Microsoft OS Descriptors for USB Devices
Microsoft provides a set of proprietary device classes and USB descriptors, which are called Microsoft OS Descriptors (MODs).
Due to the rapid emergence of devices that contain multiple hardware functions, many manufacturers find that their devices do not fit comfortably into any of the current universal serial bus (USB) device classes. This deprives such manufacturers of one of the most attractive features of USB technology: the standardization of driver software (according to the class of the device). Microsoft Windows provides native class drivers for most of the devices that belong to standard USB device classes, and these drivers allow end users to easily attach such devices to the computer without needing to install special software.
To accommodate manufacturers whose devices do not fit into the current set of USB device classes, Microsoft Corporation has developed a set of proprietary device classes and USB descriptors, which are called Microsoft OS Descriptors (MODs). Both applications and system software can identify the devices that belong to the Microsoft-defined device classes by querying the devices to determine whether they support MODs.
Microsoft OS Descriptors have important uses other than supporting the proprietary device classes. In particular, they provide a mechanism for deriving the maximum benefit from the device firmware. With the help of Microsoft OS Descriptors, you can use the firmware to deliver help files, special icons, Uniform Resource Locators (URLs), registry settings, and other data that is required to ease installation and enhance customer satisfaction. In some cases, you can forgo storage media such as floppy disks and CDs--which simplifies the delivery and support of upgrades.
Microsoft OS 1.0 Descriptors are supported by:
- Windows 8.1
- Windows 8
- Windows 7
- Windows Vista, Windows Server 2008
- Windows XP with Service Pack 1 (SP1), Windows Server 2003
Microsoft OS 2.0 Descriptors are supported by:
- Windows 8.1
Why Does Windows Issue a String Descriptor Request to Index 0xEE?
Devices that support Microsoft OS Descriptors must store a special USB string descriptor in firmware at the fixed string index of 0xEE. This string descriptor is called a Microsoft OS String Descriptor.
- Its presence indicates that the device contains one or more OS feature descriptors.
- It contains the data that is required to retrieve the associated OS feature descriptors.
- It contains a signature field that differentiates the OS string descriptor from other strings that IHVs might choose to store at 0xEE.
- It contains a version number that allows for future revisions of Microsoft OS descriptors.
If there is no string descriptor at 0xEE, or the string descriptor at that index is not a valid OS string descriptor, Windows assumes that the device does not contain any OS feature descriptors.
When a new device is attached to a computer for the first time, an operating system that supports Microsoft OS Descriptors will request the string descriptor that is at index 0xEE. The Microsoft OS String Descriptor contains an embedded signature field that the operating system uses to differentiate it from other strings that might be at index 0xEE. The presence of a string descriptor that contains the proper signature field at index 0xEE indicates to the operating system that the device supports Microsoft OS Descriptors. The Microsoft OS String Descriptor also provides the operating system with version information.
The operating system queries for the string descriptor at index 0xEE during device enumeration--before the driver for the device has loaded--which might cause some devices to malfunction. Such devices are not supported by versions of the Windows operating system that support Microsoft OS Descriptors.
If a device does not contain a valid string descriptor at index 0xEE, it must respond with a stall packet (in other words, a packet that contains a packet identifier of type STALL), which is described in the "Request Errors" section of the Universal Serial Bus Specification. If the device does not respond with a stall packet, the system will issue a single-ended zero reset packet to the device, to help it recover from its stalled state (Windows XP only).
After the operating system requests a Microsoft OS String Descriptor from a device, it creates the following registry key:
The operating system creates a registry entry, named osvc, under this registry key that indicates whether the device supports Microsoft OS Descriptors. If the device does not provide a valid response the first time that the operating system queries it for a Microsoft OS String Descriptor, the operating system will make no further requests for that descriptor.
For registry entries under that key, see USB Device Registry Entries.
For additional information, see Microsoft OS Descriptors.
What types of OS feature descriptors are supported by Windows?
Any information to be stored as a feature descriptor must comply with one of the standard formats that Microsoft has defined. Additional feature descriptors cannot be defined or implemented without Microsoft consent. Microsoft has defined the following feature descriptors:
- Extended Compat ID. Windows uses class and subclass codes to help locate the appropriate default driver for a USB device. However, the USB Device Working Group must allocate these codes. This means that devices that implement new types of features often do not yet have appropriate class and subclass codes, so Windows cannot use the codes to select a default driver. IHVs can circumvent this problem by storing the information in firmware as an extended compat ID OS feature descriptor. Windows can then retrieve this information when the device is plugged in and use it to help determine which default driver to load.
- Extended Properties. Currently, there are two levels at which properties can be declared for a USB device: class level or devnode level. The extended properties OS feature descriptor allows a vendor to store additional properties- such as help pages, URLs, and icons-in device firmware.
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