In computer hardware, GUID Partition Table (GPT) is a standard for the layout of the partition table on a physical hard disk. Typical PC hardware A typical Personal computer consists of a case or chassis in a tower shape (desktop and the following parts Motherboard A Globally Unique Identifier or GUID (ˈguːɪd or /ˈgwɪd/ is a special type of identifier used in Software applications in order to provide a reference number Disk partitioning is the creation of separate divisions of a Hard disk drive using Partition editors Once a disk is divided into several partitions directories and A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device It is a part of the Extensible Firmware Interface (EFI) standard proposed by Intel as a replacement for the obsolescent PC BIOS, one of the few remaining relics of the original IBM PC. The Extensible Firmware Interface ( EFI) is a specification that defines a software Interface between an Operating system and platform Firmware In Computing, the BIOS (ˈbaɪoʊs EFI uses GPT where BIOS uses a Master Boot Record (MBR). MBRs and disk partitioning MBRs and system bootstrapping On IA-32 IBM PC compatible machines using the MBR Partition Table scheme the bootstrapping

Diagram illustrating the layout of the GUID Partition Table scheme. Each logical block (LBA) is 512 bytes in size. LBA addresses that are negative indicate position from the end of the volume, with −1 being the last addressable block.

Features

Current PC BIOS schemes use a Master Boot Record (MBR) to begin the process of initializing the disk. The MBR begins with an entry called the Master Boot Code, which contains an executable binary for the purpose of identifying and booting the active partition. Disk partitioning is the creation of separate divisions of a Hard disk drive using Partition editors Once a disk is divided into several partitions directories and PC BIOS's replacement, EFI, itself contains this capability but to maintain backwards compatibility, GPT retains the MBR entry as the first sector on the disk followed by a Primary Partition Table Header, the actual beginning of GPT. The Extensible Firmware Interface ( EFI) is a specification that defines a software Interface between an Operating system and platform Firmware

GPT uses modern logical block addressing (LBA) in place of the cylinder-head-sector (CHS) addressing used with MBR. Logical block addressing (LBA is a common scheme used for specifying the location of blocks of data stored on Computer storage devices generally Secondary storage Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a Hard disk drive. 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. Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination In the context of Computer Disk storage, a sector is a subdivision of a track (Figure 1 item A on a Magnetic disk or Optical disc

GPT also provides for redundancy. The GPT header and partition table are written at both the beginning and end of the disk.

Legacy MBR (LBA 0)

The primary purpose of the MBR at the beginning of the disk is to prevent MBR-based disk utilities from mis-recognizing, and possibly over-writing, GPT disks. A single partition, encompassing the entire GPT drive, is indicated. The System ID for the partition is set to 0xEE, indicating that it uses GPT. Because of this, EFI ignores the MBR. Some 32-bit OSes which cannot read GPT disks nevertheless recognize this ID and present the disk as an inaccessible GPT disk. Older OSes will generally recognize the disk as containing one partition of unknown type and no empty space, and then they'll typically also refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This way, accidental erasures are prevented.

Partition table header (LBA 1)

The partition table header defines the blocks on the disk that can be utilized by the user (the usable blocks). It also defines the number and size of the partition entries that make up the partition table. On 64-bit Windows Server 2003 machines, there are 128 partition entries reserved, each 128 bytes long. Thus, 128 partitions can be created.

The header contains the disk GUID (Globally Unique Identifier). A Globally Unique Identifier or GUID (ˈguːɪd or /ˈgwɪd/ is a special type of identifier used in Software applications in order to provide a reference number It records its own size and location (always LBA 1) and the size and location of the secondary GPT header and table (always the last sectors on the disk). Importantly, it also contains a CRC32 checksum for itself and for the partition table, which is verified by EFI processes on boot. Because EFI uses and verifies this checksum, hex editors should not be used to modify the contents of the GPT. Such modification would render the checksum invalid. In this case, EFI would overwrite the primary GPT with the secondary one, or, if both GPTs contained invalid checksums, would be unable to access the disk.

