LDP/LDP/howto/linuxdoc/Plug-and-Play-HOWTO.sgml

<!doctype linuxdoc system>
<article>
<title> Plug-and-Play-HOWTO </title>
<author>David S.Lawyer
  <tt><url url="mailto:dave@lafn.org"></tt>
<date> v1.01, April 2001 

<!--
Change log: 
v1.01 April 2001 less shortage today of bus-resources, clarity in sect. 2, 
 Windows 2000 OK (even if "not a PnP OS" in CMOS)
v1.00 November 2000 "skip file" workaround for reconfiguring under Windows.
 PCI interrupt details: X, W were transposed, rewrote, mentioned MSI,
 etc.  General Introduction added.  Revised "How Linux Does PnP",
 etc..  Patching kernel to make it PnP no longer feasible.  Drivers do
 more now due to kernel-provided functions.
v0.12 June 2000  scanpci, workaround for Dos zeroing PCI IRQs.
v0.11 May 2000 scanport utility, many typos fixed, setpci hard to use
v0.10 2 March 2000 typo: /bus/pnp/devices, lspci+, book "Programming
..."
v0.09 /proc/bus/pci/devices too cryptic,
v0.08 my new email
v0.07 in Windows: remove devices
v0.06 Oct. 1999 in Windows: forced+, legacy into ESCD, pause key (bios)
v0.05 Aug. 1999 lspci, url for isapnp, tell the dev. driver, PCI still
    needs PnP
v0.04 June 1999 alternatives to reinstall Windows?, add D.H. url
v0.03 May 1999 typos, reinstall Windows, clarity, removed D.H. url
v0.02 Apr. 1999 /proc/... doesn't show hdw config (except pci)
v0.01 Mar. 1999 serial PnP spec, PCI interrupts, no DMA on PCI,
dumpregs for pnpdump , Bus Mastering DMA, clarity rewrites, pciutils
kernel 2.2 /proc tree, IO range check, Windows may not update ESCD
v0.00 Nov. 1998 
-->
<abstract>
Help with understanding and dealing with the complex Plug-and-Play
(PnP) issue.  How to get PnP to work on your PC (if it doesn't already).
It doesn't cover what's called "Universal Plug and Play"
(UPnP).  See <ref id="UPnP_" name="Universal Plug and Play (UPnP)" 
</abstract>
<toc>

<sect> Introduction

<sect1> Copyright, Trademarks, Disclaimer, & Credits

<sect2> Copyright
<p> Copyright (c) 1998-2001 by David S. Lawyer <url url="mailto:dave@lafn.org">
<!-- license.H begin -->

Please freely copy and distribute (sell or give away) this document
in any format.  Send any corrections and comments to the document
maintainer.  You may create a derivative work and distribute it
provided that you:

<enum>
<item> If it's not a translation: Email a copy of your derivative work
 (in a format LDP accepts) to the author(s) and maintainer (could be 
 the same person).  If you don't get a response them email the LDP 
 (Linux Documentation Project): submit@linuxdoc.org. 
<item>License the derivative work in the spirit of this license or use
  GPL.  Include a copyright notice and at least a pointer to the 
  license used.
<item>Give due credit to previous authors and major contributors.
</enum>

If you're considering making a derived work other than a
translation, it's requested that you discuss your plans with the
current maintainer. 

<sect2>Disclaimer
<p> While I haven't intentionally tried to mislead you, there are
likely a number of errors in this document.  Please let me know about
them.  Since this is free documentation, it should be obvious that I
cannot be held legally responsible for any errors.

<sect2>Trademarks.
<p> Any brand names (starts with a capital letter) should be assumed to
be a trademark).  Such trademarks belong to their respective owners.
<!-- copyright.H end -->


<sect2> Credits
<p>
<itemize> 
<item>  Daniel Scott proofread this in March 2000 and found many 
 typos, etc.
<item> Pete Barrett gave a workaround to prevent Windows from zeroing
 PCI IRQs.
</itemize> 

<sect1> Future Plans; You Can Help
<p> Please let me know of any errors in facts, opinions, logic,
spelling, grammar, clarity, links, etc.  But first, if the date is
over a month old, check to see that you have the latest version.
Please send me any info that you think belongs in this document.

I haven't studied the code used by various Linux drivers to implement
Plug-and-Play.  Nor do I fully understand how PnP is configured by the
BIOS (it depends on which BIOS) nor how Windows9x updates the ESCD.
Thus this HOWTO is still incomplete and may be inaccurate (let me know
where I'm wrong).  In this HOWTO I've sometimes used ??  to indicate
that I don't really know the answer.

<sect1> New Versions of this HOWTO
<p> New versions of the Plug-and-Play-HOWTO should appear every
month or two and will be available to browse and/or download at LDP
mirror sites.  For a list of mirror sites see: <url
url="http://linuxdoc.org/mirrors.html">.  Various formats are
available.  If you only want to quickly check the date of the latest
version look at: <url
url="http://linuxdoc.org/HOWTO/Plug-and-Play-HOWTO.html">.
The version you are now reading is: v1.01, April 2001 .  

<sect1> New in Recent Versions
<p> v1.01 April 2001 less shortage today of
bus-resouces, clarity in sect. 2, Windows 2000 OK (even if "not a PnP
OS" in CMOS)

The version 1.0 (Nov. 2000) was long overdue and recognized that the
kernel is doing more in helping device drivers set up PnP.  Kernel 2.4
is significantly improved in this respect.  There's still a lot of
improvements needed in both this HOWTO and the way that Linux does
PnP.

<sect1> General Introduction.  Do you need this HOWTO?
<p> Plug-and-play (PnP) is a system which automatically detects PC
devices such as disks, sound cards, ethernet cards, modems, etc.  It
also does some low-level configuring of them.  To be detected by PnP, the
device must be designed for PnP.  Non-PnP devices (or PnP devices which
have been correctly PnP-configured), can often be detected by
non-PnP methods.  

While the Linux kernel has no centralized plug-and-play system, it
does provide programs which various device drivers can use to do their
own plug-and-play.  Many drivers take advantage of this and find your
PnP devices OK.  The BIOS hardware of your PC likely may also do some
plug-and-play work.  Thus if everything works OK PnP-wise, you can use
your computer without needing to know anything about plug-and-play.
But if some devices which are supported by Linux don't work (because
they not discovered or configured correctly by PnP) then you may need
to read some of this HOWTO.  You'll learn not only about PnP but also
something about how communication takes place inside the computer.

In this document I mention so many things that can go wrong that one
who believes in Murphy's Law (If something can go wrong it will) may
become quite alarmed.  But for PnP for most people: If something can
go wrong it usually doesn't.  Remember that sometimes problems which
seem to be PnP related are actually due to defective hardware or to
hardware that doesn't conform to PnP specs.

<sect> What PnP Should Do: Allocate "Bus-Resources"
<sect1> What is Plug-and-Play (PnP)?
<p> If you don't understand this section, read the next section
 <ref id="find_dev" name="How a Computer Finds Devices (and
 conversely)">

<p> Oversimplified, Plug-and-Play automatically tells the software
(device drivers) where to find various pieces of hardware (devices)
such as modems, network cards, sound cards, etc.  Plug-and-Play's task
is to match up physical devices with the software (device drivers)
that operates them and to establish channels of communication between
each physical device and its driver.  In order to achieve this, PnP
allocates the following "bus-resources" to both drivers and hardware:
I/O addresses, IRQs, DMA channels (ISA bus only), and memory regions.
These 4 things are sometimes called 1st order resources.  If you don't
understand what these 4 bus-resources are, read the following
subsections of this HOWTO: I/O Addresses, IRQs, DMA Channels, Memory
Regions.  An article in Linux Gazette about 3 of these bus-resources
is <htmlurl url="http://www.linuxgazette.com/issue38/blanchard.html"
name="Introduction to IRQs, DMAs and Base Addresses">.  Once these
bus-resources have been assigned (and if the correct driver is
installed), the "files" for such devices in the /dev directory are
ready to use.

This PnP assignment of bus-resources is sometimes called "configuring"
but it is only a low level type of configuring.  Even with PnP fully
utilized, much configuring of devices is done by other than PnP.  For
example, for modem configuration an "init string" is sent to the modem
over the I/0 address "channel".  This "init string" has nothing to do
with PnP although the "channel" used to send it to the modem was
allocated by PnP.  Setting the speed (and many other parameters) of a
serial port is done by sending messages to the device driver from
programs run by the user, application programs, or by start-up scripts
at boot-time.  This configuring also has nothing to do with PnP.  Thus
when talking about PnP, "configuring" means only a certain type of
configuring.  While other documentation (such a for MS Windows) simply
calls bus-resources "resources",  I have used the term "bus-resources"
so as to distinguish it from the multitude of other kinds of
resources.

<sect1> How a Computer Finds Devices (and conversely) <label
 id="find_dev">
<p> A computer consists of a CPU/processor to do the computing and RAM
memory to store programs and data (for fast access).  In addition,
there are a number of devices such as various kinds of disk-drives, a
video card, a keyboard, network cards, modem cards, sound cards, the
USB bus, serial and parallel ports, etc.  There is also a power supply
to provide electric energy, various buses on a motherboard to connect
the devices to the CPU, and a case to put all this into.  

In olden days most all devices had their own plug-in cards (printed
circuit boards).  Today, in addition to plug-in cards, many "devices"
are small chips permanently mounted on the "motherboard".  Cards which
plug into the motherboard may contain more than one device.  Memory
chips are also sometimes considered to be devices but are not
plug-and-play in the sense used in this HOWTO.

