Typically, if two devices share an IRQ, like the COM ports listed in Table 13-1, only
onecanbeactiveatatime.Onearlysystems,itwasacommonproblemforthemouseand
the modem to end u pon the same IRQ, since both devices were commonly connected to
serial ports. There really wasn’t a problem, until you needed to use the mouse while the
modem was operating. On today’s PCs, it is fairly common for a scanner or a Zip drive to
share the parallel port and IRQ7 with a printer.
IRQs have priorities set by the system that determine which IRQ is to be handled if
two or more requests come in at the same time. The programmable interrupt controller
(PIC), discussed later in the chapter, manages priorities and other IRQ control issues.IRQ Assignments
The IRQ assigned to a device is usually determined by common practice and any work-
ing standards currently in use in the computing industry. There has never been a
set-in-stone standard for IRQ assignments. Manufacturers of processors, motherboards,
chipsets, and I/O adapters have more or less created the default settings currently used
in the industry.
Table 13-2 compares the IRQ settings of the three primary bus structures that are used
in PCs. Notice that even Tables 13-1 and 13-2 differ slightly. Table 13-1 shows common
IRQ settings used today and Table 13-2 shows the default settings that were or are used
on different bus structures.Chapter 13: System Resources^287
IRQ Slot Size (bits) PC XT Bus PC AT Bus Current (Pentium-class PCs)
0 n/a System timer System
timerSystem timer1 n/a Keyboard
controllerKeyboard
controllerKeyboard controller2 n/a 8-bit Available 2nd IRQ
Controller2nd IRQ Controller3 8-bit COM2/COM4 COM2/CO
M4COM2/COM4
4 8-bit COM1/COM3 COM1/CO
M3COM1/COM3
5 8-bit Hard disk
controller
(HDC)LPT2 Sound CardTable 13-2. IRQ Assignments on Bus Structures