A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBRs was publicly introduced in 1983 with PC DOS 2.0.
The MBR holds the information on how the logical partitions, containing file systems, are organized on that medium. Besides that, the MBR also contains executable code to function as a loader for the installed operating system—usually by passing control over to the loader’s second stage, or in conjunction with each partition’s volume boot record (VBR). This MBR code is usually referred to as a boot loader.
The organization of the partition table in the MBR limits the maximum addressable storage space of a disk to 2 TB (232 × 512 bytes). Therefore, the MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. A GPT can coexist with an MBR in order to provide some limited form of a backwards compatibility for older systems.
GUID Partition Table (GPT) is a standard for the layout of the partition table on a physical hard disk, using globally unique identifiers (GUID). Although it forms a part of the Unified Extensible Firmware Interface (UEFI) standard (Unified EFI Forum proposed replacement for the PC BIOS), it is also used on some BIOS systems because of the limitations of master boot record (MBR) partition tables, which use 32 bits for storing logical block addresses (LBA) and size information.
MBR-based partition table schemes insert the partitioning information for (usually) four “primary” partitions in the master boot record (MBR) (which on a BIOS system is also the container for code that begins the process of booting the system). In a GPT, the first sector of the disk is reserved for a “protective MBR” such that booting a BIOS-based computer from a GPT disk is supported, but the boot loader and O/S must both be GPT-aware. Regardless of the sector size, the GPT header begins on the second logical block of the device.
GPT uses modern logical block addressing (LBA) in place of the cylinder-head-sector (CHS) addressing used with MBR. Legacy MBR information is contained in LBA 0, the GPT header is in LBA 1, and the partition table itself follows. In 64-bit Windows operating systems, 16,384 bytes, or 32 sectors, are reserved for the GPT, leaving LBA 34 as the first usable sector on the disk.
MBR vs. GPT
Compared with MBR disk, A GPT disk can support larger than 2 TB volumes where MBR cannot. A GPT disk can be basic or dynamic, just like an MBR disk can be basic or dynamic. GPT disks also support up to 128 partitions rather than the 4 primary partitions limited to MBR. Also, GPT keeps a backup of the partition table at the end of the disk. Furthermore, GPT disk provides greater reliability due to replication and cyclical redundancy check (CRC) protection of the partition table.
The GUID partition table (GPT) disk partitioning style supports volumes up to 18 exabytes in size and up to 128 partitions per disk, compared to the master boot record (MBR) disk partitioning style, which supports volumes up to 2 terabytes in size and up to 4 primary partitions per disk (or three primary partitions, one extended partition, and unlimited logical drives). Unlike MBR partitioned disks, data critical to platform operation is located in partitions instead of unpartitioned or hidden sectors. In addition, GPT partitioned disks have redundant primary and backup partition tables for improved partition data structure integrity.
In IBM PC compatible computers, the Basic Input/Output System (BIOS), also known as System BIOS, ROM BIOS or PC BIOS, is a de facto standard defining a firmware interface. The name originated from the Basic Input/Output System used in the CP/M operating system in 1975. The BIOS software is built into the PC, and is the first software run by a PC when powered on.
The fundamental purposes of the BIOS are to initialize and test the system hardware components, and to load a bootloader or an operating system from a mass memory device. The BIOS additionally provides abstraction layer for the hardware, i.e. a consistent way for application programs and operating systems to interact with the keyboard, display, and other input/output devices. Variations in the system hardware are hidden by the BIOS from programs that use BIOS services instead of directly accessing the hardware. Modern operating systems ignore the abstraction layer provided by the BIOS and access the hardware components directly.
The Unified Extensible Firmware Interface (UEFI) (pronounced as an initialism U-E-F-I or like “unify” without the n) is a specification that defines a software interface between an operating system and platform firmware. UEFI is meant to replace the Basic Input/Output System (BIOS) firmware interface, present in all IBM PC-compatible personal computers. In practice, most UEFI images provide legacy support for BIOS services. UEFI can support remote diagnostics and repair of computers, even without another operating system.
