USB System Architecture

Device-Specific Requests

Seven device-specific requests are defined for display class devices:

•Get Display ID

•Get Max

•Get Current

•Set Current

•Get Display Status

•Degauss

•Set Display Power Mode

These requests provide the mechanism for obtaining status regarding the current settings of the display and for changing the display settings. See the Display device class specification for details.

Mass Storage Device Class

Mass storage devices differ from most device classes in that they may also be used when booting the operating system. This requires that the system BIOS must be able to initialize and access the USB storage device. The mass storage definitions then must be supported both by device drivers and the system ROM. Several types of mass storage device are defined by the USB mass storage device class specification. Five subclasses have been defined:

•General Mass Storage Subclass — This subclass defines devices that are normally access storage media in a random fashion.

•CD-ROM Subclass — CD-ROMs of course are read only. These drives may also contain interfaces beyond the mass storage interface, to support audio and video applications (not related to the mass storage class).

•Tape — Tape drives are unique due to the streaming nature of the data written to and read from the drive. Sending data on time is important since the tape drives stores data linearly as the tape moves past the heads. If data is not available on time, data corruption occurs and time-consuming retries are required.

•Solid State — These devices have no moving parts to control, but require special commands to perform the time-consuming writes, since a write follows an erase operation.

A device’s class code and subclass codes are specified in the interface descriptor. Table 21-4 shows the mass storage device class code and subclass code field

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