For the computer system to work right, each device must be under the
control of its "device driver".  This is software which is a part of
the operating system (perhaps loaded as a module) and runs on the CPU.
Device drivers are associated with "special files" in the /dev
directory although they are not really files.  They have names such as
hda1 (first partition on hard drive a), ttyS0 (the first serial port),
eth1 (the second ethernet card), etc.  To make matters more
complicated, the particular device driver selected, say for eth1, will
depend on the type of ethernet card you have.  Thus eth1 can't just be
assigned to any ethernet driver.  It must be assigned to a certain
driver that will work for the type of ethernet card you have
installed.  For modules, some of these assignments might be found in
/etc/modules.conf (called "alias") while others may reside in an
internal kernel table.

To control a device, the CPU (under the control of the device driver)
sends commands (and data) to and reads info from the various devices.
In order to do this each device driver must know the address of the
device it controls.  Knowing such an address is equivalent to setting
up a communication channel, even though the physical "channel" is
actually the data bus inside the PC which is shared with almost
everything else.

This communication channel is actually a little more complex than
described above.  An "address" is actually a range of addresses so
that sometimes the word "range" is used instead of "address".  There
could even be more that one range (with no overlapping) for a single
device.  Also, there is a reverse part of the channel (known as
interrupts) which allows devices to send an urgent "help" request to
their device driver.

<sect1> Addresses 
<p> PCs have 3 address spaces: I/O, main memory, and configuration
(except that the old ISA bus lacks a "configuration" address space).
All of these 3 types of addresses share the same bus inside the PC.
But the voltage on certain dedicated wires on the PC's bus tells which
"space" an address is in: I/O, main memory, (see <ref id="mem_"
name="Memory Ranges">),  or configuration.  See <ref
id="address_details" name="Address Details"> for more details.
Only two of these 3 address spaces are used for device I/O: I/0 and
main memory.

<sect1> I/O Addresses and Allocating Them
<p> Devices were originally located in I/O address space but today
they may use space in main memory.  An I/0 address is sometimes just
called "I/O", "IO", "i/o" or "io".  The terms "I/O port" or "I/O
range" are also used.  There are two main steps to allocate the I/O
addresses (or other bus-resources such as interrupts):

<enum> 
<item> Set the I/O address, etc. on the card (in one of its registers) 
<item> Let its device driver know what this I/O address, etc. is
</enum> 

The two step process above is something like the two part problem of
finding someone's house number on a street.  Someone must install a
number on the front of the house so that it may be found and then you
must obtain (and write down) this house number.  In computers the
device hardware must first set the address it will use in  a special
register and then the device driver must obtain this address.  Both of
these must be done, either automatically by software or by entering
the data manually into some file.  Problems occur when only one of
them gets done (or is attempted).

For manual PnP configuration some people make the mistake of doing
only one of these and then wonder why the computer can't find the
device.  For example, they may use "setserial" to assign an address
to a serial port without realizing that this only tells the driver an
address.  It doesn't set the address in the serial port hardware
itself.  If the serial port hardware doesn't have the address you told
setserial (or doesn't have any address set in it) then you're in
trouble.  

An obvious requirement is that before the device driver can use an
address it must be first set on the physical device (such as a card).
Since device drivers often start up soon after you start the computer,
they sometimes try to access a card (to see if it's there, etc.)
before the address has been set in the card by a PnP configuration
program.  Then you see an error message that they can't find the card
even though it's there (but doesn't yet have an address).

What was said in the last few paragraphs regarding I/O addresses applies
with equal force to other bus-resources: <ref id="mem_" name="Memory
Ranges">, <ref id="interrupt_over" name="IRQs --Overview"> and <ref
id="dma_" name="DMA Channels">.  What these are will be explained in
the next 3 sections.

<sect1> Memory Ranges <label id="mem_">
<p> Many devices are assigned address space in main memory.  It's
sometimes called "shared memory" or "memory-mapped I/O".  This memory
is physically located on the device.  When discussing bus-resources
it's often just called "memory".  In addition to using such "memory",
such a device might also use conventional I/O address space.  

When you plug in such a card, you are in effect also plugging in a
memory module for main memory.  A high address is selected for it by
PnP so that it doesn't conflict with main memory chips.  This memory
can either be ROM (Read Only Memory) or shared memory.  Shared memory
is shared between the device and the CPU (running the device driver)
just as IO address space is shared between the device and the CPU.
This shared memory serves as a means of data "transfer" between the
device and main memory. It's IO but it's not done in IO space.  Both
the card and the device driver need to know where it is. 

ROM is different.  It is likely a program (perhaps a device driver)
which will be used with the device.  It could be initialization code
so that a device driver is still required.  Hopefully, it will work
with Linux and not just MS Windows ??  It may need to be shadowed
which means that it is copied to your main memory chips in order to
run faster.  Once it's shadowed it's no longer "read only".
<sect1> IRQs --Overview <label id="interrupt_over"> 
<p> After reading this you may read <ref id="interrupt_detail" name=
"Interrupts --Details"> for some more details.  The following is
intentionally oversimplified:  Besides the address, there is also an
interrupt number to deal with (such as IRQ 5).  It's called an IRQ
(Interrupt ReQuest) number.  We already mentioned above that the
device driver must know the address of a card in order to be able to
communicate with it.  But what about communication in the opposite
direction?  Suppose the device needs to tell its device driver
something immediately?  For example, the device may have just received
a lot of bytes destined for main memory and the device needs to tell
its driver to fetch these bytes at once and transfer them from the
device's nearly full buffer into main memory.  Another example is to
signal the driver that the device has finished sending out a bunch of
bytes and is now waiting for some more bytes from the driver so it can
send them too.

How should the device rapidly signal its driver?  It may not be able
to use the main data bus since it's likely already in use.  Instead it
puts a voltage on a dedicated interrupt wire (part of the bus) which
is often reserved for that device alone.  This voltage signal is
called an Interrupt ReQuest (IRQ) or just an "interrupt" for short.
There are the equivalent of 16 such wires in a PC and each wire leads
(indirectly) to a certain device driver.  Each wire has a unique IRQ
(Interrupt ReQuest) number.  The device must put its interrupt on the
correct wire and the device driver must listen for the interrupt on
the correct wire.  Which wire the device sends help requests on is
determined by the IRQ number stored in the device.  This same IRQ
number must be known to the device driver so that the device driver
knows which IRQ line to listen on.

Once the device driver gets the interrupt from the device it must find
out why the interrupt was issued and take appropriate action to
service the interrupt.  On the ISA bus each device usually needs its
own unique IRQ number.  For the PCI bus and other special cases the
sharing of IRQs is allowed.

<sect1> DMA Channels <label id="dma_">
<p> DMA channels are only for the ISA bus.  DMA stands for "Direct Memory
Access".  This is where a device is allowed to take over the main
computer bus from the CPU and transfer bytes directly to main memory.
Normally the CPU would make such a transfer in a two step process: 
<enum>
<item>reading from the I/O memory space of the device and putting these
bytes into the CPU itself
<item> writing these bytes from the CPU to main memory
</enum>
<enum>
<item> With DMA it's usually a one step process of sending the bytes
directly from the device to memory  
</enum>
The device must have such capabilities built into its hardware and
thus not all devices can do DMA.  While DMA is going on the CPU can't
do too much since the main bus is being used by the DMA transfer.

The PCI bus doesn't really have any DMA but instead it has something
even better: bus mastering.  It works something like DMA and is
sometimes called DMA (for example, hard disk drives that call
themselves "UltraDMA").  It allows devices to temporarily become bus
masters and to transfer bytes almost like the bus master was the CPU.
It doesn't use any channel numbers since the organization of the PCI
bus is such that the PCI hardware knows which device is currently the
bus master and which device is requesting to become a bus master.
Thus there is no allocation of DMA channels for the PCI bus.

When a device on the ISA bus wants to do DMA it issues a DMA-request
using dedicated DMA request wires much like an interrupt request.  DMA
actually could have been handled by using interrupts but this would
introduce some delays so it's faster to do it by having a special type
of interrupt known as a DMA-request.  Like interrupts, DMA-requests
are numbered so as to identify which device is making the request.
This number is called a DMA-channel.  Since DMA transfers all use the
main bus (and only one can run at a time) they all actually use the
same channel but the "DMA channel" number serves to identify who is
using the "channel".  Hardware registers exist on the motherboard
which store the current status of each "channel".  Thus in order to
issue a DMA-request, the device must know its DMA-channel number which
must be stored in a special register on the physical device.


<sect1> "Resources" for both Device and Driver
<p> Thus device drivers must be "attached" in some way to the hardware
they control.  This is done by allocating bus-resources (I/O, Memory,
IRQ's, DMA's) to both the physical device and the device driver
software.  For example, a serial port uses only 2 (out of 4 possible)
resources: an IRQ and an I/O address.  Both of these values must be
supplied to the device driver and the physical device.  The driver
(and its device) is also given a name in the /dev directory (such as
ttyS1).  The address and IRQ number is stored by the physical device
in configuration registers on its card (or in a chip on the
motherboard).  For the case of jumpers, it's the location of the
jumpers themselves that store the bus-resource configuration in the
device hardware (on the card, etc.).  For the case of PnP, the
configuration register data is usually lost when the PC is powered
down (turned off) so that the bus-resource data must be supplied to
each device anew each time the PC is powered on.

<sect1> The Problem 
<p> The architecture of the PC provides only a limited number of
IRQ's, DMA channels, I/O address, and memory regions.  If there were
only several devices and they all had standardized bus-resource (such
as unique I/O addresses and IRQ numbers) there would be no problem of
attaching device drivers to devices.  Each device would have a fixed
resources which would not conflict with any other device on your
computer.  No two devices would have the same addresses, there would
be no IRQ conflicts, etc.  Each driver would be programmed with the
unique addresses, IRQ, etc. hard-coded into the program.  Life would
be simple.