The original EFI (Extensible Firmware Interface) specification was developed by Intel. Some of its practices and data formats mirror ones from Windows.] In 2005, UEFI deprecated EFI 1.10 (final release of EFI). The UEFI specification is managed by the Unified EFI Forum.
BIOS vs. UEFI
UEFI enables better use of bigger hard drives. Though UEFI supports the traditional master boot record (MBR) method of hard drive partitioning, it doesn’t stop there. It’s also capable of working with the GUID Partition Table (GPT), which is free of the limitations the MBR places on the number and size of partitions. GPT ups the maximum partition size from 2.19TB to 9.4 zettabytes.
UEFI may be faster than the BIOS. Various tweaks and optimizations in the UEFI may help your system boot more quickly it could before. For example: With UEFI you may not have to endure messages asking you to set up hardware functions (such as a RAID controller) unless your immediate input is required; and UEFI can choose to initialize only certain components. The degree to which a boot is sped up will depend on your system configuration and hardware, so you may see a significant or a minor speed increase.
Technical changes abound in UEFI. UEFI has room for more useful and usable features than could ever be crammed into the BIOS. Among these are cryptography, network authentication, support for extensions stored on non-volatile media, an integrated boot manager, and even a shell environment for running other EFI applications such as diagnostic utilities or flash updates. In addition, both the architecture and the drivers are CPU-independent, which opens the door to a wider variety of processors (including those using the ARM architecture, for example).
However, UEFI is still not widespread. Though major hardware companies have switched over almost exclusively to UEFI use, you still won’t find the new firmware in use on all motherboards—or in quite the same way across the spectrum. Many older and less expensive motherboards also still use the BIOS system.
Converting from MBR to GPT
One of the more unusual features of gdisk is its ability to read an MBR partition table or BSD disklabel and convert it to GPT format without damaging the contents of the partitions on the disk. This feature exists to enable upgrading to GPT in case the limitations of MBRs or BSD disklabels become too onerous—for instance, if you want to add more OSes to a multi-boot configuration, but the OSes you want to add require too many primary partitions to fit on an MBR disk.
Conversions from MBR to GPT works because of inefficiencies in the MBR partitioning scheme. On an MBR disk, the bulk of the first cylinder of the disk goes unused—only the first sector (which holds the MBR itself) is used. Depending on the disk’s CHS geometry, this first cylinder is likely to be sufficient space to store the GPT header and partition table. Likewise, space is likely to go unused at the end of the disk because the cylinder (as seen by the BIOS and whatever tool originally partitioned the disk) will be incomplete, so the last few sectors will go unused. This leaves space for the backup GPT header and partition table. (Disks partitioned with 1 MiB alignment sometimes leave no gaps at the end of the disk, which can prevent conversion to GPT format—at least, unless you delete or resize the final partition.)
The task of converting MBR to GPT therefore becomes one of extracting the MBR data and stuffing the data into the appropriate GPT locations. Partition start and end points are straightforward to manage, with one important caveat: GPT fdisk ignores the CHS values and uses the LBA values exclusively. This means that the conversion will fail on disks that were partitioned with very old software. If the disk is over 8 GiB in size, though, GPT fdisk should find the data it needs.
Once the conversion is complete, there will be a series of gaps between partitions. Gaps at the start and end of the partition set will be related to the inefficiencies mentioned earlier that permit the conversion to work. Additional gaps before each partition that used to be a logical partition exist because of inefficiencies in the way logical partitions are allocated. These gaps are likely to be quite small (a few kilobytes), so you’re unlikely to be able to put useful partitions in those spaces. You could resize your partitions with GNU Parted to remove the gaps, but the risks of such an operation outweigh the very small benefits of recovering a few kilobytes of disk space.