v0.03, May 1999
Help with understanding and dealing with the complex Plug-and-Play
issue. How to get your Linux system to support Plug-and-Play.
Table of Contents
1.1 Copyright, Trademarks, Disclaimer, & Credits
1.2 Future Plans; You Can Help
1.3 New Versions of this HOWTO
2. What PnP Should Do: Allocate "Resources"
2.1 What is Plug-and-Play (PnP)?
2.2 How a Computer Finds Devices (and conversely)
2.3 I/O Addresses, etc.
2.4 IRQs --Overview
2.5 DMA Channels
2.6 Memory Ranges
2.7 "Resources" to both Device and Driver
2.8 The Problem
2.9 PnP Finds Devices Plugged Into Serial Ports
3. The Plug-and-Play (PnP) Solution
3.1 Introduction to PnP
3.2 How It Works (simplified)
3.3 Starting Up the PC
3.5 Linux Needs to Cope Better with PnP
4. Configuring a PnP BIOS
4.1 Do you have a PnP operating system?
4.2 How are resources to be controlled?
4.3 Reset the configuration?
5. How to Deal with PnP Cards
5.1 Introduction to Dealing with PnP Cards
5.2 Disable PnP ?
5.3 BIOS Configures PnP
5.3.1 Intro to Using the BIOS to Configure PnP
5.3.2 The BIOS's ESCD Database
5.3.3 Using Windows to set ESCD
5.3.4 Adding a New Device (under Linux or Windows)
5.4 Isapnp (part of isapnptools)
5.5 PCI Utilities
5.6 Patch the Kernel to Make Linux PnP
5.7 Windows Configures
5.8 PnP Software/Documents
6. What Is My Current Configuration?
6.1 How Are My Device Drivers Configured?
6.2 How Are My Hardware Devices Configured?
7.1.1 ISA Bus Configuration Address (Read-Port etc.)
7.1.2 Address ranges
7.1.3 Address space
7.1.4 Range Check (ISA Testing for IO Address Conflicts)
7.1.5 Communicating Directly via Memory
7.2 Interrupts --Details
7.3 PCI Interrupts
1.1. Copyright, Trademarks, Disclaimer, & Credits
Copyright (c) 1998 by David S. Lawyer. Please freely copy and
distribute (sell or give away) this document. For corrections and
minor changes contact the maintainer. Otherwise you may create
derivative works and distribute them provided you:
1. Discuss it with the maintainer (if there is one). 2. Put the
derivative work at the mirrored LDP Internet site (or the like) for
free downloading. 3. License the work in the spirit of this license
or use GPL. 4. Give due credit to previous authors and major
If certain words are trademarks, the context should make it clear to
whom they belong. For example "MS Windows" (or just "Windows")
implies that "Windows" belongs to Microsoft.
Much of the info in this HOWTO was obtained from the Internet,
implications in books that may be obsolete, etc. 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 neither I nor
previous authors can be held legally responsible for any errors.
1.2. Future Plans; You Can Help
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 in detail either isapnptools nor David Howells
patches to the kernel. 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.
Would you like to improve on (rewrite) and maintain this HOWTO? I'm
looking for someone to turn it over to.
1.3. New Versions of this HOWTO
New versions of the Plug-and-Play-HOWTO should appear every month or
so and will be available to browse and/or download at LDP mirror
sites. For a list of mirror sites see:
. Various formats are
available. If you only want to quickly check the date of the latest
version look at: .
2. What PnP Should Do: Allocate "Resources"
2.1. What is Plug-and-Play (PnP)?
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
device and its driver. More precisely, PnP allocates the following
"resources" to both drivers and hardware: I/O addresses, IRQ's, DMA
channels (ISA bus only), and memory regions. If you don't understand
what these 4 items are read the following subsections. Once these
resources have been assigned, the names for such devices in the /dev
directory are ready to use.
This PnP assignment of certain 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 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 (often automatically boot-time). This
configuring also has nothing to do with PnP. Thus when talking about
PnP, "resources" means only a limited subset of resources and
"configuring" means only a certain type of configuring.
