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+><H1
+><A
+NAME="IO-PACKAGE-INTRO">Chapter 14. Introduction</H1
+><P
+>The I/O package is designed as a general purpose framework for
+supporting device drivers. This includes all classes of
+drivers from simple serial to networking stacks and beyond.</P
+><P
+>Components of the I/O package, such as device drivers, are
+configured into the system just like all other components.
+Additionally, end users may add their own drivers to this set.</P
+><P
+>While the set of drivers (and the devices they represent) may be
+considered static, they must be accessed via an opaque
+“handle”. Each device in the system has a unique name and
+the <TT
+CLASS="FUNCTION"
+>cyg_io_lookup()</TT
+> function is used to map that
+name onto the handle for the device. This “hiding” of the
+device implementation allows for generic, named devices, as well as
+more flexibility. Also, the <TT
+CLASS="FUNCTION"
+>cyg_io_lookup()</TT
+>
+function provides drivers the opportunity to initialize the device
+when usage actually starts.</P
+><P
+>All devices have a name. The standard provided devices use names such
+as <TT
+CLASS="FILENAME"
+>“/dev/console”</TT
+> and
+<TT
+CLASS="FILENAME"
+>“/dev/serial0”</TT
+>, where the
+<TT
+CLASS="FILENAME"
+>“/dev/”</TT
+> prefix indicates that this is
+the name of a device.</P
+><P
+>The entire I/O package API, as well as the standard
+set of provided drivers, is written in C. </P
+><P
+>Basic functions are provided to send data to and receive data
+from a device. The details of how this is done is left to the device [class] itself.
+For example, writing data to a block device like a disk drive may
+have different semantics than writing to a serial port. </P
+><P
+>Additional functions are provided to manipulate the state
+of the driver and/or the actual device. These functions
+are, by design, quite specific to the actual driver. </P
+><P
+>This driver model supports layering; in other words, a device
+may actually be created “on top of” another device.
+For example, the “tty” (terminal-like) devices are
+built on top of simple serial devices. The upper layer then has
+the flexibility to add features and functions not found at the lower
+layers. In this case the “tty” device provides
+for line buffering and editing not available from the simple serial
+drivers.</P
+><P
+>Some drivers will support visibility of the layers they depend
+upon. The “tty” driver allows information about
+the actual serial device to be manipulated by passing get/set
+config calls that use a serial driver “key” down
+to the serial driver itself. </P
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