Partition entries (LBA 2–33)

Partition entries are simple and straightforward. A cyclic redundancy check (CRC is a type of function that takes as input a data stream of any length and produces as output a value of a certain space commonly a 32-bit integer A Universally Unique Identifier (UUID is an identifier standard used in Software construction standardized by the Open Software Foundation (OSF as part of the The first 16 bytes designate the partition type GUID. For example, the GUID for an EFI System partition is {C12A7328-F81F-11D2-BA4B-00A0C93EC93B}. The second 16 bytes contain a GUID unique to the partition. Starting and ending 64-bit LBAs are also recorded here, and space is allocated for partition names and attributes. Unfortunately, there is no central registry for GUID partition type designators, either de jure or de facto.

Partition type GUIDs

1. ^  The GUIDs in this table are written assuming a little-endian byte order. The EFI System partition is a partition on a Data storage device that is used by machines that adhere to the Extensible Firmware Interface. The BIOS Boot Partition is a partition on a Data storage device that may be used by standard BIOS -based machines in order to boot when the partition table Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft. The Microsoft Reserved Partition, or MSR, is a partition on a Data storage device that is labelled with Globally Unique Identifier (GUID equal The Logical Disk Manager is an implementation of a logical volume manager for Microsoft Windows NT, developed by Microsoft and Veritas Software HP-UX (Hewlett Packard UniX is Hewlett-Packard 's proprietary implementation of the Unix Operating system, based on System V (initially Linux (commonly pronounced ˈlɪnəks LVM is a logical volume manager for the Linux kernel: it manages disk drives and similar mass-storage devices in particular large ones FreeBSD is a Unix-like free Operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD branch through The Unix file system ( UFS) is a File system used by many Unix and Unix-like operating systems Vinum is a logical volume manager, also called Software RAID, allowing implementations of the RAID-0, RAID-1 and RAID-5 models both individually In Computing, ZFS is a File system designed by Sun Microsystems for the Solaris Operating System. Mac OS X (mæk oʊ ɛs tɛn is a line of computer Operating systems developed marketed and sold by Apple Inc, the latest of which is pre-loaded on all currently Hierarchical File System ( HFS) is a File system developed by Apple Inc Apple Inc, ( formerly Apple Computer Inc, is an American Multinational corporation with a focus on designing and manufacturing Consumer electronics The Unix file system ( UFS) is a File system used by many Unix and Unix-like operating systems In Computing, ZFS is a File system designed by Sun Microsystems for the Solaris Operating System. Solaris is a Unix -based Operating system introduced by Sun Microsystems in 1992 as the successor to SunOS. NetBSD is a freely redistributable Open source version of the Unix -derivative Berkeley Software Distribution (BSD Computer Operating The Unix file system ( UFS) is a File system used by many Unix and Unix-like operating systems The Log-Structured File System (or LFS) is an implementation of a Log-structured file system (a concept originally proposed and implemented by John Ousterhout For example, the GUID for an EFI System partition is written as C12A7328-F81F-11D2-BA4B-00A0C93EC93B here, which corresponds to the 16 byte sequence 28 73 2A C1 1F F8 D2 11 BA 4B 00 A0 C9 3E C9 3B — only the first three blocks are byte-swapped.
2. ^a b  Linux and Windows use the same GUID for their respective data partitions.
3. ^a b  The GUID for /usr on Solaris is used as a generic GUID for ZFS by Mac OS X.
4. ^{^}  Definitions are in src/sys/sys/disklabel_gpt. h. NetBSD had used the FreeBSD GUIDs before unique NetBSD-specific GUIDs were created.

See also

• Master boot record
• GUID
• EFI
• Disk partitioning

External links

• Microsoft TechNet: Disk Sectors on GPT Disks
• Microsoft TechNet: Using GPT Drives on x86-64 Systems