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wired to them. Older ISA cards (also called “legacy” cards) have you select
which interrupt is used by configuring jumpers on the card. Newer Plug-
and-Play ISA cards have their interrupts selected by either the computer’s
BIOS (the built-in program that starts the computer) or by Windows 95/98.
The newer PCI (Peripheral Connect Interface) bus transfers data faster than
ISA and was designed to support Plug-and-Play from the start. PCI
connectors have only four interrupts wired to them (A, B, C, and D).
However, these interrupts can be shared by more than one PCI connector
slot and each connector’s interrupt can be assigned or “steered” to one of
the 11 available PC interrupts by either the BIOS or Windows 95/98. As far
as the user is concerned, PCI interrupts use the same numbers and are
assigned the same way as Plug-and-Play ISA interrupts. Note, however, that
a PCI card can share an interrupt with another PCI card but not with an ISA
card.
Plug-and-Play and Interrupts
When your computer first starts up, the BIOS will check each connector slot
to see if a Plug-and-Play card is installed. Each Plug-and-Play card has built
into it a list of what resources it requires including how many interrupts it
needs and which ones it can use. The BIOS will then configure each card
and, if needed, assign an appropriate interrupt. It does this for both ISA and
PCI Plug-and-Play cards. Windows 95/98 will also check for and configure
Plug-and-Play cards, possibly changing some of the settings to suit its
needs.
Legacy Cards and Interrupt Conflicts
Problems can arise when the system contains older “legacy” cards with their
jumper-configured (or possibly hard-wired) interrupts. The computer’s
BIOS and Windows 95/98 have no automatic way to detect these cards and
determine which interrupts, if any, are being used. For example, you may
have a MIDI card installed with its jumpers configured to use IRQ 9. Since
the BIOS cannot detect the legacy card it will consider IRQ 9 to be a free
interrupt. Consequently, it may assign IRQ 9 to Gina. The next time you try