2.2. How a Computer Finds Devices (and conversely)
A computer consists of a CPU/processor to do the computing and memory
to store programs and data. 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, 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 a single board called 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 (or 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. 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
The communication channel is actually a little more complex than
described above. An "address" is actually a range of addresses and
there is a reverse part of the channel (known as interrupts) which
allows devices to send an urgent "help" request to their device
2.3. I/O Addresses, etc.
PC's have 3 address spaces: I/O, main memory, and configuration (only
on the PCI bus). All of these 3 types of addresses share the same
address bus inside the PC. But the voltage on certain dedicated wires
tells all devices which "space" an address is in: I/O, main memory, or
configuration. See ``Addresses'' for more details. Devices were
normally located in I/O address space although today they may use
space in main memory. An I/0 address is sometimes just called "I/O",
"IO", "i/o" or "io". The term "I/O port" also used. There are two
main steps to allocate the I/O addresses (or other resources such as
1. Set the I/O address, etc. on the card (in one of its registers)
2. Let its device driver know what this I/O address, etc. is
The two step process above is something like the two part problem of
finding someone's house number on a street. You must obtain (and
write down) the house number and someone must install a number on the
front of the house so that it may be found. In computers, the device
driver must obtain the address and the device hardware must get the
same address set in one of its registers. Both of these must be done,
but 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 will
use "setserial" to assign an address to a serial port without
realizing that this only tells the driver the address. It doesn't set
the address in the serial port hardware itself. If you told the
driver the wrong address, you're in trouble.
Another obvious requirement is that the I/O address must be set on the
card before the device driver tries to use this address. 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 2 paragraphs regarding I/O addresses applies
with equal force to other resources: ``IRQs --Overview'', ``DMA
Channels'', and ``Memory Regions''. What theses are will be explained
in the next 3 sections.
2.4. IRQs --Overview
After reading this you may read ``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 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 call its driver to fetch these bytes at once
and transfer them from the device's nearly full buffer into main
How should the device call for help? It can't use the main data bus
since it's likely already in use. Instead it puts a voltage on an
dedicated interrupt wire (part of the bus) which is often reserved for
that device alone. This signal is called an interrupt. 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 it's put 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
Once the device driver gets the interrupt (a call for help) it must
find out why the interrupt was issued and take appropriate action to
service the interrupt. On the ISA bus each device needs its own
unique IRQ number. For the PCI bus and other special cases the
sharing of IRQs is allowed.
2.5. DMA Channels
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: 1.
reading from the I/O memory space of the device and putting these
bytes into the CPU itself 2. writing these bytes from the CPU to main
memory. With DMA it's usually a one step process of sending the bytes
directly from the device to memory. 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-request 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 register on the physical device.
2.6. Memory Ranges
Some devices are assigned address space in main memory. It's often
"shared memory" or "memory-mapped I/O". Sometimes it's ROM memory on
the device. When discussing PnP resources it's often just called
"memory". Such a device might also use I/O address space.
When you plug in such a card, you are in effect also plugging in a
memory module for main memory. 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). This memory can serve
as a means of direct data "transfer" between the device and main
memory. It's not really a transfer since the device puts data into
its own memory on its card which also happens to be in main memory.
Both the card and the device driver need to know where it is. The
memory address are likely to be very high so that they do not conflict
with the lower addresses of the memory chips in your computer.
ROM is different. It is likely a program (perhaps a device driver)
which will be used with the device. Hopefully, it may work with Linux
and not just 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".
2.7. "Resources" to both Device and Driver
Thus device drivers must be "attached" in some way to the hardware
they control. This is done by supplying "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 registers on the card (or in a chip on the motherboard). For the
case of jumpers, this info is always stored in the device hardware (on
the card, etc.). But for the case of PnP, the register data is
usually lost when the PC is powered down (turned off) so that the
resource data must be supplied to each device anew each time the PC is
2.8. The Problem
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 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
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 serial ports, 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 resources, there is a
problem of making a mistake in telling the device driver what the
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 resource allocation error.
The allocation of 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 it's registers. But the device can't initiate any
communication this way. To initiate the device needs an IRQ in order
to create a two-way communication channel where both the driver and
the device can initiate communication.
2.9. PnP Finds Devices Plugged Into Serial Ports
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 resources (an IRQ and I/O address) there are
no 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
3. The Plug-and-Play (PnP) Solution
3.1. Introduction to PnP
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 resources and letting the device
drivers know about it. In a more narrow sense it just setting
resources in the hardware devices. It may also mean the PnP
specifications which (among other things) specify how PnP resource
data is to be read and written to devices (often cards) on the ISA
bus. The standard PCI (and not PnP) specifications 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
resources were set in hardware devices by jumpers. Software drivers
were assigned 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 so it was trivial to
implement PnP for this bus. Since the PCI bus specifications don't
use the term PnP it's not clear whether or not the PCI bus should be
called PnP (but it supports in hardware what today is called PnP).
