Partition format for bootable hard disks for 64-bit Windows

Updated: July 8, 2008

A partition is a contiguous space of storage on a physical or logical disk that functions as though it were a physically separate disk. Partitions are visible to the system firmware and the installed operating system. Access to a partition is controlled by the system firmware and the operating system that is currently active. For 64-bit Windows, bootable hard drives must be partitioned by using the GUID partition table (GPT) mechanism that is defined in .

GPT is the default partitioning scheme that 64-bit Windows uses for all non-removable storage media. GPT complements the older master boot record (MBR) partitioning scheme that has been common to PCs.

Chapter 16 of the EFI specification defines the GPT format. Additional guidelines for implementing GPT support for Itanium-based systems is defined in Hardware Design Guide Version 3.0 for Microsoft Windows 2000 Server, which is coauthored by Intel Corporation and Microsoft Corporation.

GPT Benefits

Partition size Can be up to approximately 18 exabytes in size.
Number of partitions Not constrained by temporary schemes such as container partitions as defined by the MBR EBR.
Partition format Well defined and fully self-identifying. Data that is critical to platform operation is located in partitions and not in unpartitioned or hidden sectors.
Integrity Uses primary and backup partition tables for redundancy and CRC32 fields for improved integrity of the partitions data structure.
Partition format Uses version number and size fields for future expansion.
Unique ID Because each partition has a unique GUID and a partition content type, no coordination is necessary to prevent partition identifier collision. Each GPT partition also has a 36-character Unicode name, which means that any software can present a human-readable name for the partition.


MBR issues

MBR disks support only four partition table entries. If a user wants to create more partitions - a secondary structure - an extended partition is necessary. Extended partitions can be subdivided into one or more logical disks.

Only one extended partition can be present on any given drive, and the maximum number of logical drives is MAXULONG/4. All MBR disk partitions and logical drives must be cylinder-aligned, even on hardware RAID sets that are built from multiple different drives with no clear underlying physical geometry.

To protect GPT-partitioned disks from tools that only understand MBR - such as Windows Disk Administrator or Fdisk, which do not know how to properly access a GPT disk - each GPT disk has a Protective MBR that begins in sector 0. This sector precedes the GPT partition table and contains one type 0xEE partition that spans the disk.

Legacy software that does not know about GPT interprets only the Protective MBR when it accesses a GPT disk. These tools view a GPT disk as having a single, encompassing (possibly unrecognized) partition by interpreting the Protective MBR, instead of mistaking the disk for one that is unpartitioned.

Related topics

Designing for 64-bit Windows
Windows and GPT FAQ



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