Plug and Play Resource Allocation

Jan 16 00:36 2005 Stephen Bucaro Print This Article

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Plug and Play Resource Allocation

By Stephen Bucaro

Your computer has many devices connected to it. Your key
board is a device. Your mouse is a device. Your modem is
a device. Even a port, like a USB port, is a device. You
can add a device to your computer by installing it into an
expansion connector on the the motherboard of the computer.
In order to operate, resources need to be allocated to the

One of the most important resources a device needs is an
Interrupt Request (IRQ) line. An IRQ signals the processor
that the device needs attention. For example, each time
you press a key on your keyboard, an IRQ is sent to let
the processor know that the keyboard needs attention. A
device also needs to be allocated a set of memory addresses
where commands can be sent to it and it can send responses.

Many devices need to be allocated an area of memory to
store data and/or a Direct Memory Access (DMA) channel.
A DMA channel allows a device to create a data stream
directly between it and the computers memory without
passing through the processor.

One of the most important structures in a computer is the
system bus located on the motherbord. The processor,
chipset, memory, and expansion slot devices communicate
over the bus. Early PCs used a bus called Industry Standard
Architecture (ISA). When you installed an expansion card
into the ISA bus, you had to set IRQs and other resources
with jumpers and/or DIP switches on the card. ISA devices
cannot share resources, each ISA device must have its own

- You can have more than one ISA device configured for the
same IRQ, as long as only one of the device drivers is
loaded at any one time; otherwise, you'll get an IRQ

Since a computer has only a limited amount of resources,
the number of devices that a computer could support is
limited. One of the most limited resources is IRQ lines.
A PC has only 16 IRQs. Common devices such as the keyboard,
mouse, floppy drive, and hard drive use a standard set of

Standard IRQ aassignments

0System Timer
2IRQ Controller 2
6Floppy Drive
8Real-Time Clock
11IRQ Holder
13Math Coprocessor
14Primary IDE
15Secondary IDE

In 1993, Microsoft and Intel developed Plug and Play (PnP)
to solve this problem. One of the main structures
supporting PnP is the Peripheral Component Interconnect
(PCI) bus. Along with a PCI bus, the computers BIOS,
operating system, and hardware devices must be PnP
compliant. One of the key features of PnP is that when
installing a PCI card, you do not need to use jumpers of
DIP switches to set the IRQ or I/O address for the card,
the PCI bus controller does this for you.

The Windows 2000/XP operating system component responsible
for PnP is the Advanced Configuration and Power Interface
(ACPI). ACPI allows devices to be controlled by the
operating system to perform power management. ACPI may put
a device in a power saving state such as Standby, Suspend
or Off. ACPI also allows dynamic handling of events like
the addition or removal of a USB device.

During the boot process, the BIOS builds a table to record
which IRQs have been used by legacy ISA devices and then
assigns the unused ones to the PCI bus controller. The PCI
bus controller, which is part of the motherboard chip set,
manages the PCI bus and the expansion slots.

Each PCI device communicates its resource requirements to
ACPI. Based on the resource requests that each device makes,
ACPI assigns resources such as IRQs, I/O addresses, and
memory locations, and DMA channels. ACPI can reconfigure
resource assignments when needed, for example, when a
device is added to the system.

The PCI bus overcomes the limitation in available IRQ's
with a feature called "PCI steering". PCI steering makes
it possible for several devices to use the same IRQ. PCI
steering allocates an IRQ called the "IRQ Holder" for PCI
steering. The PCI controller steers that IRQ to its own
interrupt systems on the PCI bus. These interrupts are
called A, B, C, D, and so on to avoid confusion with the
numbered system IRQs. One interrupt is assigned to each
PCI expansion slot.

To see which IRQ has been assigned to the IRQ Holder for
PCI steering, open the Control Panel "System" utility. In
the "System Properties" dialog box, select the "Hardware"
tab and click on the "Device Manager..." button. In Device
Manager's "View" menu select "Resources by type", then
open the "Interrupt request IRQ" branch.


- Note that Windows 2000 uses IRQ 9 for PCI Steering, so
never set a device to IRQ 9.

PnP resource allocation removes the need to use jumpers of
DIP switches to set the IRQ and other resources for devices.
But PnP involves a complex interaction between the computers
BIOS, chipset, PCI bus, operating system, and PnP devices.

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Stephen Bucaro
Stephen Bucaro

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