PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

1.3 System Resources

PCs have four types of system
resourcesInterrupt Request (IRQ) lines, DMA channels, I/O
ports, and memory ranges. Many system components and peripherals
require one or more of these resources, which raises the twin
problems of resource availability and resource conflicts. Resource
availability is particularly important with regard to IRQs, which are
in high demand, and of which only 16 exist. Resource conflicts can
occur when two devices are assigned the same resource, in which case
one or both devices may not function, or may function unpredictably.
Resource conflicts may occur even with plentiful resources, such as
I/O ports, where many are available and only a few are in
use.

A frequent cause of problems when building or upgrading PCs is a
shortage of required resources or unintentional resource conflicts
that occur when a new component is installed that was inadvertently
configured to use a resource that is already in use. Two
technologies, PCI and Plug and Play, used in conjunction with recent
versions of Microsoft operating systems (Windows 95 OSR2, Windows 98,
and Windows 2000) and Linux go a long way toward extending the
availability of resources and preventing conflicts. Even in such an
ideal environment, however, resource conflicts sometimes occur,
particularly if you are using older
"legacy" hardware. The following
sections describe what you need to know about PC resources and how to
manage them.

1.3.1 Interrupt Request Line (IRQ)

When a component or peripheral, such as a
network adapter or sound card, needs to get the
CPU's attention, it does so by generating a signal
on an Interrupt Request Line
(IRQ). Table 1-2 lists IRQs
and the devices that typically use them.

An 8-bit ISA slot contains physical IRQ lines only for IRQ 02 through
IRQ 07 because IRQ 00 and IRQ 01 are reserved for system functions. A
16-bit ISA slot contains physical IRQ lines for IRQs 03 through 07,
09 through 12, 14, and 15. IRQ 09 is mapped to IRQ 02, allowing 8-bit
ISA cards to recognize IRQ 09 as IRQ 02. IRQs 00, 01, 02, 08, and 13
are not present in any slot, and so cannot be assigned to devices.

If the processor receives two or more interrupts simultaneously, it
processes them in order of priority. On 8-bit systems (PCs and XTs),
the lower-numbered IRQ always takes priority. That is, IRQ 00 is the
highest priority, and IRQ 07 is the lowest. 286 and higher systems
use a second PIC to add a second set of eight IRQs, cascaded from
IRQ02. That changes IRQ priority from the simple numerical order used
by 8-bit systems. On 16-bit and higher systems, IRQ 00 is still the
highest priority, followed by IRQ 01 and 02. But because IRQ 02 is
the cascade IRQ, the IRQs that it supportsIRQ 08 through IRQ
15are next in priority. IRQ 03 follows IRQ 15 in priority, and
then in numerical order through IRQ 07, the lowest priority. Whenever
possible, assign "important"
devices to higher-priority IRQs. For example, if you have a serial
mouse and a modem, assign the modem to COM2: (IRQ03) and the mouse to
COM1: (IRQ04). Because the modem is on the higher-priority IRQ, it is
serviced first if the modem and the mouse generate interrupts
simultaneously.

1.3.1.1 ISA interrupts versus PCI interrupts

ISA and PCI handle interrupts very
differently. ISA expansion cards are configured manually for IRQ,
usually by setting a jumper, but sometimes by running a setup
program. All ISA slots have all IRQ lines present, so it
doesn't matter which card is placed in which slot.
ISA cards use edge-sensitive interrupts, which
means that an ISA device asserts a voltage on one of the interrupt
lines to generate an interrupt. That in turn means that ISA devices
cannot share interrupts because when the processor senses voltage on
a particular interrupt line, it has no way to determine which of
multiple devices might be asserting that interrupt. For ISA slots and
devices, the rule is simple:

two devices cannot share an
IRQ if there is any possibility that those two devices may be used
simultaneously . In practice that means that you cannot
assign the same IRQ to more than one ISA device.