3.2. How It Works (simplified)
Here's an oversimplified view of how PnP works. The PnP configuration
program (perhaps a program in the BIOS) finds all PnP devices and asks
each what resources it needs. Then it checks what resources (IRQs,
etc.) it has to give away. Of course if it has reserved 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 resources so that there are no
conflicts and so that all devices get what they need. It then tells
each physical device what resources are assigned to it and the devices
set themselves up to use only the assigned resources. Then the device
drivers somehow find out what 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 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" and a PnP BIOS stores it in non-volatile memory in your PC
(known as ESCD; see ``The BIOS's ESCD Database'').
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 resources to this new
device might involve taking some resources away from an existing
device and assigning the existing device alternative resources.
3.3. Starting Up the PC
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 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 "boot" (load the operating system from the
hard-disk). If you've told the BIOS that you have a PnP operating
system, it should boot the PC and let the operating system finish the
PnP configuring. Otherwise, a PnP-BIOS will likely try to do the rest
of the PnP configuring of devices (but not their drivers).
ISA is the old bus of the old IBM PC's while PCI is a newer and faster
bus from Intel. Eventually, the ISA bus should become extinct. When
it does, there should be almost no PnP problem in Linux provided: 1.
the only bus in use is PCI. 2. The BIOS does a good job of PnP
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 resources have
been assigned to hardware devices. To see what has happened look at
the /proc/pci "file" (/proc/bus/pnp/devices for kernel 2.2+), the
boot-up messages on your display (use shift-PageUp to back up), or use
PCI Utilities (for kernel 2.2+).
For the ISA bus there is 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 configure info
to physical device. As a result, the way PnP was shoehorned onto the
ISA bus is very complicated. A whole book has been written about it.
See ``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 ``Isolation'' for the complex details.
3.5. Linux Needs to Cope Better with PnP
PnP was invented by Compaq, Intel, and Phoenix. Microsoft has been a
leading promoter of it. Linux would have been better off if PnP had
never been "invented". Eventually the ISA bus would have become
extinct and the PnP-like PCI bus would prevail so that we would have
in effect gotten an easy-to-implement PnP. But like it or not, most
all new ISA hardware today is PnP and Linux has no choice but to deal
effectively with PnP. But standard Linux (as of early 1999) makes
dealing with PnP complicated (especially on the ISA bus) while the
purpose of PnP was to make it simple.
In a sense, Linux is already somewhat PnP for the PCI bus. 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
resources the BIOS has assigned them). But there still could be
conflicts with the ISA bus.
Linux users should not need to delve into the details of PnP to
configure ISA PnP devices as they now need to. One solution would be
a standardized version of the Linux kernel that supports Plug-and-Play
on both the ISA and PCI buses. A patch to the kernel has been written
although most drivers don't support it. It's not part of standard
Linux. See ``Patch Kernel''.
4. Configuring a PnP BIOS
When the computer is first turned on, the BIOS runs before the
operating system is loaded. Newer BIOSs are PnP and will configure
some or all of the PnP devices. For most PnP BIOSs there is no way to
disable PnP so you have to live with it. Here are some of the choices
which may exist in your BIOS's CMOS menu:
· ``Do you have a PnP operating system?''
· ``How are resources to be controlled?''
· ``Reset the configuration?''
4.1. Do you have a PnP operating system?
If you say yes, then the PnP BIOS will PnP-configure the hard-drive,
video card, and keyboard to make the system bootable. But it will
leave it up to the operating system to finish the configuration job.
It may do an ``Isolation'' on the ISA bus leaving the devices disabled
but ready to be configured by the operating system. You should
probably tell it that you don't have a PnP operating system. If you
don't do this, the BIOS might leave the ISA devices it hasn't
configured in a disabled state ?? Also PCI devices might not get
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 ``The BIOS's ESCD Database'' If the last session
on your computer was with Linux, then there should be no change in
configuration. See ``BIOS Configures PnP''. But if the last session
was with Windows9x (which are PnP) then Windows could have set up the
configuration differently and possibly saved some of it in the ESCD.
See ``Using Windows to set ESCD''. If you are using the isapnp or PCI
Utilities program(s) to do configuring, they will run after the BIOS
runs and change things the way you told them to.
