127 lines
6.1 KiB
HTML
127 lines
6.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Devices, Drivers, and all that</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node43.html">Kernel Configuration</A>
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<B>Up:</B> <A HREF="node41.html">Configuring the Networking Hardware</A>
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<B> Previous:</B> <A HREF="node41.html">Configuring the Networking Hardware</A>
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<BR> <P>
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<H1><A NAME="SECTION005100000">Devices, Drivers, and all that</A></H1>
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<P>
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<A NAME="hardwaredrivers"></A>
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<A NAME="1835"></A>
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<A NAME="1836"></A>
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<P>
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Up to now, we've been talking quite a bit about network interfaces and
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general TCP/IP issues, but didn't really cover exactly <em>what</em>
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happens when ``the networking code'' in the kernel accesses a piece of
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hardware. For this, we have to talk a little about the concept of
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interfaces and drivers.
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<P>
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First, of course, there's the hardware itself, for example an Ethernet
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board: this is a slice of Epoxy, cluttered with lots of tiny chips
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with silly numbers on them, sitting in a slot of your PC. This is
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what we generally call a device.
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<P>
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For you to be able to use the Ethernet board, special functions have
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to be present in your kernel that understand the particular
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way this device is accessed. These are the so-called device drivers.
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For example, has device drivers for several brands of
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Ethernet boards that are very similar in function. They are known as
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the ``Becker Series Drivers'', named after their author, Donald
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Becker. A different example is the D-Link driver that handles a D-Link
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pocket adaptor attached to a parallel port.
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<P>
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But, what do we mean when we say a driver ``handles'' a device? Let's
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go back to that Ethernet board we examined above. The driver has to be
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able to communicate with the peripheral's on-board logic somehow: it
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has to send commands and data to the board, while the board should
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deliver any data received to the driver.
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<P>
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<P><A NAME="2203"></A><BR>
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<STRONG>Figure:</STRONG>
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<A NAME="hardwarefigdrivers"></A>
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The relationship between drivers, interfaces, and the
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hardware.
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<BR>
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<P>
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<P>
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In PCs, this communication takes place through an area of I/O-memory
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that is mapped to on-board registers and the like. All commands and
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data the kernel sends to the board have to go through these registers.
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I/O memory is generally described by giving its starting or <em>base
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address</em>. Typical base addresses for Ethernet boards are 0x300,
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or 0x360.
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<P>
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Usually, you don't have to worry about any hardware issues such as the
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base address, because the kernel makes an attempt at boot time to
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detect a board's location. This is called autoprobing, which means
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that the kernel reads several memory locations and compares the data
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read with what it should see if a certain Ethernet board was
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installed. However, there may be Ethernet boards it cannot detect
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automatically; this is sometimes the case with cheap Ethernet cards
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that are not-quite clones of standard boards from other manufacturers.
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Also, the kernel will attempt to detect only one Ethernet device when
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booting. If you're using more than one board, you have to tell the
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kernel about this board explicitly.
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<P>
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<A NAME="1903"></A>
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Another such parameter that you might have to tell the kernel about is
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the interrupt request channel. Hardware components usually interrupt
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the kernel when they need care taken of them, e.g. when data has
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arrived, or a special condition occurs. In a PC, interrupts may occur on
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one of 15 interrupt channels numbered 0, 1, and 3 through 15. The
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interrupt number assigned to a hardware component is called its
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<em>interrupt request number</em>, or IRQ.<A HREF="footnode.html#1905"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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<A NAME="1906"></A>
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<A NAME="1907"></A>
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As described in chapter-<A HREF="node23.html#tcpip"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>, the kernel accesses a device
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through a so-called interface. Interfaces offer an abstract set of
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functions that is the same across all types of hardware, such as
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sending or receiving a datagram.
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<P>
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Interfaces are identified by means of names. These are names defined
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internally in the kernel, and are not device files in the /dev
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directory. Typical names are eth0, eth1, etc, for
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Ethernet interfaces. The assignment of interfaces to devices usually
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depends on the order in which devices are configured; for instance the
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first Ethernet board installed will become eth0, the next will
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be eth1, and so on. One exception from this rule are SLIP
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interfaces, which are assigned dynamically; that is, whenever a SLIP
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connection is established, an interface is assigned to the serial
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port.
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<P>
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The picture given in figure-<A HREF="node42.html#hardwarefigdrivers"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> tries to show
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the relationship between the hardware, device drivers and interfaces.
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<P>
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When booting, the kernel displays what devices it detects, and what
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interfaces it installs. The following is an excerpt of a typical
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boot screen:
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<P>
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<P><P>
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<P>
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This shows that the kernel has been compiled with TCP/IP enabled, and
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drivers for SLIP, CSLIP, and PPP included. The third line from below says
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that a D-Link pocket adaptor was detected, and installed as interface
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dl0. If you have a different type of Ethernet card, the kernel will
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usually print a line starting with eth0, followed by the type of
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card detected. If you have an Ethernet card installed but don't see any
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such message, this means that the kernel is unable to detect your board
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properly. This is dealt with in a later section.
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node43.html">Kernel Configuration</A>
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<B>Up:</B> <A HREF="node41.html">Configuring the Networking Hardware</A>
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<B> Previous:</B> <A HREF="node41.html">Configuring the Networking Hardware</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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