In the disconnect function, it is also important to retrieve from the interface any data
that was previously set with a call to usb_set_intfdata. Then set the data pointer in
the struct usb_interface structure to NULL to prevent any further mistakes in access-
ing the data improperly

In the probe function the USB driver should initialize any local structures
that it might use to manage the USB device. It should also save any information that
it needs about the device to the local structure, as it is usually easier to do so at this
time. As an example, USB drivers usually want to detect what the endpoint address
and buffer sizes are for the device, as they are needed in order to communicate with
the device.

• • Control Transfers
  • Interrupt Transfers
  • Isochronous Transfers
  • Bulk Transfer

Control transfer

Control transfers are typically used for command and status operations. They are essential to set up a USB device with all enumeration functions being performed using control transfers. They are typically bursty, random packets which are initiated by the host and use best effort delivery. The packet length of control transfers in low speed devices must be 8 bytes, high speed devices allow a packet size of 8, 16, 32 or 64 bytes and full speed devices must have a packet size of 64 bytes.

Type A plugs always face upstream. Type A sockets will typically find themselves on hosts and hubs. For example type A sockets are common on computer main boards and hubs. Type B plugs are always connected downstream and consequently type B sockets are found on devices.

void (*setup)(structnet_device *));

Here, sizeof_priv is the size of the driver’s “private data” area; with network devices, that area is allocated along with the net_device structure. In fact, the two are allocated together in one large chunk of memory, but driver authors should pretend that they don’t know that. name is the name of this interface.

This function allocates a network device using eth%d for the name argument. It provides its own initialization function (ether_setup) that sets several net_device fields with appropriate values for Ethernet devices.

Designed to easily support encapsulation/decapsulation of data
through the protocol layers.
In addition to the data itself, an sk_buff maintains
head : the start of the packet
data : the start of the packet payload
tail : the end of the packet payload
end : the end of the packet

The socket created is one end point of a communication channel. The domain parameter specifies the address family, the type parameter specifies the type of communication to be used with this socket, and protocol specifies the protocol to be employed.

int bind(int socket, const struct sockaddr *address, size_t address_len);

The bind system call assigns the address specified in the parameter, address, to the unnamed
socket associated with the file descriptor socket. The length of the address structure is passed as address_len.

int listen(int socket, int backlog);

A Linux system may limit the maximum number of pending connections that may be held in a queue.Subject to this maximum, listen sets the queue length to backlog. Incoming connections up to this queue length are held pending on the socket; further connections will be refused and the client’s connection will fail.

int accept(int socket, struct sockaddr *address, size_t *address_len);

The accept system call returns when a client program attempts to connect to the socket specified by the parameter socket. The client is the first pending connection from that socket’s queue. The accept function creates a new socket to communicate with the client and returns its descriptor.

int connect(int socket, const struct sockaddr *address, size_t address_len);

The socket specified by the parameter socket is connected to the server socket specified by the parameter address, which is of length address_len. The socket must be a valid file descriptor obtained by a call to socket

five main functions that r used are:

socket() , bind(), listen(), accept() and close()

struct resource *request_region(unsigned long first, unsigned long n, const char *name);

We can also check whether region available or not by this function

int check_region(unsigned long first, unsigned long n);

• Create a node by  mknod /dev/parlelport c 61 0

• The module can now be installed, parlelport. You can check that it is effectively reserving the input/output port addresses 0x378 with the command:

cat /proc/ioports

• To turn on the LEDs and check that the system is working, execute the command

echo -n A >/dev/parlelport

This should turn on LED zero and six, leaving all of the others off.

• You can check the state of the parallel port issuing the command:

cat /dev/parlelport

Segmentation unit allows the use of two address components, viz. segment and offset for relocability and sharing of code and data.
•Segmentation unit allows segments of size 4Gbytes at max.
•The Paging unit organizes the physical memory in terms of pages     of        4kbytes size each.

Paging unit works under the control of the segmentation unit, i.e. each     segment is further divided into pages. The virtual memory is also organizes in terms of segments and pages by the memory management unit.