PCI cards use level-sensitive interrupts, which
means that different PCI devices can assert different voltages on the
same physical interrupt line, allowing the processor to determine
which device generated the interrupt. PCI cards and slots manage
interrupts internally. A PCI bus normally supports a maximum of four
PCI slots, numbered 1 through 4. Each PCI slot can access four
interrupts, labeled INT#1 through INT#4 (or INT#A through INT#D).
Ordinarily, INT#1/A is used by PCI Slot 1, INT#2/B by Slot 2, and so
on.

AGP cards support only INT#1/A and INT#2/B, and share with PCI Slot 1. If a PCI Slot 5 exists, it shares with Slot 4. In either case, with slots that share resources, avoid installing cards in both slots if both cards require the same shared resource. If you must use both slots, install only cards that can share an IRQ. If you encounter a conflict on a PCI system, simply moving cards to different slots often solves the problem.

Bridging circuitry within the chipset
allows additional PCI or other busses to be cascaded from the primary
PCI bus. For example, the PCI-to-ISA bridge present in most current
chipsets allows cascading an ISA bus from the primary PCI bus. One
function of the bridging circuitry is to convert PCI interrupts to
ISA interrupts when a PCI device needs to get the
processor's attention. PCI interrupts do not
correspond directly to ISA IRQs, although an INT# can be mapped to an
IRQ via the PC's interrupt handler if the card using
that INT# requires an IRQ. Some configuration firmware restricts
mapping PCI interrupts to IRQ 09, 10, or 11 or to Auto, while others
allow mapping any INT# to any available IRQ.

In
general, leave INT-IRQ mapping for all PCI slots set to Auto unless
you have good reason to assign a specific IRQ. Sometimes a card with
a dynamically mapped IRQ may work fine with some programs and not
others. For example, many older games expect to find a sound card at
IRQ 05. If you have a PCI/Plug and Play sound card installed in PCI
Slot 3, you can use INT-IRQ mapping to assign IRQ 05 to that slot and
card, keeping the old games happy.

PCI expansion cards
are normally assigned an IRQ dynamically, either by the BIOS or by
the operating system, depending on the version of Windows (or Linux)
being used and the PCI/Plug and Play configuration options in effect.
On bridged PCI-ISA systems, ISA IRQs 00, 01, 02, 08, and 13 are
reserved for critical system functions. IRQs 03 through 07, 09
through 12, 14, and 15 can each be programmed using the CMOS Setup
PCI/Plug and Play configuration utility as being owned by either the
PCI bus

or the ISA bus, but not both. The
terminology for this varies. Some utilities allow you to specify each
IRQ as

PCI/ISA Plug and Play or Legacy
ISA (or similar words). Others allow you to specify each
IRQ as

Level-Sensitive or
Edge-Sensitive (or similar words). In either
case, the effect is the same.

If you are installing a
"legacy" card (i.e., a non-Plug and
Play ISA card), you can use static IRQ mappings to assign a specific
IRQ to that card. For example, if you install an old ISA sound card
that requires IRQ 05, use the PCI/Plug and Play configuration utility
to set IRQ 05 for Legacy ISA or Edge-Sensitive, thereby reserving
that IRQ for that card.

Do not confuse mapping PCI INT# interrupts to ISA IRQs with allocating IRQs to the ISA or PCI bus. The two are entirely unrelated. Use the former to "lock down" a PCI slot/card to a specific IRQfor example, to allocate IRQ 05 to a PCI sound card. Use the latter to reserve IRQs for ISA devicesfor example, to reserve IRQ 05 for an ISA sound card. Confusing these functions may cause lockups or other strange behavior. In general, the best way to prevent conflicts is to avoid installing ISA cards in PCI systems.

1.3.1.2 PCI Bus IRQ Steering

PCI Bus IRQ Steering

is a function built into Windows 95
OSR2 or higher and Windows 98 (but not NT4 or Windows 2000/XP). IRQ
Steering allows Windows itself to assign IRQs to PCI devices. With
earlier versions of Windows 95, the BIOS assigns IRQs to PCI devices,
and Windows must accept the decisions made by the BIOS IRQ Steering.
If Windows IRQ Steering is enabled, Windows can override those BIOS
decisions, although it seldom does so. OSR2 disables IRQ Steering by
default; Windows 98 enables it by default.

