MBR (Legacy) | GPT (Modern) |
| Max Partitions | 4 Primary (or 3 + Extended) | 128 (Windows) |
| Max Disk Size | 2.2 Terabytes | 9.4 Zettabytes |
| Redundancy | None | Backup copy at end of disk |
| Firmware Requirement | BIOS or UEFI in CSM mode | UEFI |
| Integrity | No checksums | CRC32 error checking |
Common Use Cases
- Legacy Systems: Computers running Windows XP or older versions of Linux.
- Small USB Drives: Often used for 32GB or smaller flash drives to ensure maximum compatibility with older TVs, car stereos, and legacy PCs.
- 32-bit Operating Systems: Necessary for older 32-bit systems that cannot boot from GPT.
Technical Note: When you initialize a new disk in Windows Disk Management, it will ask if you want MBR or GPT. For any modern system or drive larger than 2TB, GPT is the correct choice.