UPGRADING REPAIRING PCs

Table 2.20 16/32-Bit ISA/VL-Bus/PCI Default Interrupt
Assignments
Other Add-On Card Configuration Issues 51
Standard Recommended
IRQ Function Bus Slot Card Type Use
0 System timer No — —
1 Keyboard
controller
No — —
21 Second IRQ
controller
cascade
No — —
8 Real-time clock No — —
9 Available Yes 8-/16-bit Network
(appears as Interface
IRQ 2) Card
10 Available Yes 16-bit USB
11 Available Yes 16-bit SCSI host adapter
12 Motherboard Yes 16-bit Motherboard
mouse port
available
mouse port
13 Math
coprocessor
No — —
14 Primary IDE Yes 16-bit Primary IDE (hard
disks)
15 Secondary IDE/ Yes 16-bit Secondary IDE
available (CD-ROM/ tape)
34 Serial Port 2 Yes 8-/16-bit COM 2:/internal
(COM 2:) modem
43 Serial Port 1
(COM 1:)
Yes 8-/16-bit COM 1:
52 Sound/Parallel
Port 2 (LPT2:)
Yes 8-/16-bit Sound card
6 Floppy disk Yes 8-/16-bit Floppy
controller controller
7 Parallel Port
1 (LPT1:)
Yes 8-/16-bit LPT1:
1. The original IBM PC/XT and compatible systems with 8-bit ISA slots did not assign any standard
device to IRQ 2. When the 16-bit ISA slot was introduced, along with a second range of
IRQs (8–15), this permitted the “cascading” of these interrupts via IRQ 2. Older cards that
have IRQ 2 as a setting actually use IRQ 9 instead on 286-based and higher systems.
2. On original XT-class systems with 8-bit ISA slots, IRQ 5 was assigned to the hard disk controller
card. Even though IRQ 5’s “official” assignment is to handle LPT2 on systems with
16-bit ISA slots, only EPP and ECP (IEEE-1284) parallel port modes actually use an IRQ. This
permits the use of IRQ 5 for sound cards in most systems without interfering with the use of
LPT2.
3. Systems with COM 3 default to “sharing” COM 1’s IRQ 4. This will cause system lockups in
Windows if a serial mouse is used on COM 1 with a modem on COM 3. Use the modem, and
the IRQ conflict crashes the system. To avoid problems, set the device using COM 3 to a different
IRQ, or disable COM 2 and use COM 2 for the modem.