UPGRADING REPAIRING PCs

50
Other Add-On Card Configuration
Issues
When a card is installed into an expansion slot or a PCMCIA/PC
card device is installed into a PC card slot, the card must use at
least one of four hardware resources to be accessible to the system.
All add-on cards must use at least an I/O port address range or
ranges; most cards use an IRQ (interrupt request line); fewer cards
use DMA (Direct Memory Access); and memory addresses are used
least of all. Many cards use two or more of these hardware
resources.
Note
Chapter 2—System Components and Configuration
For more information, see the section “Hardware and Firmware
Devices That Use Memory Addresses,” earlier in this chapter.
If an add-on card is set to use the same hardware resource as an
existing card, it will not work unless that resource is designed to be
shared between cards. Although the capability to share IRQs has
existed (at least in theory) since the Micro Channel Architecture of
the late 1980s, even today the best rule of thumb for adding cards
is “each card has its own settings.”
Plug-and-Play (PnP) configuration—introduced with Windows 95
and also present with Windows 98, Windows Me, and Windows
2000—is designed to minimize much of the grief of adding cards,
but this technology has been in a state of flux since it was introduced.
To help you add cards, the following tables of standard settings
also list software and hardware tools that can help you find
the settings already in use before you install your next card.
IRQs
Interrupt request channels (IRQs), or hardware interrupts, are used by
various hardware devices to signal the motherboard that a request
must be fulfilled. Most add-on cards use IRQs, and because systems
today have the same number of IRQs available with the first IBM
PC/AT systems built in 1984, IRQs frequently cause trouble in addon
card installations.
Table 2.20 shows IRQ assignments for 16-bit ISA and 32-bit
VL-Bus/PCI expansion slots, listed by priority. Technically speaking,
PCI interrupts can be shared, but in practice, many older Pentium
systems must use a unique IRQ value for each PCI card, as with ISA
and VL-Bus cards.