If you are running Windows on the same PC, how do you answer the
question: Do you have a PnP OS? Normally (and truthfully) you would
say no for standard Linux and yes for Windows9x. But it's a lot of
bother to have to set up the CMOS menu manually each time you want to
change OSs. One solution is set the CMOS for no PnP OS, including
when you start Windows. This should work OK if the configuration in
the ESCD is the same as in the Windows' Registry. One way (the only
way??) to try to get them the same is to install (or reinstall)
Windows when the BIOS is set for "not a PnP OS". This should present
Windows with a completely configured system (except for the devices
drivers). If this configuration is without conflicts, Windows will
hopefully leave it alone and save it in it's registry. 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.
4.2. How are resources to be controlled?
This may involve just deciding how to allocate IRQ and DMA 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) cards.
The BIOS may or may not otherwise know about your legacy cards. The
BIOS will only know about your 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.
4.3. Reset the configuration?
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.
5. How to Deal with PnP Cards
5.1. Introduction to Dealing with PnP Cards
Today most all new internal boards (cards) are Plug-and-Play (PnP).
Although some software exists in Linux to handle PnP, it is not always
easy to use. There are 6 different methods listed below to cope with
PnP (but some may not be feasible in your situation). Which one(s)
you should use depends on your goals. What may be most expedient to
do now may not be the easiest and best in the long run. A seemingly
simple way is to do nothing and just let a PnP-BIOS configure it but
then you may need to do some exploring to to find out what the BIOS
has done. A comparison of these methods needs to be written by
someone who has tried them all. You may need to use more than one
method to do the job.
· ``Disable PnP'' by jumpers (but many cards can't do this) or
· ``BIOS Configures PnP'' (For the PCI bus you only need a PCI BIOS,
otherwise you need a PnP BIOS)
· ``Isapnp'' is a program you can always use to configure PnP devices
on the ISA bus only
· ``PCI Utilities'' is for configuring the PCI bus
· ``Patch Kernel'' to transform Linux into a PnP operating system
· ``Windows Configures'' and then you boot Linux from within
Windows/DOS. Use as a last resort.
5.2. Disable PnP ?
Many devices are PnP only with no option for disabling it. 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).
This will avoid the often complicated task of configuring PnP. Don't
forget to tell the BIOS the resources that it uses. There are also
some reasons why you might not want to disable PnP:
1. 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.
2. The range of selection for IRQ numbers (or port addresses) etc.
may be quite limited unless you use PnP.
3. If it requires the use of Dos/Windows software to disable/enable
PnP, then someday you might not have Dos/Windows around anymore and
will thus have difficulty changing the configuration.
4. 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.
Once configured as non-PnP devices, they can't be configured by PnP
software or the BIOS (until you move jumpers and/or use the
Dos/Windows configuration software again).
5.3. BIOS Configures PnP
5.3.1. Intro to Using the BIOS to Configure PnP
This means that your BIOS reads the resource requirements of all
devices and configures them (allocates resources to them). It is a
substitute for a PnP OS except that the BIOS doesn't match up the
devices with their drivers nor tell the drivers how it has done the
configuring. It should give preference to using the configuration it
has stored in its non-volatile memory (ESCD).
Your BIOS must support such configuring but 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). But
it's not always that simple to determine what the BIOS has done. See
``What Is My Current Configuration?'' Another advantage is that the
BIOS does its work before Linux starts so that all the resources are
ready to be used (and found) by the device drivers that start up
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 try to turn over the difficult parts of the
configuration task to Window utilities. If this happens you'll either
have to find another method (such as isapnptools) or try to set up the
ESCD database if the BIOS has one. See the next section.
5.3.2. The BIOS's ESCD Database
The BIOS's 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 put 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 what isapnp has
set. A good PnP OS might update the ESCD so you can use it later on
for a non-PnP OS (like standard Linux). Windows may do this in some
cases. See ``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. 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 then
allocate resources to the card and update the ESCD. It may even have
to change the resources assigned to existing PnP cards and modify ESCD
If devices saved their last configuration in their 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 on which
resources have been reserved for use by non-PnP cards. Many BIOSs
5.3.3. Using Windows to set ESCD
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 isn't any. Thus one may resort to
attempting to use Windows (if you have it on the same PC) to do this.