But it's not.  Not only are there so many different devices today that
conflicts are frequent, but one sometimes needs to have more than one of
the same type of device.  For example, one may want to have a few
different disk-drives, a few network cards, etc.  For these reasons
devices need to have some flexibility so that they can be set to
whatever address, IRQ, etc. is needed to avoid conflicts.  But some
IRQ's and addresses are pretty standard such as the ones for the clock
and keyboard.  These don't need such flexibility.

Besides the problem of conflicting allocation of bus-resources, there
is a problem of making a mistake in telling the device driver what the
bus-resources are.  For example, suppose that you enter IRQ 4 in a
configuration file when the device is actually set at IRQ 5.  This is
another type of bus-resource allocation error.

The allocation of bus-resources, if done correctly, establishes
channels of communication between physical hardware and their device
drivers.  For example, if a certain I/O address range (resource) is
allocated to both a device driver and a piece of hardware, then this
has established a one-way communication channel between them.  The
driver may send commands and info to the device.  It's actually a
little more than one-way since the driver may get information from the
device by reading its registers.  But the device can't initiate any
communication this way.  To initiate communication the device needs an
IRQ so it can send interrupts to its driver.  This creates a two-way
communication channel where both the driver and the physical device
can initiate communication.

<sect1> PnP Finds Devices Plugged Into Serial Ports
<p> External devices that connect to the serial port via a cable (such
as external modems) can also be called Plug-and-Play.  Since only the
serial port itself needs bus-resources (an IRQ and I/O address) there are
no bus-resources to allocate to such plug-in devices.  Thus PnP is not
really needed for them.  Even so, there is a PnP specification for
such external serial devices.

A PnP operating system will find such an external device and read its
model number, etc.  Then it may be able to find a device driver for it
so that you don't have to tell an application program that you have a
certain device on say /dev/ttyS1.  Since you should be able to
manually inform your application program (via a configuration file,
etc.) what serial port the device is on (and possibly what model
number it is) you should not really need this "serial port" feature of
PnP.  I don't think Linux supports it ??

<sect> The Plug-and-Play (PnP) Solution
<sect1> Introduction to PnP
<p> The term Plug-and-Play (PnP) has various meanings.  In the broad
sense it is just auto-configuration where one just plugs in a device
and it configures itself.  In the sense used in this HOWTO, the
configuration is only that of configuring PnP bus-resources and letting
the device drivers know about it.  In a narrower sense it is just
setting bus-resources in the hardware devices.  For the case of Linux,
it is often just a driver detecting how the bus-resources have been
set in its device by the BIOS, etc.  "PnP' may just mean the PnP
specifications which (among other things).  There are long specs for
PnP on the resource the ISA bus.  The standard PCI specifications
(which are not called "PnP") do the same for the PCI bus.

PnP matches up devices with their device drivers and specifies their
communication channels.  On the ISA bus before Plug-and-Play the
bus-resources were formerly set in hardware devices by jumpers.
Software drivers were assigned bus-resources by configuration files
(or the like) or by probing the for the device at addresses where it's
expected to reside.  The PCI bus was PnP-like from the beginning but
at first it wasn't called PnP.  While the PCI bus specifications don't
use the term PnP it supports in hardware what today is called PnP.

<sect1> How It Works (simplified)
<p> Here's an oversimplified view of how PnP should work.  The PnP
configuration program (in Linux only the BIOS  does this) finds all PnP
devices and asks each what bus-resources it needs.  Then it checks what
bus-resources (IRQs, etc.) it has to give away.  Of course, if it has
reserved bus-resources used by non-PnP (legacy) devices (if it knows about
them) it doesn't give these away.  Then it uses some criteria (not
specified by PnP specifications) to give out the bus-resources so that
there are no conflicts and so that all devices get what they need (if
possible).  It then tells each physical device what bus-resources are
assigned to it and the devices set themselves up to use only the
assigned bus-resources.  Then the device drivers somehow find out what
bus-resources their devices use and are thus able to communicate
effectively with the devices they control.  

For example, suppose a card needs one interrupt (IRQ number) and 1 MB
of shared memory.  The PnP program reads this request from the card.
It then assigns the card IRQ5 and 1 MB of memory addresses space,
starting at address 0xe9000000.  It's not always this simple as the
card may specify that it can only use certain IRQ numbers (ISA only)
or that the 1 MB of memory must lie within a certain range of
addresses.  The details are different for the PCI and ISA buses with
more complexity on the ISA bus.

There are some shortcuts that PnP software may use.  One is to keep
track of how it assigned bus-resources at the last configuration (when the
computer was last used) and reuse this.   Windows9x and PnP BIOSs do
this but standard Linux doesn't.  Windows9x stores this info in its
"Registry" on the hard disk and a PnP BIOS stores it in non-volatile
memory in your PC (known as ESCD; see <ref id="escd_" name="The BIOS's
ESCD Database">).

While MS Windows is a PnP OS, Linux isn't.  But PnP still often works
in Linux due to the BIOS doing the configuring of bus-resources and the
device drivers finding out (using programs supplied by the Linux
kernel) what the BIOS has done.  Drivers can also change bus-resource
assignments using such programs (but they might take a bus-resource
needed by another device).  Some device drivers store the last
configuration they used and use it the next time the computer is
powered on.

If the device hardware remembered their previous configuration, then
there wouldn't be any hardware to configure at the next boot-time, but
they seem to forget their configuration when the power is turned off.
Some devices contain a default configuration (but not necessarily the
last one used).  Thus a PnP configuration program needs to be run each
time the PC is powered on.  Also, if a new device has been added, then
it too needs to be configured.  Allocating bus-resources to this new
device might involve taking some bus-resources away from an existing
device and assigning the existing device alternative bus-resources
that it can use instead.  At present, Linux can't do this.

<sect1> Starting Up the PC
<p> When the PC is first turned on the BIOS chip runs its program to
get the computer started (the first step is to check out the
hardware).  If the operating system is stored on the hard-drive (as it
normally is) then the BIOS must know about the hard-drive.  If the
hard-drive is PnP then the BIOS may use PnP methods to find it.  Also,
in order to permit the user to manually configure the BIOS's CMOS and
respond to error messages when the computer starts up, a screen (video
card) and keyboard are also required.  Thus the BIOS must always
PnP-configure these devices on its own.

Once the BIOS has identified the hard-drive, the video card, and the
keyboard it is ready to start booting (loading the operating system
into memory from the hard-disk).  If you've told the BIOS that you
have a PnP operating system (PnP OS), it should start booting the PC
as above and let the operating system finish the PnP configuring.
Otherwise, a PnP-BIOS will (prior to booting) likely try to do the
rest of the PnP configuring of devices (but not informing their
drivers).  This is what usually happens when running Linux.

<sect1> Buses
<p> ISA is the old bus of the old IBM PCs while PCI is a newer and
faster bus from Intel.  The PCI bus was designed for what is today
called PnP.  It makes it easy (as compared to the ISA bus) to find out
how PnP bus-resources have been assigned to hardware devices.  To see
what has happened use the commands <tt/lspci/ or scanpci (Xwindows)
and/or look at <tt>/proc/pci</tt> or <tt>/proc/bus/pci</tt>.  The
boot-up messages on your display are useful (use shift-PageUp to back
up thru them).  See <ref id="boot_time_msgs" name="Boot-time
Messages">

For the ISA bus there was a real problem with implementing PnP since no
one had PnP in mind when the ISA bus was designed and there are almost
no I/O addresses available for PnP to use for sending configuration info
to physical device.  As a result, the way PnP was shoehorned onto the
ISA bus is very complicated.  Whole books have been written about it.
See <ref id="pnp_book" name="PnP Book">.  Among other things, it
requires that each PnP device be assigned a temporary "handle" by the
PnP program so that one may address it for PnP configuring.  Assigning
these "handles" is call "isolation".  See <ref id="isolation_"
name="ISA Isolation"> for the complex details.

Eventually, the ISA bus should become extinct.  When it does, PnP will
be easier since it will be easy to find out how the BIOS has
configured the hardware.  There will still be the need to match up
device drivers with devices and also a need to configure devices that
are added when the PC is up and running.  These needs would be better
satisfied if Linux was a PnP operating system.

<sect1> How Linux Does PnP <label id="how_linux_pnps"> 
<p> Linux has had a serious problem dealing with PnP and still has a
problem but it's not as severe as it once was.  Linux still is not
really a PnP operating system and seems to mainly rely on and device
drivers and the PnP BIOS to configure bus-resources for devices.  But
the kernel provides help for the drivers in the form of PnP programs
they may call on.  In many cases the device driver does all the needed
configuring.  In other cases the BIOS may configure and then the device
driver may find out how the BIOS has configured it.  The kernel
provides the drivers with some functions (program code) that the
drivers may use to find out if their device exists, how it's been
configured, and functions to modify the configuration.  Kernel 2.2
could do this only for the PCI bus but Kernel 2.4 has this feature for
both the ISA and PCI buses (provided that the PNP options have been
selected when compiling the kernel).  This by no means guarantees that
all drivers will fully and correctly use these features.

In addition, the kernel helps avoid resource conflicts by not allowing
two devices to use the same bus-resources at the same time.
Originally this was only for IRQs, and DMAs but now it's for address
resources as well.

Prior to Kernel 2.4, the standalone program: isapnp was often run to
configure and/or get info from PnP devices on the ISA bus.  isapnp is
still needed for cases where the device driver is not fully PnP for
the ISA bus..  There was at least one attempt to make Linux a true PnP
operating system.  See <url
url="http://www.astarte.free-online.co.uk">.  But it never was put
into the kernel.

