Shulou(Shulou.com)06/02 Report--

In the previous detailed explanation of the MBR structure of Windows disks, we introduced the Master Boot Record structure in Basic Disk. GPT Disk is a partition structure introduced after Windows 2003. The GUID partition table structure is used, which has the advantage of supporting larger and more partitions and improving fault tolerance compared with MBR. This article introduces the structure of the GUID partition table and the meaning of each field.

Protective MBR of GPT Disk:

In the structure of GPT Disk, the first LBA location (LBA 0) holds Protective MBR, and then the location of LBA1 is the GUID partition header of GPT. The role of Protective MBR is to prevent some legacy MBR disk tools from breaking GUID partition tables for compatibility reasons. This structure of storing Protective MBR in the location of LBA 0 is based on the Extensible Firmware Interface (EFI) specification. Protective MBR and MBR have the same structure, and also include a partition record that contains a System ID with a value of 0xEE (shown in the following figure). This value indicates that the partition is a GPT partition. If the disk containing the partition is moved to Windows 2000 or accessed by some legacy disk tools, the partition will be marked with the morning GPT Protective partition and cannot be deleted.

Globally Unique Identifier Partition Table (GUID) structure:

As shown in the following figure, the GUID records that make up GPT Disk are as follows:

GUID Partition Table Header: GPT Header defines the area of the Logical Block Address used by the partition record. It also defines the location of the partition on disk, its own GUID, and a set of CRC32 check values. Primary GPT Header is located in the LBA1 location of the disk, followed by Protective MBR. Backup GPT Header is located on the last track of the disk. The following figure shows several fields in GPT Header:

The first 0x00 is an 8-byte signature file, which must be a fixed value to define EFI compatibility.

0x08 begins with four sets of fields with a length of 4 bytes. Revision marks the version information of the EFI specification, and Header Size records the size of the entire GPT Head, as well as fixed values, CRC32 check information, and Reserved reserved bits.

Then the 0x18 starts with five 8-byte fields and a 16-byte field. Primary LBA records the location of the Primary GPT Head, the location of the Backup LBA record Backup GPT Header, and the First Usable LBA records the start position of the first partition, such as 64-bit Windows Server 2003, which starts at 34, where the partition of the Last Usable LBA record ends. The length of Disk GUID is 16 bytes, which is used to identify the partition header and disk, Partition Entry LBA records the location of GUID partition records, fixed as LBA2.

Starting with 0x50, there are 3 4-byte records, Number of Partition Entries records the largest number of disk partitions, for example, 64-bit Windows Server is 128, Size of Partition Entry records the size of each GUID partition record, its value is 128bytes, and Partition Entry Array CRC32 records the check value of a set of partition records.

Finally, the reserved space is 420 bytes long from the 0x5C position, with a value of 0.

GUID Partition Entry Array: similar to the partition table in MBR, GUID partition entry array contains a record of each partition on the disk. For example, if 64-bit Windows Server 2003 creates an array value of 16384 bytes, the first available data is greater than or equal to LBA34. GUID Partition Entry Array also has a copy at the end of the partition, after the last available LBA and before GUID Partition table header.

GUID Partition Entry: the GUID Partition Entry is 128 bytes long and is used to define a single partition structure. Each GUID Partition entry starts with the Partition Type record. The 16-byte Partition Type GUID, similar to System ID in the MBR disk partition table, defines the data contained in the partition and the role of the partition. The following figure shows a typical GUID Partition entry array record of GPT Disk, which shows three kinds of partition records

The first part {45464920-5041-5254-0000-01005C000000} is the EFI System partition, the middle part {16E3C9E3-5C0B-B84D-817D-F92DF00215AE} is the Microsoft Reserved partition, and the bottom {A2A0D0EB-E5B9-3344-87C0-68B6B72699C7} is a primary partition in the Windows Basic Disk.

Some fields are mainly recorded in the middle of each section. The 0x10 position after Partition Type GUID begins with a 16-byte Unique Partition GUID record that identifies the uniqueness of each record. 0x20 starts with three 8-byte fields, Starting LBA and Ending LBA record the start and end of the partition, Attribute Bit describes how the partition is used, such as whether it is hidden and read-only, etc., and the final 72-byte field at the beginning of 0x38 is used for the name of the Unicode partition, with a maximum length of 36 characters.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.