The USB Mass Storage Class - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 53

col (50h) for all new devices. CBI is approved for use only with full-speed
floppy drives.
iInterface. Index to a string descriptor that contains a string describing the
interface. This value is zero if there is no interface string descriptor.
Endpoint Descriptors
Each endpoint specified in an interface descriptor has an endpoint descrip-
tor. Endpoint zero never has a descriptor because every device must support
endpoint zero, the device descriptor contains the maximum packet size, and
the USB specification defines everything else about the endpoint.
Table 3-4 shows the fields in the descriptors. Here are more details about the
fields and how they're used in a mass-storage device:
bLength. The number of bytes in the descriptor. Always 07h.
bDescriptorType. The constant ENDPOINT (05h).
bEndpointAddress. The endpoint number and direction. Bits 0..3 are the
endpoint number, which can be any value from 1 to 15 supported by the
device's hardware. Bit 7 is the direction: Out = 0, In = 1 Bits 6..4 are unused
and must be zero. For example, an interface could use endpoint 1 OUT
(01h) and endpoint 1 IN (81h), or endpoint 2 OUT (02h) and endpoint 3
in (83h).
bmAttributes. Bits 1..0 specify the type of transfer the endpoint supports.
Bits 7..2 are zero. For bulk transfers, set to 02h.
wMaxPacketSize. The value in bits 10..0 is the maximum number of data
bytes the endpoint can transfer in a transaction. The allowed values vary
with the device speed and type of transfer. A full-speed bulk endpoint can
have a maximum packet size of 08h, 10h, 20h, or 40h bytes. For best perfor-
mance, use 40h. If a full-speed bulk endpoint's wMaxPacketSize is less than
40h, some host controllers schedule no more than one transaction per
frame. For high speed, the maximum packet size must be 200h. Bits 15..11
are zero for bulk endpoints.
bInterval. The host ignores this value for full-speed bulk endpoints and
high-speed bulk IN endpoints. For high-speed bulk OUT endpoints, the