To see what help the kernel may provide to device drivers see the
kernel documentation.   This documentation (if you have it) is a
directory /usr/.../.../Documentation where one of the ... contains the
word "kernel".  Use the "locate" command to find it.  In this
documentation directory see pci.txt ("How to Write Linux PCI Drivers")
and the file: /usr/include/linux/pci.h.  Unless you are a driver guru
and know C Programming, these files are written so tersely that they
will not actually teach you how to write a driver.  But it will give
you some idea of what PnP type functions are available for drivers to
use.  For the ISA bus see isapnp.txt and possibly (for kernel 2.4)
/usr/include/linux/isapnp.h.

When the PC starts up you may note from the messages on the screen
that some Linux device drivers often find their hardware devices (and
the bus-resources the BIOS has assigned them).  But there are
situations that a real PnP operating system could handle better: 

<itemize>
<item>A shortage of bus-resources
<item>More than one driver for a physical device  
<item>Finding a driver for a detected device (instead of making
  drivers do the searching)
<item>Saving a lot of work for the programmers of device drivers
</itemize>

The "shortage of bus-resources" problem is becoming less of a problem
for two reasons:  One reason is that the PCI bus is replacing the ISA
bus.  Under PCI there is no shortage of IRQs since IRQs may be shared
(even though sharing is less efficient).  Also, PCI doesn't use DMA
resources (although it does the equivalent of DMA without needing such
resources).

The second reason is that more and more physical devices
are using main memory addresses instead of IO address space.  On
32-bit PCs there is 4GB of main memory address space and much of this
bus-resource is available for device IO (unless you had 4GB of main
memory installed).  Compare this to the IO address space which is
limited to 64KB.  So the memory space for device IO is not (yet ?) in
short supply.

<sect> Setting up a PnP BIOS <label id="conf_pnp_bios">
<p> When the computer is first turned on, the BIOS runs before the
operating system is loaded.  Modern BIOSs are PnP and can configure
some or all of the PnP devices.  Here are some of the choices
which may exist in your BIOS's CMOS menu:

<itemize>
<item> <ref id="bios_pnp_os" name="Do you have a PnP operating
 system?">
<item> <ref id="bios_resources" name="How are bus-resources to be controlled?">
<item> <ref id="bios_reset" name="Reset the configuration?">
</itemize>

<sect1> Do you have a PnP operating system? <label id="bios_pnp_os">
<p> In any case the PnP BIOS will PnP-configure the hard-drive, video
card, and keyboard to make the system bootable.  If you said you had a
PnP OS it will leave it up to the operating system (or device drivers)
to finish the configuration job.  If you said no PnP OS then the BIOS
should configure everything.  If you only run Linux on your PC, you
should probably tell it truthfully that you don't have a PnP operating
system If you also run MS Windows on your PC and said it was a PnP OS
when you installed Windows, then you might try saying that you have a
PnP OS to keep Windows 95/98 happy (but it might cause problems for
Linux.  For Windows 2000 it's claimed that Windows worked OK even if you
say you don't have a PnP OS.  In this case Windows 2000 will report
finding new hardware (even though it already knew about the hardware
but didn't know how the BIOS Pnp-configured it).

If you say you have a PnP OS then you rely on the Linux device drivers
and possibly the program isapnp to take care of the bus-resource
configuring.  This often works OK but sometimes doesn't.  Fibbing to
Linux this way has sometimes actually fixed problems.  This could be
because the BIOS didn't do it's job right but Linux alone did.

If you tell the BIOS you don't have a PnP OS, then the BIOS will do
the configuring itself.  Unless you have added new PnP devices, it
should use the configuration which it has stored in its non-volatile
memory (ESCD).  See <ref id="escd_" name="The BIOS's ESCD Database">.
If the last session on your computer was with Linux, then there should
be no change in configuration.  See <ref id="bios_conf" name= "BIOS
Configures PnP">.  But if the last session was with Windows9x (which
is PnP) then Windows could have modified the ESCD.  It supposedly does
this only if you "force" a configuration or install a legacy device.
See <ref id="W9x_ESCD" name="Using Windows to set ESCD">.  Device
drivers that do configuring may modify what the BIOS has done.  So
will the isapnp or PCI Utilities programs.


<sect2> Interoperability with Windows
<p> If you are running both Linux and Windows on the same PC, how do
you answer the BIOS's question: Do you have a PnP OS?  Normally (and
truthfully) you would say no for Linux and yes for Windows.  If you're
truthful, everything should work OK.  But it's a lot of bother to have
to set up the BIOS's CMOS menu manually each time you want to switch
OSs.  To fix this you need to fib about either Linux or Windows.  If
you fib, everything may not work OK but probably can (with some effort)
be made to work OK.

If you intend on fibbing, should you tell the truth about Windows or
Linux?   This depends on how good your Linux of Windows is about
configuring itself if you fib about it.  Both Windows 2000 and Linux
seem to be getting better about coping with such fibs.

<sect2> Fibbing to Linux
If you say that you have a PnP OS, then Linux may work OK if all the
drivers and isapnp (if you use it) are able to configure OK.  Perhaps
updating of the Linux OS and/or drivers will help.

<sect2> Fibbing to Windows9x
<p>Another solution is to set the CMOS for no PnP OS, including when
you start Windows.  Now you are fibbing to Windows and since Windows
is seemingly much more sophisticated in handling PnP.  One would
expect Windows9x to be able to cope with with hardware that has been
fully configured by the BIOS but it can't.  But it's reported that
Windows 2000 can cope with this.  One might expect that even if
Windows9x didn't realize that the hardware was already configured, it
would set the configuration of the physical devices per it's registry
and then then everything would work OK.  

But it doesn't seem to happen this way.  It seems that Windows9x may
just tell its device drivers what has been stored in the Windows'
Registry.  But the actual hardware configuration (done by the BIOS) is
what was stored in the ESCD and may not be the same as the Registry =>
trouble.  So for Windows to work OK you need to get the Registry to
contain the bus-resource configuration which the BIOS creates from the
ESCD.

One way to try to get the Registry and the ESCD the same is to install
(or reinstall) Windows when the BIOS is set for "not a PnP OS".  This
should present Windows with hardware configured by the BIOS.  If this
configuration is without conflicts, Windows will hopefully leave it
alone and save it in it's Registry.  Then the ESCD and the registry
are in sync.  If this works for you (and this is the latest version of
this HOWTO), let me know as I only have one report of this working out
OK.  

Another method is to remove devices that are causing problems in
Windows by clicking on "remove" in the Device Manager.  Then reboot with
"Not a PnP OS" (set it in the CMOS as you start to boot).  Windows
will then reinstall the devices, hopefully using the bus-resource
settings configured by the BIOS.  Be warned that Windows will likely
ask you to insert the Window installation CD since it sometimes can't
find the driver files (and the like) even though they are still
there.  A workaround for this is to select "skip file" and continue.

As a test I "removed" a NIC card which used a Novell compatible
driver.  Upon rebooting, Windows reinstalled it with Microsoft
Networking instead of Novell.  This meant that the Novell Client
needed to be reinstalled --a lot of unnecessary work.

<sect2> Fibbing to Windows 2000
<p>If you have Windows 2000, it's claimed that fibbing to it will work
out OK (if you said it was a PnP-OS when you first installed Windows
2000).  When you change to not a PnP-OS, Windows 2000 is reported to
automatically PnP-reconfigure it's devices and tell you that it's
finding new hardware and installing new devices.

<sect1> How are bus-resources to be controlled? <label
id="bios_resources"> <p> This may involve just deciding how to
allocate IRQ and DMA bus-resources.  If set to "auto", the BIOS will
do the allocation.  If set to manual, you manually reserve some IRQ's
for use on "legacy" (non-pnp) ISA cards.  The BIOS may or may not
otherwise know about such legacy cards.  The BIOS will only know about
these legacy cards if you ran ICU (or the like) under Windows to tell
the BIOS about them.  If the BIOS knows about them, then try using
"auto".  If it doesn't know about them, then manually reserve the
IRQ's needed for the legacy ISA cards and let the rest be for the BIOS
PnP to allocate.

<sect1> Reset the configuration? <label id="bios_reset">
<p> This will erase the BIOSs ESCD data-base of how your PnP devices
should be configured as well as the list of how legacy (non-PnP)
devices are configured.  Never do this unless you are convinced that
this data-base is wrong and needs to be remade.  It was stated
somewhere that you should do this only if you can't get your computer
to boot.  If the BIOS loses the data on legacy devices, then you'll
need to run ICA again under DOS/Windows to reestablish this data.

<sect> How to Deal with PnP Cards
<sect1> Introduction to Dealing with PnP Cards
<p> Today most all new internal boards (cards) are Plug-and-Play
(PnP).  There are 5 different methods listed below to cope with PnP
(but some may not be feasible in your situation).  If the device
driver configures it, then you don't need to do anything.  If the BIOS
configures it, you hope that the driver can find out what the BIOS did
(you may need to tell it this in a configuration file or the like).

<itemize>
<item> <ref id="dev_d_conf" name="Device Driver Configures">
<item>  <ref id="bios_conf" name= "BIOS Configures PnP"> (For the PCI bus
 you only need a PCI BIOS, otherwise you need a PnP BIOS)
<item> <ref id="disable_pnp" name="Disable PnP">  by jumpers or
 DOS/Windows software (but many cards can't do this) 
<item> <ref id="isapnp_" name="Isapnp"> is a program you can always 
 use to configure PnP devices on the ISA bus only
<item> <ref id="pciutils_" name="PCI Utilities"> is for configuring the PCI
 bus but the device driver should handle it
<item> <ref id="windows_conf" name="Windows Configures"> and then you
 boot Linux from within Windows/DOS.  Use as a last resort
</itemize>

Any of the above will set the bus-resources in the hardware but only
the first one tells the driver what has been done.  How the driver gets
informed depends on the driver.  You may need to do something to
inform it.  See <ref id="tell_driver_config" name="Tell the Driver the
Configuration">

<sect1> Device Driver Configures <label id="dev_d_conf">
<p> Many device drivers (with the help of code provided by the kernel)
will use PnP methods to set the bus-resources in the hardware but only
for the device that they control.  Since the driver has done the
configuring, it obviously knows the configuration and there is no need
for you to tell it this info.  This is obviously the easiest way to do
it since you don't have to do anything if the driver does it all.

