Information Technology Reference
In-Depth Information
Transport and UFI Command Specification, are relevant only to some
floppy drives.
Devices must also support one or more industry-standard command-block
sets to exchange data, control devices, and read status information. Chapter
6 has more about these command blocks.
Logical Block Addressing
A USB mass-storage host specifies locations to read and write to in the stor-
age media using the logical block addressing (LBA) method described in
Chapter 1. Every USB mass-storage device must support accessing its media
via LBA.
Mass Storage Requests
The bulk-only transport protocol has two defined control requests. Bulk
Only Mass Storage Reset requests the device to become ready to receive a
new mass-storage command block. Get Max LUN requests the highest logi-
cal-unit number supported by the device. In Windows, each logical unit, or
volume, is represented by a drive letter. A device with a single logical unit
can return zero or stall the request. A device with LUN 0 and LUN 1
returns 1. The maximum is 15. All other mass-storage data travels in bulk
transfers.
The control/bulk/interrupt (CBI) protocol has one defined control request:
Accept Device-Specific Command (ADSC). The Data stage of the request
carries the command. A device can use an interrupt transfer to indicate that
the device has completed a command's requested action.
A mass-storage host can also use control transfers to clear halt conditions on
bulk endpoints. To do so, the host sends the standard USB request Clear
Feature with the feature specified as ENDPOINT_HALT.
Descriptors
As Chapter 2 explained, every USB device has a series of descriptors that
provide information about the device's capabilities. Every mass-storage
device has a device descriptor, a configuration descriptor, an interface
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