Windows IRQ Steering allows Windows to reassign PCI interrupts
automatically to accommodate the inflexible requirements of ISA
devices. For example, assume that a PC with a BIOS that does not
recognize non-Plug and Play ISA cards (that is, IRQ Steering is not
implemented in BIOS) is running Windows 98 with IRQ Steering
disabled. The PC is properly configured with all PCI devices, and the
BIOS has assigned IRQ 11 to a Creative SoundBlaster AudioPCI 128
sound card. You then open the case and install a 3Com 3C509 network
adapter (a non-Plug and Play ISA card), which is also configured for
IRQ 11. When you restart the system, a conflict exists between the
sound card on IRQ 11 and the network card, also on IRQ 11. If you
enable Windows 98 IRQ Steering and restart the system, IRQ Steering
takes the following actions during boot:

Detects that IRQ 11 is in use by both the PCI sound card and the ISA
network card.

Disables the PCI sound card.

Maps a free IRQone that is not being used by an ISA
deviceto a PCI interrupt and assigns an IRQ holder to it. If
IRQ 10 is available, for example, PCI Steering may assign it to a PCI
interrupt.

Reprograms the sound card to use IRQ 10.

Resets the IRQ mapping table to specify that IRQ 11 is now assigned
to ISA and removes the PCI IRQ holder for IRQ 11.

When the system restarts, the sound card is now assigned to IRQ 10,
the network card is still IRQ 11, and both devices work. Note that
IRQ Steering does nothing that you cannot do for yourself. It simply
automates the process of resolving IRQ conflicts when ISA devices are
present in a PCI system.

To view the IRQ assignments made by IRQ Steering, right-click the My
Computer icon, choose Properties, click the Device Manager tab, and
double-click the Computer icon at the top of the tree to display the
View Resources page of the Computer Properties dialog, shown in Figure 1-3. IRQs which IRQ Steering has assigned to PCI
are flagged with an entry labeled IRQ Holder for PCI Steering. This
flag does not indicate that another device is assigned to the IRQ,
but simply that IRQ Steering has reserved that IRQ for PCI, making it
unavailable to ISA devices even if no PCI devices are currently using
that IRQ.

Figure 1-3. The View Resources page of the Computer Properties dialog displaying global resource allocations for IRQ, DMA, I/O ports, and memory ranges

IRQs which both the BIOS and Windows 98 have assigned to PCI are
flagged twice. In Figure 1-3 IRQ Steering has
assigned IRQ 10 as a PCI interrupt, which is being shared by a SCSI
host adapter, a network adapter, and the USB host controller. Both
BIOS IRQ Steering and Windows 98 IRQ Steering have assigned IRQ 10 to
PCI. The Matrox video card on IRQ 11 is the only device assigned to
that IRQ, and only BIOS IRQ Steering has assigned an IRQ Holder.

To view or change settings for IRQ Bus Steering itself, right-click
the My Computer icon, choose Properties, and click the Device Manager
tab. Double- click System Devices to expand the tree, and then
double-click PCI Bus to display the PCI bus Properties dialog. Click
the IRQ Steering tab to display the IRQ Steering page of the PCI bus
Properties dialog, shown in Figure 1-4.

Figure 1-4. The IRQ Steering page of the PCI bus Properties dialog, which allows you to enable or disable Windows 98 IRQ Steering, configure it, and view its current status

The IRQ Routing Status pane at the bottom of the dialog displays the
current status of IRQ Steering. Windows 98 enables IRQ Steering using
the defaults shown. Leaving this checkbox marked means that Windows
98 manages IRQ Steering. To disable Windows 98 IRQ Steering and allow
the BIOS to manage IRQ Steering, clear the Use IRQ Steering checkbox
and restart the PC. If you do that, the Windows Find New Hardware
Wizard runs after the restart, locates the
"new" devices, and installs drivers
for them.