For PCI devices, most drivers will configure PnP but for ISA devices
it's problematical.  This is because PCI has always been inherently
PnP even though PnP was called "PCI Configuration" (and still is).
For ISA, the kernel provided no functions for PnP configuring until
version 2.4.  So if you have a late version of both the kernel and the
driver then the driver is more likely to configure PnP
(bus-resources).  But if you have older versions (or if the driver
maintainer didn't add PnP to it) then the driver will likely not
configure PnP.

Unfortunately, a driver may grab bus-resources that are needed by other
devices (but not yet allocated to them by the kernel).  Thus a true
PnP Linux kernel would be better where the kernel did the allocation
after all requests were in.  See <ref id="how_linux_pnps" name="How
Linux Does PnP">.

<sect1> BIOS Configures PnP <label id="bios_conf">
<sect2> Intro to Using the BIOS to Configure PnP
<p> If you have a PnP BIOS, it can configure the hardware.  This means
that your BIOS reads the resource requirements of all devices and
configures them (allocates bus-resources to them).  It is a substitute
for a PnP OS except that the BIOS doesn't match up the drivers with
their devices nor tell the drivers how it has done the configuring.
It should normally use the configuration it has stored in its
non-volatile memory (ESCD).  If it finds a new device or if there's a
conflict, the BIOS should make the necessary changes to the
configuration and may not use the same configuration as was in the
ESCD.

Your BIOS must support such configuring and there have been cases
where it doesn't do it correctly or completely.  An advantage of using
the BIOS is that it's simple since in most cases there is nothing to
set up (except to tell the BIOS's CMOS menu it's not a PnP OS).  While
many device drivers will be able to automatically detect what the
BIOS has done, in some cases you may need to determine it (not always
easy).  See <ref id="current_config" name="What Is My Current
Configuration?"> Another possible advantage is that the BIOS does its
work before Linux starts so that all the bus-resources are ready to be
used (and found) by the device drivers that start up later.

According to MS it's only optional (not required) that a PnP BIOS be
able to PnP-configure the devices (without help from MS Windows).  But
it seems that most of the ones made after 1996 ?? or so can do it.  We
should send them thank-you notes if they do it right.  They configure
both the PCI and ISA buses, but it has been claimed that some older
BIOSs can only do the PCI.  To try to find out more about your BIOS,
look on the Web.  Please don't ask me as I don't have data on this.
The details of the BIOS that you would like to know about may be hard
to find (or not available).  Some BIOSs may have minimal PnP
capabilities and seemingly expects the operating system to do it
right.  If this happens you'll either have to find another method or
try to set up the ESCD database if the BIOS has one.  See the next
section.

<sect2> The BIOS's ESCD Database <label id="escd_">
<p> The BIOS maintains a non-volatile database containing a
PnP-configuration that it will try to use.  It's called the ESCD
(Extended System Configuration Data).  Again, the provision of ESCD is
optional but most PnP-BIOSs have it.  The ESCD not only stores the
resource-configuration of PnP devices but also stores configuration
information of non-PnP devices (and marks them as such) so as to avoid
conflicts.  The ESCD data is usually saved on a chip and remains
intact when the power is off, but sometimes it's kept on a hard-drive??  

The ESCD is intended to hold the last used configuration, but if you
use a program such as Linux's isapnp or pci utilities (which doesn't
update the ESCD) then the ESCD will not know about this and will not
save this configuration in the ESCD.  A good PnP OS might update the
ESCD so you can use it later on for a non-PnP OS (like standard
Linux).  MS Windows does this only in special cases.  See <ref
id="W9x_ESCD" name="Using Windows to set ESCD">.

To use what's set in ESCD be sure you've set "Not a PnP OS" or the
like in the BIOS's CMOS.  Then each time the BIOS starts up (before
the Linux OS is loaded) it should configure things this way.  If the
BIOS detects a new PnP card which is not in the ESCD, then it must
allocate bus-resources to the card and update the ESCD.  It may even
have to change the bus-resources assigned to existing PnP cards and
modify the ESCD accordingly. 

If each device saved its last configuration in its hardware, hardware
configuring wouldn't be needed each time you start your PC.  But it
doesn't work this way.  So all the ESCD data needs to be kept correct
if you use the BIOS for PnP.  There are some BIOSs that don't have an
ESCD but do have some non-volatile memory to store info regarding
which bus-resources have been reserved for use by non-PnP cards.  Many
BIOSs have both.

<sect2> Using Windows to set the ESCD  <label id="W9x_ESCD">
<p> If the BIOS doesn't set up the ESCD the way you want it (or the
way it should be) then it would be nice to have a Linux utility to
set the ESCD.  As of early 1999 there wasn't any and now in late 2000
no one has told me about any.   Thus one may resort to attempting to
use Windows (if you have it on the same PC) to do this.

There are three ways to use Windows to try to set/modify the ESCD.  One
way is to use the ICU utility designed for DOS or Windows 3.x.  It
should also work OK for Windows 9x/2k ??  Another way is to set up
devices manually ("forced") under Windows 9x/2k so that Windows will
put this info into the ESCD when Windows is shut down normally.  The
third way is only for legacy devices that are not plug-and-play.  If
Windows knows about them and what bus-resources they use, then Windows
should put this info into the ESCD.

If PnP devices are configured automatically by Windows without the
user "forcing" it to change settings, then such settings probably will 
not make it into the ESCD.  Of course Windows may well decide on its
own to configure the same as what is set in the ESCD so they could
wind up being the same by coincidence.

Windows 9x are PnP operating systems and automatically PnP-configure
devices.  They maintain their own PnP-database deep down in the
Registry (stored in binary Windows files).  There is also a lot of
other configuration stuff in the Registry besides PnP-bus-resources.
There is both a current PnP resource configuration in memory and
another (perhaps about the same) stored on the hard disk.  To look at
this in Windows98 or to force changes to it you use the Device Manager.

In Windows98 there are 2 ways to get to the Device Manager: 1. My
Computer --> Control Panel --> System Properties --> Device Manager.
2. (right-click) My Computer --> Properties --> Device Manager.  Then
in Device Manager you select a device (sometimes a multi-step process
if there are a few devices of the same class).  Then click on
"Properties" and then on "Resources".  To attempt to change the
resource configuration manually, uncheck "Use automatic settings" and
then click on  "Change Settings".  Now try to change the setting, but
it may not let you change it.  If it does let you, you have "forced" a
change.  A message should inform you that it's being forced.  If you
want to keep the existing setting shown by Windows but make it
"forced" then you will have to force a change to something else and
then force it back to its original setting.

To see what has been "forced" under Windows98 look at the "forced
hardware" list: Start --> Programs --> Accessories --> System Tools
--> System Information --> Hardware Resources --> Forced Hardware.
When you "force" a change of bus-resources in Windows, it should put
your change into the ESCD (provided you exit Windows normally).
From the "System Information" window you may also inspect how IRQs and
IO ports have been allocated under Windows.

Even if Windows shows no conflict of bus-resources, there may be a
conflict under Linux.  That's because Windows may assign bus-resources
differently than the ESCD does.  In the the rare case where all
devices under Windows are either legacy devices or have been "forced",
then Windows and the ESCD configurations should be identical.

<sect2> Adding a New Device (under Linux or Windows)
<p> If you add a new PnP device and have the BIOS set to "not a PnP
OS", then the BIOS should automatically configure it and store the
configuration in ESCD.  If it's a non-PnP legacy device (or one made
that way by jumpers, etc.) then here are a few options to handle it:

You may be able to tell the BIOS directly (via the CMOS setup menus)
that certain bus-resources it uses (such as IRQs) are reserved and are
not to be allocated by PnP.  This does not put this info into the
ESCD.  But there may be a BIOS menu selection as to whether or not to
have these CMOS choices override what may be in the ESCD in case of
conflict.  Another method is to run ICU under DOS/Windows.  Still
another is to install it manually under Windows 9x/2k and then make
sure its configuration is "forced" (see the previous section).  If
it's "forced" Windows should update the ESCD when you shut down the
PC.  

<sect1> Disable PnP ? <label id="disable_pnp">
<p> Many devices are PnP only with no option for disabling PnP.  But
for some, you may be able to disable PnP by jumpers or by running a
Windows program that comes with the device (jumperless configuration).
If the device driver can't configure it this will avoid the possibly
complicated task of doing PnP configuring.   Don't forget to tell the
BIOS that these bus-resources are reserved.  There are also some
reasons why you might not want to disable PnP:

<enum>
<item> If you have MS Windows on the same machine, then you may want
 to allow PnP to configure devices differently under Windows from what
 it does under Linux.
<item> The range of selection for IRQ numbers (or port addresses) etc.
 may be quite limited unless you use PnP.
<item> You might have a Linux device driver that uses PnP methods to
 search for the device it controls.
<item> If you need to change the configuration in the future, it may
 be easier to do this if it's PnP (no setting of jumpers or running a
 Dos/Windows program).
<item> You may have (or will have) other PnP devices that need
 configuring so that you'll need to provide for (or learn about) PnP
 anyway.
</enum>
Once configured as non-PnP devices, they can't be configured by PnP
software or a PnP-BIOS (until you move jumpers and/or use the
Dos/Windows configuration software again).