There are two 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. If
devices are configured automatically by Windows (without the user
telling it to "change setting") the setting will probably 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-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
(the one in memory?) in Windows98 you use the Device Manager, 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
change the configuration manually click on Change Setting. This
should put you change into the ESCD. In Windows98 There are 2 ways to
get to the Device Manager: 1. Control Panel --> System Properties -->
Device Manager. 2. My Computer --> Properties --> Device Manager.
5.3.4. Adding a New Device (under Linux or Windows)
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 there are a few options to handle it.
You may be able to tell the BIOS directly (via the CMOS setup menus)
that certain 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. Since this
configuration is "forced" Windows should update the ESCD when you shut
down the PC.
5.4. Isapnp (part of isapnptools)
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. 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 you set in /etc/isapnp.conf. Its possible to create
this configuration file automatically but you then must edit it
manually to chose between various options. With isapnp, a device
driver which is part of 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 trys 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 "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. If you use
"isapnp" like this and have a PnP BIOS, you should probably tell the
BIOS (when you set it up) that you don't have a PnP OS since you still
want the BIOS to configure the PCI devices. While the BIOS may also
configure the ISA devices, isapnp will redo it.
5.5. PCI Utilities
The new package PCI Utilities (= pciutils, incorrectly called
"pcitools"), should let you manually PnP-configure the PCI bus.
"lspci" lists resources while "setpci" sets resource allocations in
the hardware devices.
5.6. Patch the Kernel to Make Linux PnP
David Howells has created a patch to do this called "Linux Kernel
Configuration/Resource Manager" (sometimes just called Configuration
Manager). It may be hard to locate and may not be against the most
recent kernel. His old website is defunct but he's been creating a
new one (url unknown) The resulting kernel is is claimed to be stable
but bugs have been reported. It includes documentation: serial.txt to
show how to deal with the serial port. It provides "files" in the
/proc tree so that you can see what is going on and can echo commands
into one of these files for custom configuration. One problem is that
most device drivers don't know about it so that you still may have to
use the traditional configuration files, etc. for configuration.
5.7. Windows Configures
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). It
been reported that Windows erased the IRQs from PCI devices registers.
Then Linux complained that it found a zero IRQ. Thus you may not be
able to use this method.
5.8. PnP Software/Documents
· Isapnptools homepage
· PnP driver project
· PnP Specs. from Microsoft
· Book: PCI System Architecture, 3rd ed. by Tom Shanley +, MindShare
1995. Covers PnP-like features on the PCI bus.
· 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.
6. What Is My Current Configuration?
Here "configuration" means the assignment of PnP resources (addresses,
IRQs, and DMAs). There are two parts to this question for each
device. Each part should have the same answer.
1. What is the configuration of the device driver software? I.e.:
What does the driver think the hardware configuration is?
2. What configuration (if any) is set in the device hardware?
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, there may be a difference. This means the the driver
has incorrect information about the actually 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
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
But some drivers 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 (even if you've told it to probe for resources).
Thus "setserial" may only be telling you the configuration of the
driver and not the hardware.
Some info on configuration may be obtained from the messages from the
BIOSs and Linux that appear on the screen when you first start the
computer. After all the messages have flashed by, type shift-PageUp
to scroll back to them. 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 a PnP OS, isapnp, or pci utilities, may have changed
it since then.
6.1. How Are My Device Drivers Configured?
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. /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
/proc/interrupts shows only interrupts currently in use and many 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, it may mean that this interrupt doesn't exist on that
device does exist on some other device which is not in use, but which
somehow has issued an interrupt or two. As of kernel 2.2 the /proc
tree has changed.
6.2. How Are My Hardware Devices Configured?
It's easy to find out what resources have been assigned to devices on
the PCI bus: Just look at /proc/pci or /proc/bus/pci/devices (in
kernel 2.2+). For the ISA bus you may try running pnpdump --dumpregs
but it's not a sure thing. The results may be difficult to decipher.
Don't confuse the read-port address which pnpdump "trys" (and finds
something there) with the I/O address of the found device. They are
not the same.
Messages from the BIOS at boot-time tell you how the hardware
configuration was then. If you rely on the BIOS for configuring, then
it should still be the same. Messages from Linux may be from drivers
that have checked to see that the hardware is there (and possibly
checked the IRQ and DMA). Of course, if the device works fine, then
it's likely configured the same as the driver.
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.
