GUID Partition Table

The layout of a disk with the GUID Partition Table. In this example, each logical block is 512 bytes in size and each entry has 128 bytes. The corresponding partition entries are assumed to be located in LBA‑byte sectors, the MBR partition table entries allow a maximum size of 2 TiB (2 × 512‑bytes) or 9.44 ZB (9.44 × 10 bytes). For disks with 4,096‑byte sectors (Advanced Format). For compatibility with legacy hardware and software, those drives include an emulation technology (512e) that presents 512‑byte write operation. Since April 2014, enterprise-class drives without emulation technology (4K native) have been available on the market.

Readiness of the support for 4 KB logical sectors within operating systems differs among their types, vendors and versions. For example, Microsoft Windows supports 4K native drives since Windows 8 and Windows Server 2012 (both released in 2012) in UEFI.

MBR vs. GPT partitioning and boot sequence visualized (systems using BIOS firmware). See GRUB2.

Like MBR, GPT uses logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing. The protective MBR is stored at LBA 0, and the GPT header is in LBA 1. The GPT header has a pointer to the partition table (Partition Entry Array), which is typically at LBA 2. Each entry in the partition table has the same size, which is 128 or 256 or 512, etc., bytes; typically this size is 128 bytes. The UEFI specification stipulates that a minimum of 16,384 bytes, regardless of sector size, are allocated for the Partition Entry Array. Thus, on a disk with 512-byte sectors, at least 32 sectors are used for the Partition Entry Array, and the first usable block is at LBA 34 or higher, while on a 4,096-byte sector disk, at least 4 sectors are used for the Partition Entry Array, and the first usable block is at LBA 6 or higher. In addition to the primary GPT header and Partition Entry Array, stored at the beginning of the disk, there is a backup GPT header and Partition Entry Array, stored at the end of the disk. The backup GPT header must be at the last block on the disk (LBA -1) and the backup Partition Entry Array is placed between the end of the last partition and the last block.

For limited backward compatibility, the space of the legacy Master Boot Record (MBR) is still reserved in the GPT specification, but it is now used in a way that prevents MBR-based disk utilities from misrecognizing and possibly overwriting GPT disks. This is referred to as a protective MBR.

A single partition of type EEh, encompassing the entire GPT drive (where "entire" actually means as much of the drive as can be represented in an MBR), is indicated and identifies it as GPT. Operating systems and tools which cannot read GPT disks will generally recognize the disk as containing one partition of unknown type and no empty space, and will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures. Furthermore, GPT-aware OSes may check the protective MBR and if the enclosed partition type is not of type EEh or if there are multiple partitions defined on the target device, the OS may refuse to manipulate the partition table.

If the actual size of the disk exceeds the maximum partition size representable using the legacy 32-bit LBA entries in the MBR partition table, the recorded size of this partition is clipped at the maximum, thereby ignoring the rest of the disk. This amounts to a maximum reported size of 2 TiB, assuming a disk with 512 bytes per sector (see 512e). It would result in 16 TiB with 4 KiB sectors (4Kn), but since many older operating systems and tools are hard coded for a sector size of 512 bytes or are limited to 32-bit calculations, exceeding the 2 TiB limit could cause compatibility problems.

In operating systems that support GPT-based boot through BIOS services rather than EFI, the first sector may also still be used to store the first stage of the bootloader code, but modified to recognize GPT partitions. The bootloader in the MBR must not assume a sector size of 512 bytes.

GPT header format

Offset	Length	Contents
0 (0x00)	8 bytes	Signature ("EFI PART", 00h 00h 01h 00h) for UEFI 2.10
12 (0x0C)	4 bytes	Header size in little endian (in bytes, usually +0h in partition entries of the MBR partition table)
3–47	Reserved for future use
48–63	Defined and used by the individual partition type

Microsoft defines the type-specific attributes for basic data partition as:

Basic data partition attributes

Bit	Content
60	Read-only
61	Shadow copy (of another partition)
62	Hidden
63	No drive letter (i.e. do not automount)

Google defines the type-specific attributes for ChromeOS kernel as:

ChromeOS kernel partition attributes

Bit	Content
56	Successful boot flag
55–52	Tries remaining
51–48	Priority (15: highest, 1: lowest, 0: not bootable)

Yes Yes Tools such as gdisk, GNU Parted, util-linux v2.23+ fdisk, SYSLINUX, GRUB 0.96 + patches and GRUB 2 have been GPT-enabled. Limited to 256 partitions per disk. macOS Since 10.4.0 (some features since 10.4.6) IA-32, x86-64, PowerPC, Apple silicon Yes Yes OpenBSD Since 5.9 IA-32, x86-64, ARM Yes Yes

Windows: 32-bit versions

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Windows 7 and earlier do not support UEFI on 32-bit platforms, and therefore do not allow booting from GPT partitions.

Details of GPT support on 32-bit editions of Microsoft Windows

OS version	Release date	Platform	Read or write support	Boot support	Note
Windows 9x	1995-08-24	IA-32	No	No
Windows Server 2003 SP1	2005-03-30	IA-32	Yes	No	MBR takes precedence in hybrid configuration.
Windows 7	2009-10-22	IA-32	Yes	[https://technet.microsoft.com/en-us/library/hh824898.aspx Windows 8 32-bit supports booting from UEFI-based PC (x86-32 only) using GPT-based disks.]</ref>"]}'>Requires UEFI	MBR takes precedence in hybrid configuration.
Windows 8.1	2013-08-27	IA-32	Yes	[https://technet.microsoft.com/en-us/library/hh824898.aspx Windows 10 32-bit supports booting from UEFI-based PC (x86-32 only) using GPT-based disks.]</ref>"]}'>Requires UEFI	MBR takes precedence in hybrid configuration.

Windows: 64-bit versions

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Limited to 128 partitions per disk.

Details of GPT support on 64-bit editions of Microsoft Windows

OS version	Release date	Platform	Read and write support	Boot support	Note
Windows XP 64-Bit Edition for Itanium systems, Version 2002	2001-10-25	IA-64	Yes	No	MBR takes precedence in hybrid configuration.
Windows Server 2003	2005-04-25	IA-64	Yes	Requires UEFI	MBR takes precedence in hybrid configuration.
Windows Server 2008	2008-02-27	IA-64	Yes	Yes	MBR takes precedence in hybrid configuration.
Windows 8 Windows Server 2012	2012-08-01	x64	Yes	[https://technet.microsoft.com/en-us/library/hh824898.aspx Windows 8.1 64-bit supports booting from UEFI-based PC (x86-64 only) using GPT-based disks.]</ref>"]}'>Requires UEFI	MBR takes precedence in hybrid configuration.
Windows 10	2015-07-29	x64, ARM64	Yes	Requires UEFI	MBR takes precedence in hybrid configuration.
Windows Server 2022	2021-08-18	x64	Yes	Yes	UEFI is a system requirement for Windows Server 2025.

Partition type GUIDs

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"Partition type GUID" means that each partition type is strictly identified by a GUID number unique to that type, and therefore partitions of the same type will all have the same "partition type GUID". Each partition also has a "partition unique GUID" as a separate entry, which as the name implies is a unique id for each partition.

Operating system	Partition type		Globally unique identifier (GUID)
OS-independent	Unused entry		C12A7328-F81F-11D2-BA4B-00A0C93EC93B
	BIOS boot partition		View original on Wikipedia | Back | FazBrowse Home