<sect1> Isapnp (part of isapnptools) <label id="isapnp_"> 
<p> Unfortunately, much of the documentation for isapnp is still
difficult to understand unless you know the basics of PnP.  This HOWTO
should help you understand it as well the FAQ that comes with it.
<tt/isapnp/ is only for PnP devices on the ISA bus (non-PCI).  Running
the Linux program "isapnp" at boot-time will configure such devices to
the resource values specified in /etc/isapnp.conf.  Its possible to
create this configuration file automatically but you then must edit it
manually to choose between various options.  

With isapnp there's a danger that a device driver which is built into
the kernel may run too early before isapnp has set the address, etc.
in the hardware.  This results in the device driver not being able to
find the device.  The driver tries the right address but the address
hasn't been set yet in the hardware.

If your Linux distribution automatically installed isapnptools, isapnp
may already be running at startup.  In this case, all you need to do
is to edit /etc/isapnp.conf per "<tt/man isapnp.conf/".  Note that
this is like manually configuring PnP since you make the decisions as
to how to configure as you edit the configuration file.  You can use
the program "pnpdump" to help create the configuration file.  It
almost creates a configuration file for you but you must skillfully
edit it a little before using it.  It contains some comments to help
you edit it.  If you use "isapnp" for configuring and have a PnP BIOS,
you you may want to tell the BIOS (when you set it up) that you don't
have a PnP OS since you may want the BIOS to configure the PCI
devices.  While the BIOS may also configure the ISA devices, isapnp
will redo it.

The terminology used in the /etc/isapnp.conf file may seem odd at
first.  For example for an I0 address of 0x3e8 you might see "(IO 0
(BASE 0x3e8))" instead.  The "IO 0" means this is the first (0th) IO
address-range that this device uses.   Another way to express all this
would be: "IO[0] = 0x3e8" but isapnp doesn't do it this way.  "IO 1"
would mean that this is the second IO address range used by this
device, etc.  "INT 0" has a similar meaning but for IRQs (interrupts).
A single card may contain several physical devices but the above
explanation was for just one of these devices.

<sect1> PCI Utilities  <label id="pciutils_">
<p> The package PCI Utilities (= pciutils, incorrectly called
"pcitools"), should let you manually PnP-configure the PCI bus.
"lspci" or "scanpci" (Xwindows) lists bus-resources while "setpci"
sets resource allocations in the hardware devices.  It appears that
setpci is mainly intended for use in scripts and presently one needs to
know the details of the PCI configuration registers in order to use it.
That's a topic not explained here nor in the manual page for setpci.

<sect1> Windows Configures  <label id="windows_conf">
<p> If you have Windows9x (or 2k) on the same PC, then just start
Windows and let it configure PnP.  Then start Linux from Windows (or
DOS).  But there may be a problem with IRQs for PCI devices.  As
Windows shuts down to make way for Linux, it may erase (zero) the IRQ
which is stored in one of the PCI device's configuration registers.
Linux will complain that it has found an IRQ of zero.

The above is reported to happen if you start Linux using a shortcut
(PIF file).  But a workaround is reported where you still use the
shortcut PIF.  A shortcut is something like a symbolic link in Linux
but it's more than that since it may be "configured".  To start Linux
(from DOS you create a batch file (script) which starts Linux.  (The
program that starts Linux is in the package called "loadlin").  Then
create a PIF shortcut to that batch file and get to the "Properties"
dialog box for the shortcut.  Select "Advanced" and then check "MS-DOS
mode" to get it to start in genuine MS-DOS.

Now here's the trick to prevent zeroing the PCI IRQs.  Check "Specify
a new MS-DOS configuration".  Then either accept the default
configuration presented to you or click on "Configuration" to change
it.  Now when you start Linux by clicking on the shortcut, new
configuration files (Config.sys and Autoexec.bat) will be created per
your new configuration.

The old files are stored as "Config.wos and Autoexec.wos".  After you
are done using Linux and shut down your PC then you'll need these
files again so that you can run DOS the next time you start your PC.
You need to ensure that the names get restored to *.sys and *.bat.
When you leave Windows/DOS to enter Linux, Windows is expecting that
when you are done using Linux you will return to Windows so that
Windows can automatically restore these files to their original names.
But this doesn't happen since when you exit Linux you shut down your
PC and don't get back to Windows.  So how do you get these files
renamed?  It's easy, just put commands into your "start-Linux" batch
file to rename these files to their *.bat and *.sys names.  Put these
renaming commands into your batch file just before the line that loads
Linux.

Also it's reported that you should click on the "General" tab (of the
"Properties" dialog of your shortcut) and check "Read-only".
Otherwise Windows may reset the "Advanced Settings" to "Use current
MS-DOS configuration" and PCI IRQs get zeroed.  Thus Windows erases
the IRQs when you use the current MS-DOS configuration but doesn't
erase when you use a new configuration (which may actually configure
things identical to the old configuration).  Windows does not seem to
be very consistent.


<sect1> PnP Software/Documents  <label id="sw_and_docs">
<p>
<itemize>
<item><url url="http://www.roestock.demon.co.uk/isapnptools/"
 name="Isapnptools homepage">
<item><url url="http://www.astarte.free-online.co.uk" name="Patch to
 make the Linux kernel PnP">
<item> <url url="http://www.io.com/~cdb/mirrors/lpsg/pnp-linux.html"
name="PnP driver project">
<item <url url="http://www.microsoft.com/hwdev/respec/pnpspecs.htm"
 name="PnP Specs. from Microsoft">
<item> Book: PCI System Architecture, 3rd ed. by Tom Shanley +,
 MindShare 1995.  Covers PnP-like features on the PCI bus.
<item> <label id="pnp_book"> Book: Plug and Play System Architecture,
 by Tom Shanley, Mind Share 1995.  Details of PnP on the ISA bus.  Only
 a terse overview of PnP on the PCI bus.
<item> Book: Programming Plug and Play, by James Kelsey, Sams 1995.
 Details of programming to communicate with a PnP BIOS.  Covers ISA,
 PCI, and PCMCIA buses.
</itemize>

<sect> Tell the Driver the Configuration <label id="tell_driver_config">
<sect1> Introduction
<p> Just how this is done depends upon the driver.  Some drivers have
more than one way to find out how their physical device is configured.
At one extreme is the case where you must hard-code the bus-resources into
the kernel and recompile.  At the other extreme, the driver does
everything automatically and you have nothing to do.  It may even set
the bus-resources in the hardware using PnP methods.

In the middle are cases where you run a program to give the resource
info to the driver or put the info in a file.  In some cases the
driver may probe for the device at addresses where it suspects the
device resides.  It may then try to test various IRQs to see which one
works.   It may or may not automatically do this.  In other cases the
driver may use PnP methods to find the device and how the bus-resources
have been set, but will not actually set them.  It may also look in
some of the files in the /proc directory.

One may need to give the bus-resources as a parameter to the kernel or to
a loadable module.  See /usr/lib/modules_help/descr.gz for a list of
possible parameters.  The module to load is listed in /etc/modules
along with its parameters.  In some other case the bus-resources may be
given as parameters to the kernel.  These are put into the lilo.conf
file as append="...".   Then the lilo program must be run to save this
in the kernel boot code.

While there is great non-uniformity about how drivers find out about
bus-resources, the end goal is the same.  If you're having problems
with a driver you may need to look at driver documentation (check the
kernel documentation tree).  Some brief info on a few drivers is
presented in the following sections:

<sect1> Serial Port Driver: setserial
<p> For the standard serial port driver (not for multiport cards) you
use setserial to configure the driver.  It is often run from a
start-up file.  In newer versions there is a /etc/serial.conf file
that you "edit" by simply using the setserial command in the normal
way and what you set using <tt/setserial/ is saved in the
<tt>serial.conf</tt> configuration file.  The <tt>serial.conf</tt>
file should be consulted when the <tt/setserial/ command runs from a
start-up file.  Your distribution may or may not set this up for you.

There are two different ways to use <tt/setserial/ depending on the
options you give it.  One way is used to manually tell the driver the
configuration.  The other way is to probe at a given address and
report if a serial port exists there.  It can also probe this address
and try to detect what IRQ is used for this port.  The driver runs
something like <tt/setserial/ at start-up but it doesn't probe for
IRQs, it just assigns the "standard" IRQ which may be wrong.  It does
probe for the existence of a port.  See Serial-HOWTO for more details.

For PCI serial, the serial driver may detect certain modems and
configure the bus-resources.

<sect1> Sound Card Drivers 

<sect2> OSS-Lite
<p> You must give the IO, IRQ, and DMA as parameters to a module
or compile them into the kernel.  But some PCI cards will get
automatically detected (likely by using the lspci command or the like).
RedHat supplies a program "sndconfig" which detects ISA PnP cards and
automatically sets up the modules for loading with the detected
bus-resources.

<sect2> OSS (Open Sound System) and ALSA
<p> These will detect the card by PnP methods and then select the
appropriate driver and load it.  It will also set the bus-resources on an
ISA-PnP card.  You may need to manually intervene to avoid conflicts.
For the ALSA driver, support for ISA-PnP is optional and you may use
isapnp tools if you want to.

<sect> What Is My Current Configuration? <label id="current_config">
<p> Here "configuration" means the assignment of PnP bus-resources
(addresses, IRQs, and DMAs).  There are two parts to this question for
each device.  Each part should have the same answer.
<enum>
<item> What is the configuration of the device driver software?
 I.e.: What does the driver think the hardware configuration is?
<item> What configuration (if any) is set in the device hardware?
</enum>

Of course the configuration of the device hardware and its driver
should be the same (and it normally is).  But if things are not
working right, it could be because there's a difference.  This means
the the driver has incorrect information about the actual
configuration of the hardware.  This spells trouble.  If the software
you use doesn't adequately tell you what's wrong (or automatically
configure it correctly) then you need to investigate how your hardware
devices and their drivers are configured.  While Linux device drivers
should "tell all" in some cases it's not easy to determine what has
been set in the hardware.

