The USB Mass Storage Class - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 52

Table 3-3: The interface descriptor specifies the mass-storage function.
Byte

Field Description

0 bLength Descriptor size in bytes (09h).

1 bDescriptorType The constant INTERFACE (04h).

2 bInterfaceNumber Number identifying this interface.

3 bAlternateSetting Set to 00h for the default setting.

4 bNumEndpoints Number of endpoints supported, not counting endpoint

zero. Set to 02h for a bulk-only mass-storage device.

5 bInterfaceClass Class code. Mass storage = 08h.

6 bInterfaceSubclass Subclass code. Mass-storage values:

01h: Reduced Block Commands (RBC).
02h: SFF-8020i, MMC-2 (ATAPI) (CD/DVD drives)
03h: QIC-157 (tape drives).
04h: USB Floppy Interface (UFI).
05h: SFF-8070i (ATAPI removable rewritable media
devices).
06h: SCSI transparent command set. Use the SCSI
INQUIRY command to determine the peripheral device
type. Recommended value for most devices.

7 bInterfaceProtocol Protocol code. Mass storage values:

00h: CBI with command completion interrupt transfers
01h: CBI without command completion interrupt transfer
50h: bulk only. Recommended value for most devices.

8 iInterface Index of string descriptor for the interface.

command code systems transported by the interface.” In reality, bInterface-
Subclass names the command-code system, or command blocks, that a
device uses (either explicitly or by leaving the issue to the INQUIRY com-
mand), and the bInterfaceProtocol field (described below) names the trans-
port protocol.
bInterfaceProtocol. In mass-storage devices, the bInterfaceProtocol field
specifies a mass-storage transport protocol. The transport protocol defines
structures and specifies USB transfer types for carrying mass-storage com-
mands, data, and status information on the bus. A device may use either of
two protocols: bulk-only transport (BOT, sometimes called BBB because all
three phases use bulk transfers) or control/bulk/interrupt (CBI). The
mass-storage specification recommends using the bulk-only transport proto-