New HOWTO: Plug-and-Play-HOWTO - page 3
Table of Contents
2. What PnP Should Do: Allocate "Bus-Resources"
2.1. What is Plug-and-Play (PnP)?
If you don't understand this section, read the next section ``How a
Computer Finds Devices (and conversely)''
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 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
2.2. How a Computer Finds Devices (and conversely)
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
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.
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 ``Memory Ranges''), or
configuration. See ``Address Details'' for more details. Only two of
these 3 address spaces are used for device I/O: I/0 and main memory.
2.4. I/O Addresses and Allocating Them
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):
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. 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
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: ``Memory Ranges'',
``IRQs --Overview'' and ``DMA Channels''. What these are will be
explained in the next 3 sections.
2.5. Memory Ranges
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".
2.6. 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 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.
2.7. 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
1. 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
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.
2.8. "Resources" for both Device and Driver
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.
2.9. 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 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
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.
2.10. 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 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 ??