Another problem is that when you view configuration messages on the
screen, it's sometimes not clear whether the reported configuration is
that of the device driver, the device hardware, or both.  If the
device driver is assigned a configuration and then checks the hardware
out to see if it's configured the same, then the configuration
reported by the driver should be that of both the hardware and the
driver. 

But some drivers which don't do this may accept a configuration that
doesn't check out.  For example, "setserial" will accept a
configuration that doesn't check out and thus may only be telling you
the configuration of the driver and not the hardware.

<sect1> Boot-time Messages <label id="boot_time_msgs">
<p> Some info on configuration may be obtained by reading the messages
from the BIOS and Linux that appear on the screen when you first start
the computer.  These messages often flash by too fast to read but once
they stop type Shift-PageUp a few times to scroll back to them.  To
scroll forward thru them type Shift-PageDown.  Typing "dmesg" at any
time to the shell prompt will show only the Linux kernel messages and
miss some of the most important ones (including ones from the BIOS).
The messages from Linux may sometimes only show what the device driver
thinks the configuration is, perhaps as told it via an incorrect
configuration file.  

The BIOS messages will show the actual hardware configuration at that
time, but isapnp, or pci utilities, or device drivers may change it
later.  The BIOS messages are displayed first before the ones from
Linux.  As an alternative to eventually using Shift-PageUp to read
them, try freezing them by hitting the "Pause" key.  Press any key to
resume.  But once the messages from Linux start to appear, it's too
late to use "Pause" since it will not freeze the messages from Linux.

<sect1> How Are My Device Drivers Configured? 
<p> There may be a programs you can run from the command line (such as
"setserial" for serial ports) to determine this.   The /proc directory
tree is useful.  It seems that there are many files buried deep in
this tree that contain bus-resource info.  Only a couple of them will
be mentioned here.  /proc/ioports shows the I/O addresses that the
drivers use (or try if it's wrong).  They might not be set this way in
hardware.

/proc/interrupts shows only interrupts currently in use.  Many
interrupts that have been allocated to drivers don't show at all since
they're not currently being used.  For example, even though your
floppy drive has a floppy disk in it and is ready to use, the
interrupt for it will not show unless its in use.  Again, just because
an interrupt shows up here doesn't mean that it exists in the
hardware.  A clue that it doesn't exist in hardware will be if it
shows that 0 interrupts have been issued by this interrupt.  Even if
it shows some interrupts have been issued there is no guarantee that
they came from the device shown.  It could be that some other device
which is not currently in use has issued them.  A device not in use (per
the kernel) may still issue some interrupts for various reasons.

<sect1> How Are My Hardware Devices Configured? 
<p> It's easy to find out what bus-resources have been assigned to
devices on the PCI bus with the "lspci" or "scanpci" commands.  For
for kernels &lt 2.2: see <tt>/proc/pci</tt> or <tt>/proc/bus/pci</tt>
for later kernels.  Note that IRQs for <tt>/proc/pci</tt> are in
hexadecimal.  Don't bother trying to decipher
<tt>/proc/bus/pci/devices</tt> since "lspci" will do that for you.  

For the ISA bus you may try running <tt>pnpdump --dumpregs</tt>
but it's not a sure thing.  The results may be seem cryptic but they
can be deciphered.  Don't confuse the read-port address which
<tt/pnpdump/ "tries" (and finds something there) with the I/O address
of the found device.  They are not the same.  To try to find missing
hardware on the ISA bus (both PnP and legacy) try the program
"scanport" but be warned that it might hang your PC.  It will not show
the IRQ nor will it positively identify the hardware.

Messages from the BIOS at boot-time tell you how the hardware
configuration was then.  If only the BIOS does the configuring, then
it should still be the same.  Messages from Linux may be from drivers
that used kernel PnP functions to inspect and/or set bus-resources.
These should be correct, but beware of messages that only show what is
in some configuration file (what the driver thinks).  Of course, if
the device works fine, then it's likely configured the same as the
driver.

Some people use Windows to see how bus-resources have be set up.
Unfortunately, since the hardware forgets its bus-resource
configuration when powered down, the configuration may not be the same
under Linux.  It sometimes turns out the same, but Windows and Linux
do not necessarily use the same logic in configuring.  Many devices
have bus-resource preferences that are often adhered to by both Linux
and Windows.  But don't count on devices being configured the same.

<sect> Appendix
<sect1> Universal Plug and Play (UPnP) <label id="UPnP_">
<p> This is actually a sort of network plug-and-play developed by
Microsoft but usable by Linux.  You plug something into a network and
that something doesn't need to be configured provided it will only
communicate with other UPnP enabled devices on the network.  Here
configure is used in the broad sense and doesn't mean just configuring
bus-resources.  One objective is to allow people who know little about
networks or configuring to install routers, gateways, network
printers, etc.  A major use for UPnP would be in wireless networking.

UPnP uses:
<itemize>
<item> Simple Service Discovery Protocol to find devices
<item> General Event Notification Architecture
<item> Simple Object Access Protocol for controlling devices
</itemize>

This HOWTO doesn't cover UPnP.  UPnP for Linux is supported by Intel
which has developed software for it.  There are other programs which
do about the same thing as UPnP.  A comparison of some of them is at
<url url="http://www.cs.umbc.edu/~dchakr1/papers/mcommerce.html">

<sect1> Address Details <label id="address_details">
<p> There are three types of addresses: main memory addresses, I/O
addresses and configuration addresses.  On the PCI bus, configuration
addresses constitute a separate address space just like I/O addresses
do.  Except for the complicated case of ISA configuration addresses
whether or not an address on the bus is a memory address, I/O address,
or configuration address depends only on the voltage on other wires
(traces) of the bus.  For the ISA configuration addresses see <ref
id="isa_conf_addresses" name="ISA Bus Configuration Addresses
(Read-Port etc.)"> for details

<sect2> Address ranges
<p> The term "address" is sometimes used in this document to mean a
contiguous range of addresses.  Since addresses are in units of bytes,
a single address only contains one byte but I/O (and main memory)
addresses need more than this.  So a range of say 8 bytes is often
used for I/O address while the range for main memory addresses
allocated to a device is much larger.  For a serial port (an I/O
device) it's sufficient to give the starting I/O address of the device
(such as 3F8) since it's well known that the range of addresses for
serial port is only 8 bytes.  The starting address is known as the
"base address".  Sometimes just the word "range" is used to mean
"address range".

<sect2> Address space 
<p> For ISA, to access both I/O and (main) memory address "spaces"
the same address bus is used (the wires used for the address are
shared).  How does the device know whether or not an address which
appears on the address bus is a memory address or I/O address?  Well,
there are 4 dedicated wires on the bus that convey this information
and more.  If a certain one of these 4 wires is asserted, it says that
the CPU wants to read from an I/O address, and the main memory ignores
the address on the bus.  The other 3 wires serve similar purposes.
Thus read and write wires exist for both main memory and I/O addresses
(4 wires in all).  

For the PCI bus it's the same basic idea (also using 4 wires) but it's
done a little differently.  Instead of only one or the four wires
being asserted, a binary number is put on the wires (16 different
possibilities).  Thus more info may be conveyed.   Four of these 16
numbers serve the I/O and memory spaces as in the above paragraph.  In
addition there is also configuration address space which uses up two
more numbers.  Ten extra numbers are left over for other purposes.

<sect2> Range Check (ISA Testing for IO Address Conflicts)
<p> On the ISA bus, there's a method built into each PnP card for
checking that there are no other cards that use the same I/O address.
If two or more cards use the same IO address, neither card is likely
to work right (if at all).  Good PnP software should assign
bus-resources so as to avoid this conflict, but even in this case a
legacy card might be lurking somewhere with the same address.

The test works by a card putting a test number in its own IO
registers.  Then the PnP software reads it and verifies that it reads
the same test number.  If not, something is wrong (such as another
card with the same address.  It repeats the same test with another
test number.  Since it actually checks the range of IO addresses
assigned to the card, it's called a "range check".  It could be better
called an address-conflict test.  If there is an address conflict you
get an error message and need to resolve it yourself.

<sect2> Communicating Directly via Memory
<p> Traditionally, most I/O devices used only I/O memory to
communicate with the CPU.  For example, the serial port does this.
The device driver, running on the CPU would read and write data
to/from the I/O address space and main memory.  A faster way would be
for the device itself to put the data directly into main memory.  One
way to do this is by using <ref id="dma_" name="DMA Channels"> or bus
mastering.  Another way is to allocate some space in main memory to
the device.  This way the device reads and writes directly to main
memory without having to bother with DMA or bus mastering.  Such
a device may also use IO addresses.

<sect1> ISA Bus Configuration Addresses (Read-Port etc.)
<label id="isa_conf_addresses">
<p> These addresses are also known as the "Auto-configuration Ports".
For the ISA bus, there is technically no configuration address space,
but there is a special way for the CPU to access PnP configuration
registers on the PnP cards.  For this purpose 3 @ I/O addresses are
allocated and each addresses only a single byte (there is no "range").
This is not 3 addresses for each card but 3 addresses shared by all
ISA-PnP cards.

These 3 addresses are named read-port, write-port, and address-port.
Each port is just one byte in size.  Each PnP card has many
configuration registers so that just 3 addresses are not even
sufficient for the configuration registers on a single card.  To solve
this problem, each card is assigned a card number (handle) using a
technique called "isolation".  See <ref id="isolation_" name="ISA
Isolation"> for the complex details.