The

Get IRQ table .. . checkboxes are a ranked
priority list of the methods Windows can use to obtain the data it
needs to manage IRQ Steering. Clearing one of these checkboxes causes
Windows not to attempt that method. In Figure 1-4
Windows first attempts to obtain this data using the ACPI BIOS. That
fails, so it next attempts to get the data using the MS Specification
table, which also fails. The Protected Mode PCIBIOS 2.1 method is not
checked, so Windows does not attempt to use that method. Finally,
Windows attempts to get the data using a Real Mode PCIBIOS 2.1 call,
which succeeds.

In general, leave IRQ Steering enabled. If problems occur with a PCI
device being recognized or configured properly, take the following
steps in order until the problem is resolved:

Clear the Get IRQ table using ACPI BIOS checkbox and restart the
system.

Clear the Get IRQ table using MS Specification table checkbox and
restart the system.

Clear the Get IRQ table using Real Mode PCIBIOS 2.1 call checkbox,
mark the Get IRQ table using Protected Mode PCIBIOS 2.1 call and
restart the system.

Clear the Use IRQ Steering checkbox and restart the system to allow
the BIOS to manage IRQ steering.

If IRQ Steering cannot be enabled, the system BIOS may not support IRQ Steering (which is to say it will not allow Windows 98 IRQ Steering to change assignments), or the BIOS IRQ routing table may be missing or corrupt. In either case, contact the system or motherboard maker for an updated BIOS or additional assistance.

1.3.2 Direct Memory Access (DMA)

Direct Memory Access


(DMA)
is a means by which devices can exchange data with memory or with
each other without requiring intervention by the processor. Standard
DMA allows a device to exchange data with memory, but not with
another device. Bus Mastering DMA allows two
devices to communicate directly with each other. The advantage of
using DMA is that it reduces the load on the processor, allowing it
to perform other tasks. There are even fewer DMA channels than
IRQseight versus 16but DMA channels are much less in
demand than IRQs, so DMA channel availability is almost never an
issue. Table 1-3 lists DMA channels and the uses
to which they are typically put.

DMA 2 is used by nearly all systems for the floppy disk drive
controller. Excluding DMA 4, which is a dedicated cascade channel
(used to access the secondary DMA controller), the other DMA channels
are available for use with expansion cards. DMA 0 is almost never
used because, although it appears only in 16-bit slots, it supports
only 8-bit transfers. Most ISA sound cards require two DMA channels,
with 8-bit sound using DMA 1 and 16-bit sound using DMA 5. Note that
these DMA channels pertain only to ISA cards. PCI devices do not
require one of these DMA channels to use DMA. For example, if you
enable DMA transfer mode on one or both of the embedded PCI IDE
controllers, you will find that they operate in DMA mode without
occupying ISA DMA channels.

The only time DMA conflicts are likely to arise is if you install an ISA sound card and an ISA SCSI host adapter. Nearly all ISA sound cards use both DMA 1 and DMA 5, and some ISA SCSI cards are configured by default to use DMA 5, which causes a conflict. The easy answer is to configure the SCSI host adapter to use DMA 6 or DMA 7. The better answer, as usual, is to avoid ISA cards whenever possible.

1.3.3 I/O Ports

Input/Output ports



(I/O ports) are ranges of addresses that
function like mailboxes, allowing programs and components to exchange
messages and data. An I/O port has a base
address, which is the hexadecimal address of the first
byte allocated to that I/O port, and a length, which is also
expressed in hexadecimal. For example, many network adapters default
to base address 300h and are 20h bytes (32 decimal bytes) long, and
so occupy the range 300-31Fh.

There's no shortage of I/O ports, because thousands
exist. We have never seen I/O port conflicts with PCI devices
operating in a Plug and Play environment, but I/O port conflicts
commonly occur when two ISA devices are unintentionally assigned
overlapping ranges. For example, another common base address for
network adapters is 360h (range 360-37Fh). Unfortunately, that range
overlaps the range of LPT1: (base address 378h), so setting a network
card to 360h results in conflicts with the parallel port.

