PORT ADDRESSES

7/19/97

Port Addresses: The IBM PC accesses I/O ports separately from memory. The 80x8x CPU can address 65536 ports, but the original PC only used 10 bits -- 1024 possible ports -- for port addressing and the ISA bus itself provides only 12 bits. A lot of hardware was designed that assumed only 10 or 12 bits of addressing and might interfere (by "address aliasing") with ports above address 3FF or FFF. So for practical purposed, only 1024 ports of 4096 are fully available. Full 16 bit addresses are available and are used on the PCI bus. Port addresses are conventionally given in hexadecimal.

The original PC assigned Ports 000-0FF to the motherboard and 200-3FF to peripherals. This distinction is no longer important, but is somewhat adhered to. Ports 100-1FF were write only in the original PC but became readable in most later machines.

Many Port addresses are fixed by convention at those used in the IBM PC, the PC XT or the PC AT. Others are restricted to certain address ranges set by jumpers or software. The following are some of the more common Port Assignments:

Port Device

entry1a entry1b

000-00F DMA Controller

010 POST Checkpoint in PC (See Port 80)

020-021 Interrupt Controller

022-023 CPU and Chipset

040-043 Programmable Timer

060-064 Keyboard Controller and Misc functions incl PS/2 mouse

071 NMI Control - CMOS Access

080 POST Checkpoint in AT and above (See Port 10)

081-083 DMA Page Registers

0A0 NMI on PC/PC XT?

0C2 DMA Ch3 Device Select

170-177 Secondary IDE Controller

1F0-1F7 Primary IDE Controller

201 Gameport

210-215 Extender cards in original PC

278-27F LPT3

279 ISA PNP

2E8- Some Video Cards (also 12E8, 22E8, etc)

2E8-2EF COM4

2F8-2FF COM2

300 EISA BIOS Checkpoint (Award)

300-3?? Default port address on many network cards

320-330 MFM/RLL Disk Controllers on PC/PC XT

374 IEEE 1149 Boundary Scan Test

378-37F LPT2

380-389 SDLC2 Port (rare)

380-38C SDLC1 Port (rare)

3A0-3A9 Binary Synchronous Adapter (rare)

3B0-3BB Mono Video

3BC-3BF LPT1

3D0-3DC CGA/EGA Video

3E8-3EF COM3

3F0-3F5 Floppy Disk Controller

3F8-3FF COM1

A79 ISA PNP write only

Port	Device
entry1a	entry1b
000-00F	DMA Controller
010	POST Checkpoint in PC (See Port 80)
020-021	Interrupt Controller
022-023	CPU and Chipset
040-043	Programmable Timer
060-064	Keyboard Controller and Misc functions incl PS/2 mouse
071	NMI Control - CMOS Access
080	POST Checkpoint in AT and above (See Port 10)
081-083	DMA Page Registers
0A0	NMI on PC/PC XT?
0C2	DMA Ch3 Device Select
170-177	Secondary IDE Controller
1F0-1F7	Primary IDE Controller
201	Gameport
210-215	Extender cards in original PC
278-27F	LPT3
279	ISA PNP
2E8-	Some Video Cards (also 12E8, 22E8, etc)
2E8-2EF	COM4
2F8-2FF	COM2
300	EISA BIOS Checkpoint (Award)
300-3??	Default port address on many network cards
320-330	MFM/RLL Disk Controllers on PC/PC XT
374	IEEE 1149 Boundary Scan Test
378-37F	LPT2
380-389	SDLC2 Port (rare)
380-38C	SDLC1 Port (rare)
3A0-3A9	Binary Synchronous Adapter (rare)
3B0-3BB	Mono Video
3BC-3BF	LPT1
3D0-3DC	CGA/EGA Video
3E8-3EF	COM3
3F0-3F5	Floppy Disk Controller
3F8-3FF	COM1
A79	ISA PNP write only

Several common peripherals such as Network cards and Sound Cards do not have fixed Port addresses and are configured by selecting from a list of possible base addresses. For example, many network cards allow a variety of addresses at 32 bit intervals e.g. 300, 320, 340, 360.