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USB System Architecture (USB 2.0).pdf
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Chapter 21: Device Classes

within the interface descriptor.

Table 21-4: Mass Storage Class Code and Subclass Code

Offset

Field

Size

Value

 

Description

 

 

(bytes)

 

 

 

 

 

 

 

 

 

 

 

 

 

5

InterfaceClass

1

Class

Class code for mass storage

 

 

 

 

device = 01h.

 

 

 

 

 

6

InterfaceSubClass

1

SubClass

SubClass code for mass storage

 

 

 

 

devices defined as:

 

 

 

 

01h

= General Mass Storage

 

 

 

 

02h

= CD-ROM

 

 

 

 

03h

= Tape

 

 

 

 

04h

= Solid State

 

 

 

 

05 - FEh Reserved

 

 

 

 

 

 

Standard Mass Storage Interface

The standard interfaced used by mass storage devices (except CD-ROM) consists of four endpoints:

Control endpoint (default endpoint zero) not defined by interface.

Bulk transfer endpoint supporting IN transactions.

Bulk transfer endpoint supporting OUT transactions.

Interrupt transfer endpoint.

Control Endpoint

Mass storage device drivers and devices use the device’s default control endpoint to deliver commands to the mass storage function. These commands are sent using SCSI command structures. The device typically responds to the command when the host subsequently accesses the endpoint specified (or targeted) by the command. For example, depending on the command, the device might return status when the interrupt endpoint is accessed, or transfer data to or from the media when the host accesses one of the bulk transfer endpoints.

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