1.3.4 Memory Ranges

The original IBM PC used an
8088 processor, which supported up to 1 MB of physical memory,
addressed as sixteen 64 KB segments. Memory locations are enumerated
in hexadecimal, so the first segment includes the addresses 00000h
through 0FFFFh (0 through 65,535 decimal) and the 16th includes the
addresses F0000h through FFFFFh (983,040 through 1,048,575 decimal).
The first 10 of those segments00000h through
9FFFFhcomprise the base 640 KB of memory addresses that are
accessible by the operating system and programs. The last six
segmentsA0000h through FFFFFcomprise the 384 KB of
upper memory addresses (the Upper Memory Area or
UMA) reserved for system use. The first two UMA
segments (A0000h through BFFFFh) are reserved for video memory. The
second two UMA segments (C0000h through DFFFFh) are reserved address
space for ROM BIOSs that reside on some adapters, such as video
cards, SCSI host adapters, and network adapters. The final two UMA
segments (E0000h through FFFFF) are reserved for the motherboard
BIOS.

Modern processors use a flat (unsegmented) 32-bit address space,
which allows them to access up to 4 GB (4096 MB, or 4,294,967,296
bytes) of distinct memory addresses. That additional address space
means that memory addresses are expressed as eight rather than five
hexadecimal characters (e.g., addresses for the first MB are
expressed as 00000000h through 000FFFFFh). Because few systems have
anywhere near 4 GB of physical memory installed, huge ranges of
unused memory addresses are available for assignment to devices that
require memory ranges. Which of those ranges are used depends on how
much physical memory is installed and which operating system you run.

Windows NT/2000/XP uses address ranges from the UMA of the first
megabyte (000A0000h through 000FFFFFh) for the original purposes of
addressing video memory, adapter ROMs, and so on. It uses address
ranges at the top of its address space, F0000000h and above (up near
4 GB), to provide additional memory ranges for which there is
inadequate room in UMA. Windows 98 does the same, but also uses
memory ranges immediately above the end of the range occupied by
physical RAM.

Memory range conflicts are seldom a problem on modern computers
running recent versions of Windows.

1.3.5 Viewing and Reserving System Resources

Windows 9X/2000/XP and Linux all provide convenient means to view the
resources that are in use. Windows 98 also allows you to reserve
resources manually for non-Plug and Play ISA devices on systems with
a BIOS that does not support IRQ Steering.

1.3.5.1 Viewing resources with Windows 2000 or Windows XP

To view system resources with Windows
2000 or Windows XP, use the Control Panel to display System
Properties, click the Hardware tab, and then click the Device Manager
button to display the dialog shown in Figure 1-5,
which lists all installed devices. Clicking the + icon (or
double-clicking a branch name) expands the list to show individual
devices within that branch. If a problem exists with a device (a
resource conflict, missing driver, etc.), Windows 2000/XP
automatically expands the branch that contains that device and flags
the device with an alert icon.

Figure 1-5. The Windows XP Device Manager displaying all installed devices

To view a global list of resources, click the View menu and select
the Resources by Type option to display the Device Manager window
shown in Figure 1-6. Expand the listing for the
type of resource you want to view. Figure 1-6 shows
that ISA IRQs 02, 05, 06, 07, 10, and 11 are available for use by new
devices.

Figure 1-6. The Windows XP Device Manager displaying used and available IRQs

To view all resources being used by a particular device, expand the
Device Manager tree (see Figure 1-5), double-click
the device name to display the Properties sheet for that device, and
display the Resources tab. Figure 1-7 shows the
Properties sheet for an ATI RAGE 128 PRO AGP video card. The Resource
type pane displays all resources assigned to that device, although
you may have to scroll the list to see all items. If a resource
conflict exists, Windows 2000/XP displays a list of other devices
using the same resource(s) in the Conflicting device list pane.

Figure 1-7. The Properties sheet

1.3.5.2 Viewing resources with Windows 9X

To view
system resources with Windows 98, right-click My Computer, choose
Properties, and click the Device Manager tab to display the System
Properties dialog shown in Figure 1-8, which lists
all installed devices. Clicking the + icon (or double-clicking a
branch name) expands the list to show individual devices within that
branch. If a problem exists with a device (a resource conflict,
missing driver, etc.), Windows 98 automatically expands the branch
that contains that device and flags the device with an alert icon.

