Windows® Internals, Sixth Edition, Part 2

As part of an initiative to provide a standardized and extensible firmware platform for operating systems to use during their boot process, Intel designed the Extensible Firmware Interface (EFI) specification, originally for the Itanium processor. Intel donated EFI to the Unified EFI Forum, which has continued to evolve UEFI for x86, x64, and ARM CPUs. UEFI includes a mini–operating system environment implemented in firmware (typically flash memory) that operating systems use early in the system boot process to load system diagnostics and their boot code. UEFI defines a partitioning scheme, called the GUID (globally unique identifier) Partition Table (GPT) that addresses some of the shortcomings of MBR-style partitioning. For example, the sector addresses that the GPT structures use are 64 bits wide instead of 32 bits. A 32-bit sector address is sufficient to access only 2 terabytes (TB) of storage, while a GPT allows the addressing of disk sizes into the foreseeable future. Other advantages of the GPT scheme include the fact that it uses cyclic redundancy checksums (CRC) to ensure the integrity of the partition table, and it maintains a backup copy of the partition table. GPT takes its name from the fact that in addition to storing a 36-byte Unicode partition name for each partition, it assigns each partition a GUID.

Figure 9-6 shows a sample GPT partition layout. As in MBR-style partitioning, the first sector of a GPT disk is an MBR (protective MBR) that serves to protect the GPT partitioning in case the disk is accessed from a non-GPT-aware operating system. However, the second and last sectors of the disk store the GPT headers with the actual partition table following the second sector and preceding the last sector. With its extensible list of partitions, GPT partitioning doesn’t require nested partitions, as MBR partitions do.

Figure 9-6. Example GPT partition layout

Note

Because Windows doesn’t support the creation of multipartition volumes on basic disks, a new basic disk partition is the equivalent of a volume. For this reason, the Disk Management MMC snap-in uses the term partition when you create a volume on a basic disk.

Basic Disk Volume Manager

The volume manager driver (%SystemRoot%\System32\Drivers\Volmgr.sys) creates disk device objects that represent volumes on basic disks and plays an integral role in managing all basic disk volumes, including simple volumes. For each volume, the volume manager creates a device object of the form \Device\HarddiskVolumeX, in which X is a number (starting from 1) that identifies the volume.

The volume manager is actually a bus driver because it’s responsible for enumerating basic disks to detect the presence of basic volumes and report them to the Windows Plug and Play (PnP) manager. To implement this enumeration, the volume manager leverages the PnP manager, with the aid of the partition manager (Partmgr.sys) driver to determine what basic disk partitions exist. The partition manager registers with the PnP manager so that Windows can inform the partition manager whenever the disk class driver creates a partition device object. The partition manager informs the volume manager about new partition objects through a private interface and creates filter device objects that the partition manager then attaches to the partition objects. The existence of the filter objects prompts Windows to inform the partition manager whenever a partition device object is deleted so that the partition manager can update the volume manager. The disk class driver deletes a partition device object when a partition in the Disk Management MMC snap-in is deleted. As the volume manager becomes aware of partitions, it uses the basic disk configuration information to determine the correspondence of partitions to volumes and creates a volume device object when it has been informed of the presence of all the partitions in a volume’s description.

Windows volume drive-letter assignment, a process described shortly, creates drive-letter symbolic links under the \Global?? object manager directory that point to the volume device objects that the volume manager creates. When the system or an application accesses a volume for the first time, Windows performs a mount operation that gives file system drivers the opportunity to recognize and claim ownership for volumes formatted with a file system type they manage. (Mount operations are described in the section Volume Mounting later in this chapter.)

Dynamic Disks