Then to configure a certain card, its card number (handle) is sent 
out via the write-port address to tell that card that it is to listen
at its address port.  All other cards note that this isn't their card
number and thus don't listen.  Then the address of a configuration
register (for that card) is sent to the address-port (for all cards
--but only one is listening).  Next, data transfer takes place with
that configuration register on that card by either doing a read on the
read-port or a write on the write-port.

The write-port is always at A79 and the address-port is always at 279
(hex).  The read-port is not fixed but is set by the configuration
software at some address (in the range 203-3FF) that will hopefully
not conflict with any other ISA card.  If there is a conflict, it will
change the address.  All PnP cards get "programmed" with this address.
Thus if you use say isapnp to set or check configuration data it must
determine this read-port address.

<sect1> Interrupts --Details <label id="interrupt_detail">
<p> Interrupts convey a lot of information but only indirectly.  The
interrupt request signal (a voltage on a wire) just tells a chip
called the interrupt controller that a certain device needs attention.
The interrupt controller then signals the CPU.  The CPU then
interrupts whatever it was doing, finds the driver for this device and
runs a part of it known as an "interrupt service routine" (or
"interrupt handler").  This "routine" tries to find out what has
happened and then deals with the problem.  For example, bytes may need
to be transferred from/to the device.   This program (routine) can
easily find out what has happened since the device has registers at
addresses known to the the driver software (provided the IRQ number
and the I/O address of the device has been set correctly).  These
registers contain status information about the device .  The software
reads the contents of these registers and by inspecting the contents,
finds out what happened and takes appropriate action.

Thus each device driver needs to know what interrupt number (IRQ) to
listen to.  On the PCI bus (and for some special cases on the ISA bus)
it's possible for two (or more) devices to share the same IRQ number.
When such an interrupt is issued, the CPU runs all interrupt service
routines for all devices using that interrupt.  The first thing the
first service routine does is to check its device registers to see if
an interrupt actually happened for its device.  If it finds that its
device didn't issue an interrupt (a false alarm) it likely will
immediately exit and the service routine begins for the second device
using that same interrupt, etc, etc.

The putting of a voltage on the IRQ line is only a request that the
CPU be interrupted so it can run a device driver.  In almost all cases
the CPU is interrupted per the request.  But interrupts may be
temporarily disabled or prioritized so that in rare cases the actual
interrupt doesn't happen (or gets delayed).  Thus what was above
called an "interrupt" is more precisely only an interrupt request
and explains why IRQ stands for Interrupt ReQuest.

<sect1> PCI Interrupts
<p> There are two newer developments in PCI interrupts that are not
covered here.  They are especially important for cases of more than
one CPU per computer.  One is the Advanced Programmable Interrupt
Controller (APIC).  Another is Message Signalled Interrupts (MSI)
where the interrupt is just a message sent to a special address over
the main computer bus (no interrupt lines needed).  But the device
that sends such a message must first gain control of the main bus so
that it can send the interrupt message.  Such a message contains more
info than just "I'm sending an interrupt".

Ordinary PCI interrupts are different than ISA interrupts, but since
they are normally mapped to IRQ's they behave in about the same way.
One major difference is that the BIOS does this mapping.  Under Linux
it's not feasible to change it ?? unless the CMOS menu will let you do
it.  Another major difference is that PCI interrupts may be shared.
For example IRQ5 may be shared between two PCI devices.  This sharing
ability is built into the hardware and all device drivers are supposed
to support it.  Note that you can't share the same interrupt between
the PCI and ISA bus.  However, illegal sharing will work provided the
devices which are in conflict are not in use at the same time.  "In
use" here means that a program is running which "opened" the device in
its C programming code.  

Here are some of the details of the PCI interrupt system.  Each PCI
card (and device mounted on the motherboard) has 4 possible
interrupts: INTA#, INTB#, INTC#, INTD#.  From now on we may call them
just A, B, C, and D.  Each has its own pin on the edge connector of a
card.  Thus for a 7-slot system there could be 7 x 4 = 28  different
interrupt lines for the cards.  But the specs permit a fewer number of
interrupt lines so many PCI buses seem to be made with only 4
interrupt lines.  This is not too restrictive since interrupts may be
shared.  Call these lines (wires or traces) W, X, Y, Z.  Suppose we
designate the B interrupt from slot 3 as interrupt 3B. 

One simple method of connecting these lines to the interrupts would be
to connect all A interrupts (INTA#) to line W, all B's to X,
etc.  This method was used several years ago but it is not a
good solution.  Here's why.  If a card only needs one interrupt, it's
required that it use A.  If it needs two interrupts, it must use both
A and B, etc.  Thus INTA# is used much more often than INTD#.  So one
winds up with an excessive number of interrupts sharing the first line
(W connected to all the INTA#).  To overcome this problem one may
connect them in a more complicated way so that each of the 4 interrupt
lines (W, X, Y, Z) will share about the same number of interrupts.

One method of doing this would be to have wire W share interrupts 1A,
2B, 3C, 4D, 5A, 6B, 7C.  This is done by physically connecting wire W
to wires 1A, 2B, etc.  Likewise wire X could be connected to wires 1B,
2C, 3D, 4A, 5B, 6C, 7D, etc.  Then on startup, the BIOS maps the X, W,
Y, Z to IRQs.  After that it writes the IRQ that each device uses into
a hardware configuration register in each device.  From then on any
program interrogating this register can find out what IRQ the device
uses.

A card in a slot may have up to 8 devices on it but there are only 4
PCI interrupts for it (A, B, C, D).  This is OK since interrupts may
be shared so that each of the 8 devices (if they exist) can have an
interrupt.  The PCI interrupt letter of a device is often fixed and
hardwired into the device.  The assignment of interrupts is done by
the BIOS mapping the ISA interrupts to the PCI interrupts as mentioned
above.  If there are only 4 lines (W, X, Y, and Z) as in the above
example, the choices the PCI BIOS has are limited.  Some motherboards
may use more lines and thus have more choices.  The BIOS knows about
how this is wired.

The BIOS assigns IRQs (interrupts) so as to avoid conflicts with the
IRQs it knows about on the ISA bus.  Sometimes in the CMOS BIOS menu
one may allow one to assign IRQs to PCI cards. 

You might think that since the PCI is using IRQ's (ISA bus) it might
be slow since the ISA bus is slow.  Not really.  The ISA Interrupt
Controller Chip(s) has a direct interrupt wire going to the CPU so it
can get immediate attention.  While signals on the ISA address and
data buses may be slow to get to the CPU, the IRQ interrupt signals
get there almost instantly.

<sect1> ISA Isolation <label id="isolation_">
<p> This is only for the ISA bus.  Isolation is a complex method of
assigning a temporary handle (id number or Card Select Number = CSN)
to each PnP device on the ISA bus.  Since there are more efficient
(but more complex) ways to do this, some might claim that it's a
simple method.  Only one write address is used for PnP writes to all
PnP devices so that writing to this address goes to all PnP device
that are listening.  This write address is used to send (assign) a
unique handle to each PnP device.  To assign this handle requires that
only one device be listening when the handle is sent (written) to this
common address.  All PnP devices have a unique serial number which
they use for the process of isolation.  Doing isolation is something
like a game.  It's done using the equivalent of just one common bus
wire connecting all PnP devices to the isolation program.

For the first round of the "game" all PnP devices listen on this wire
and send out simultaneously a sequence of bits to the wire.  The
allowed bits are either a 1 (positive voltage) or an "open 0" of no
voltage (open circuit or tri-state).  To do this, each PnP device just
starts to sequentially send out its serial number on this wire,
bit-by-bit, starting with the high-order bit.  If any device sends a
1, a 1 will be heard on the wire by all other devices.  If all devices
send an "open 0" nothing will be heard on the wire.  The object is to
eliminate (by the end of this first round) all but highest serial
number device.  "Eliminate" means to drop out of this round of the
game and thus temporarily cease to listen anymore to the wire.  (Note
that all serial numbers are of the same length.)  When there remains
only one device still listening, it will be given a handle (card
number).

First consider only the high-order bit of the serial number which is
put on the wire first by all devices which have no handle yet.  If any
PnP device sends out a 0 (open 0) but hears a 1, this means that some
other PnP device has a higher serial number, so it temporarily drops
out of this round.  Now the devices remaining in the game (for this
round) all have the same leading digit (a 1) so we may strip off this
digit and consider only the resulting "stripped serial number" for
future participation in this round.  Then go to the start of this
paragraph and repeat until the entire serial number has been examined
for each device (see below for the all-0 case). 

Thus it's clear that only cards with the lower serial number get
eliminated during a round.  But what happens if all devices in the
game all send out a 0 as their high-order bit?  In this case an "open
0" is sent on the line and all participants stay in the game.  If they
all have a leading 0 then this is a tie and the 0's are stripped off
just like the 1's were in the above paragraph.  The game then
continues as the next digit (of the serial number) is sent out.

At the end of the round (after the low-order bit of the serial number
has been sent out) only one PnP device with the highest serial number
remains in the game.  It then gets assigned a handle and drops out of
the game permanently.  Then all the dropouts from the previous round
(that don't have a handle yet) reenter the game and a new round begins
with one less participant.  Eventually, all PnP devices are assigned
handles.  It's easy to prove that this algorithm works.  The actual
algorithm is a little more complex than that presented above since
each step is repeated twice to ensure reliability and the repeats are
done somewhat differently (but use the same basic idea).

Once all handles are assigned, they are used to address each PnP
device for sending/reading configuration data.  Note that these
handles are only used for PnP configuration and are not used for
normal communication with the PnP device.  When the computer starts
up a PnP BIOS will often do such an isolation and then a PnP
configuration.  After that, all the handles are "lost" so that if one
wants to change (or inspect) the configuration again, the isolation
must be done over again.

END OF Plug-and-Play-HOWTO
</article>