Motherboards and Buses - Upgrading and Repairing PCs - page 287

Therefore, in many cases you must assign a nonconflicting interrupt for each card, even
PCI cards. The conflict between assigning ISA IRQs for PCI interrupts caused many con-
figuration problems for early users of PCI motherboards and continued to cause problems
even after the development of Windows 95 and its PnP technology.
The solution to the interrupt sharing problem for PCI cards was something called PCI
IRQ Steering , which has been supported in OSs (starting with Windows 95 OSR 2.x) and
BIOSs for more than a decade. PCI IRQ Steering allows a plug-and-play operating sys-
tem such as Windows to dynamically map or “steer” PCI cards (which almost all use PCI
INTA#) to standard PC interrupts and allows several PCI cards to be mapped to the same
interrupt. More information on PCI IRQ Steering is found in the section “ PCI Interrupts ,”
later in this chapter.
Hardware interrupts are sometimes referred to as maskable interrupts , which means you
can mask or turn off the interrupts for a short time while the CPU is used for other critical
operations. It is up to the system BIOS and programs to manage interrupts properly and
efficiently for the best system performance.
The following sections discuss the IRQs that any standard devices use, as well as what
might be free in your system.
8-Bit ISA Bus Interrupts
The PC and XT (the systems based on the 8-bit 8086 CPU) provide for eight different ex-
ternal hardware interrupts. Table 4.57 shows the typical uses for these interrupts, which
are numbered 0-7.
Table 4.57 8-Bit ISA Bus Default Interrupt Assignments
If you have a system that has one of the original 8-bit ISA buses, the IRQ resources that
the system provides present a severe limitation. Installing several devices that need the
services of system IRQs in a PC/XT-type system can be a study in frustration because the