Device Descriptor
This is the
device descriptor of the mouse we are examining. The analyser is
showing the significance of the 18 bytes which comprise the descriptor.
The device
descriptor is the top level set of information which can be read
from a device. It is the first item which the host will attempt
to retrieve.
The first two
bytes show the length and descriptor type, as with any descriptor.
This is followed by a word which says the device conforms to USB
specification V1.1.
The next three
bytes describe the device class. In the case of a mouse which is
a Human Interface Device class, the class is not defined here, but
defined for each interface in the device in the interface descriptor(s).
(We will see that our mouse has one interface.)
bMaxPacketSize0
specifies that the maximum input and output packet size for endpoint
0 is 8 bytes.
The next three
words specify the exact identification of the device. We can see
who made it, what it is, and which version it is.
The next three
items define indexes to strings which the device may or may not
specify. This device has a manufacturer string (which the host never
asked for, so we don't know what it is), a product string (shown
here because the analyser noted what it contains when it was asked
for later), but no serial number string.
A mouse does
not usually have a serial number.
Finally we
are told that this device has just 1 configuration, which in practice
is the most common number of configurations for a device to have.
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