Figure 1-8. Windows 98 Device Manager displaying all installed devices

To view a global list of resources, double-click the Computer branch
to display the View Resources page of the Computer Properties dialog,
shown in Figure 1-9. Choosing any of the four
option buttons immediately displays a global list of assignments for
that resource, allowing you to determine easily which resources are
unassigned. Figure 1-9 shows that IRQ 03, normally
assigned to Communications Port (COM2), is available for use by a new
device.

Figure 1-9. The View Resources page of the Computer Resources dialog lists resources in use

To view all resources being used by a particular device, expand the
Device Manager tree (see Figure 1-8) and
double-click the device name to display the Properties sheet for that
device. Figure 1-10 shows the Properties sheet for a
Matrox Millenium II PowerDesk video card. The Resource type pane
displays all resources assigned to that device. If a resource
conflict exists, Windows 98 displays a list of other devices using
the same resource(s) in the Conflicting device list pane.

Figure 1-10. The Properties sheet listing all the resources allocated to the Matrox MGA Millenium II PowerDesk

1.3.5.3 Reserving resources with Windows 9X

If the
system BIOS is up to date and all expansion cards are Plug and
Play-compliant, Windows 98 and Plug and Play normally configure the
system properly without further ado. However, if the system has an
older BIOS and/or you need to install one or more cards that are not
Plug and Play-compliant, conflicts may occur because the BIOS and
Windows cannot determine which resources those older cards need. For
such situations, Windows allows you to specify manually which
resources these older cards require, removing them from the pool of
resources that Windows manages automatically.

To reserve resources, first examine the documentation and settings
for the card to determine which resources (IRQ, DMA, I/O ports, and
memory ranges) it requires. Display the Device Manager and click the
Reserve Resources tab to display the dialog shown in Figure 1-11. This dialog lists any resource reservations
already in effect, and allows you to modify existing reservations and
add new reservations. Mark one of the four option buttons to select
the type of resource for which you want to add a reservation or view
existing reservations.

Figure 1-11. The Reserve Resources dialog, which allows you to remove resources from the pool available to Windows and assign those resources manually to legacy devices

To add a resource reservation, click Add to display the Edit Resource
Setting dialog, whose appearance varies depending on the type of
resource for which you are adding a reservation. Figure 1-12 shows the dialog for reserving an IRQ. Use the
up and down arrows to specify a value for the resource to be reserved
and click OK. You can reserve multiple resources in a single session
by repeatedly selecting the resource type and adding reservations.
When you finish reserving resources, click OK to store the resource
reservations and then restart the system to put the changes into
effect.

Figure 1-12. The Edit Resource Setting dialog, where you can specify the resource to be reserved

Device Manager initially displays reserved resources as System
Reserved, as shown for IRQ03 in Figure 1-13.
However, once you restart the computer, that resource will no longer
be displayed in the Device Manager.

Figure 1-13. Device Manager listing reserved resources as System Reserved, as for IRQ 03

Be very careful when reserving resources. Windows 98 allows you to reserve any resource, including ones that are already in use. Reserving an in-use resource may disable the device that is currently using that resource. If that occurs, use the Device Manager to remove the device, and then run the Add New Hardware Wizard from the Control Panel to reinstall the device.

1.3.5.4 Viewing resources with Linux

Viewing resources with Linux is
straightforward. If you use KDE, simply open the KDE Control Center,
expand the Information branch in the left panel, and double-click an
item to view the details. (If you use Gnome, simply open a terminal
window and type kcontrol to start the KDE Control
Center. Figure 1-14 shows the KDE Control Center
displaying the I/O ports in use on this system.

Figure 1-14. Using the KDE Control Center to list I/O ports in use

If you run Linux without a GUI, do not have KDE installed, or simply
prefer using a command line, change to the /proc
directory, which contains numerous descriptively named hardware
configuration files. Use the cat command to
display the appropriate file. For example, the command
cat interrupts lists the
interrupts in use. For larger files, use the more
or less command to prevent data from scrolling off
the screen.