7.1.1. ISA Bus Configuration Address (Read-Port etc.)
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. This is not 3 addresses for each card but 3 addresses
shared by all 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 these registers on a single card. To communicate with
a certain card, a specially assigned card number (handle) is sent to
all cards at the write-port address. After that only that one card is
listening. Then the address of the configuration register (of that
card) is sent to the address-port (of all cards --but only one is
listening). Next communication takes place with one 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). But the read-port is not fixed but is set by the configuration
software at some address that will supposedly 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 the read-
7.1.2. Address ranges
The term "address" is sometimes used in this document to mean a
contiguous range of addresses. Since addresses are given in 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
7.1.3. Address space
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. In summary:
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.
7.1.4. Range Check (ISA Testing for IO Address Conflicts)
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 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 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 it's 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.
7.1.5. Communicating Directly via Memory
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 ``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.
7.2. Interrupts --Details
Interrupts convey a lot of information but only indirectly. The
interrupt 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 find 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 such a transferring
bytes from (or 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 the serial ports on the ISA bus
starting with Kernel 2.2) 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
to see if an interrupt actually happened for its device. If there was
no interrupt (false alarm) it likely will exit and the next service
routine starts, etc.
7.3. PCI Interrupts
PCI interrupts are different but since they are normally mapped to
IRQ's they behave in about the same way. A major difference is that
PCI interrupts may be shared. For example IRQ5 may be shared between
two PCI devices. This sharing ability is automatic: you don't need
special hardware or software. There have been some reports of
situations where such sharing didn't work, but it's likely due to a
defect in the device driver software. All device drivers for PCI
devices are supposed to provide for interrupt sharing. 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 it's C programming code.
You may need to know some of the details of the PCI interrupt system
in order to set up the BIOS's CMOS or to set jumpers on old PCI cards.
Each PCI card has 4 possible interrupts: INTA#, INTB#, INTC#, INTD#.
Thus for a 7-slot system there could be 7 x 4 = 28 different
interrupt lines. But the specs permit a fewer number of interrupt
lines. This is not too restrictive since interrupts may be shared.
Many PCI buses seem to be made with only 4 interrupt lines. Call
these lines (wires or traces) W, X, Y, Z. Suppose we designate the B
interrupt from slot 3 as interrupt 3B. Then wire W could be used to
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. Then on
startup, the BIOS maps the X, W, Y, Z to IRQ's. After that it writes
the IRQ that each device is mapped to into a hardware register in each
device. Then and anything interrogating the device can find out what
IRQ it uses.
The above mentioned wires X, W, Y, Z are labeled per PCI specs as
INTA#, INTB#, INTC# and INTD#. This official PCI notation is
confusing since now INTA# has 2 possible meanings depending on whether
we are talking about a slot or the PCI bus. For example, if 3C is
mapped to X then we say that INTC# of slot 3 is cabled to INTA# (X) of
the PCI bus. Confusing notation.
There's another requirement also. A PCI slot must use the lower
interrupt letters first. Thus if a slot only uses one interrupt, it
must be INTA#. If it uses 2 interrupts they must be INTA# and INTB#,
etc. A card in a slot may have up to 8 devices on it but there are
only 4 PCI interrupts for it. 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 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 assign IRQs to PCI cards (but it's not simple as explained
above). There's a situation where Windows zeroed out all the IRQ
numbers in the PCI cards after the IRQ mappings had been set. Then
someone running Windows booted Linux from Windows with the result that
Linux only found only incorrect IRQs of zero.
You might reason that since the PCI is using IRQ's (ISA bus) it might
be slow, etc. 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 need to go
thru the PCI bus to get to the CPU, the IRQ interrupt signals go there
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 and 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). Each PnP device just starts to
sequentially send out its serial number, bit-by-bit, starting with the
high-order bit, on this wire. 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 cease to listen anymore to the write address that
all devices still in the game are still listening to. This is also
called "dropping out". (Note that all serial numbers are of the same
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 and doesn't listen anymore until the round is
finished (when a handle is assigned to the winner: the highest serial
number). Now the devices still in the game 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 the highest serial number will not be eliminated
from the game. But what happens if the leading digits (of the
possibly stripped serial numbers) are all 0? 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 by whatever participants remain) only one PnP device
with the highest serial number remains. It then gets assigned a
handle and drops out of the game permanently. Then all the dropouts
from the last 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
Once all handles are assigned, they are used to address each PnP
device and send it a configuration as well as to read configuration
info from the PnP device. 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, all of the handles are lost
so that a PnP BIOS usually does the isolation process again each time
you start your PC.
END OF Plug-and-Play-HOWTO