Legacy I/O Assignments

A P P E N D I X   D

Legacy I/O Assignments

This appendix summarizes assignments for IRQ, DMA, and I/O port addresses used by built-in devices on legacy system boards. Contents

Fixed ISA Interrupts

The following IRQs used by ISA devices are considered to be fixed assignments.

Fixed ISA Interrupts
Hardware IRQ Default assignment

IRQ 0 System timer
IRQ 1 Keyboard
IRQ 2 Second programmable interrupt controller
(PIC) cascade
IRQ 3 COM 2
IRQ 4 COM 1
IRQ 5 Sometimes LPT 2 - not considered fixed
IRQ 6 Standard FDC
IRQ 7 LPT 1
IRQ 8 Real-time clock / CMOS
IRQ 9 -
IRQ 10 Sometimes COM 4 - not considered fixed
IRQ 11 Sometimes COM 3 - not considered fixed
IRQ 12 PC/2-style mouse
IRQ 13 Coprocessor
IRQ 14 Primary IDE controller
IRQ 15 Secondary IDE controller

Legacy ISA DMA Assignments

The following table lists DMA channel assignment used by legacy ISA devices and therefore considered fixed.

Legacy ISA DMA considered fixed
Hardware DMA System function (default)

DMA 0 ISA expansion
DMA 1 -
DMA 2 FDC
DMA 3 ECP parallel port on LPT 1
DMA 4 DMA controller cascading
DMA 5 -
DMA 6 -
DMA 7 -

Legacy ISA I/O Address Assignments

The following table lists I/O addresses used by legacy ISA devices and therefore considered fixed.

Legacy ISA System I/O
I/O Address Default system function

0000 - 000F Slave DMA
0010 - 0018 System
0001F System
0020 - 0021 Master 8259
0040 - 0043, 0048 - 004B PIT #1, PIT #2
0050 - 0052 System
0060 Keyboard / mouse controller
0061 System control port B
0064 Keyboard / mouse status
0070 - 0071 NMI Enable / real-time clock
0081 - 008B DMA page registers
0090 - 0091 System
0092 System control port A
0093 - 009F System
00A0 - 00A1 Slave interrupt controller
00C0 - 00DE Master DMA controller
00F0 - 00F1 Coprocessor busy clear/reset
0170 - 0177 Secondary IDE controller
01F0 - 01F7 Primary IDE controller
0201 Joystick interface
0220 - 022F Sound Blaster
0278 - 027A LPT 2 (XT parallel port 3)
02E8 - 02EF Alternate COM (4)
02F8 - 02FF COM 2
0330 - 0331 MPU-401
0376 IDE Controller
0378 - 037A LPT 1 (XT parallel port 2)
0388 - 038B FM synthesis
03B0 - 03BB MDA, EGA / VGA
03BC - 03BE LPT 3 (XT parallel port 1)
03C0 - 03DF EGA / VGA
03E0 - 03E7 PCIC PCMCIA controllers
03E8 - 03EF Alternate COM (3)
03F0 - 03F7 FDC
03F8 - 03FF COM 1
0534 - 0537 Windows Sound System-compatible
0CF8 - 0CFB PCI ports