Alan Freedman -- The Computer Language Company - Computer Desktop Encyclopedia
Computer Desktop Encyclopedia
Longest-Running Tech Reference on the Planet

A CDE Definition

You'll love The Computer Desktop Encyclopedia (CDE) for Tech Term of the Day (TTOD)


I/O address

(1) On x86-based PCs, a number used to identify a peripheral device, such as a disk, sound card or serial port. The address, which is expressed as a hex number (1F0, 37F, etc.), must be unique. Until the late 1990s, controller cards typically contained a small range of selectable addresses to avoid conflicts with other devices. However, the Plug and Play feature, introduced with Windows 95, resolved conflicts automatically. See PC I/O addressing.

(2) The identifying address of a peripheral device.

PC I/O addressing

The method used to pass signals from the CPU to the controller boards of peripheral devices on x86 machines. An I/O address, also called a "port address," references a separate memory space on PC peripheral boards, a little bit similar to memory-mapped peripherals that use blocks of memory. Peripherals often use both methods: an I/O address for passing control signals and memory for transferring data.

Tiny Scattered Memory Banks
There is a 64K address space for I/O addresses, although typically less than 1K is used. Each board that uses an I/O address contains a few bytes of memory (16, 32, etc.) set to a default address range. One or more alternate addresses is also provided to resolve conflicts with other boards. These I/O spaces are a bunch of tiny memory banks scattered over different devices. As long as each one is set to a different address, the CPU can transmit signals to the appropriate boards without conflict.

The I/O Address Operation
If a program needs to send a byte to the serial port, it issues an OUT instruction to the CPU with the address of that serial port. The CPU notifies the address bus to activate the I/O space, not regular memory, and the address bus signals the appropriate byte location on the board. The CPU then sends the data character over the data bus to that memory location. Following are the default I/O addresses for the serial and parallel ports in a PC. See Plug and Play.

           Port     PC      PS/2

           COM1     3F8h
           COM2     2F8h
           COM3     2E8h
           COM4     2E0h
           LPT1     378h    3BCh
           LPT2     278h    378h
           LPT3     3BCh    278h

I/O Addresses
Windows provides ways to view the details of I/O addresses and other internal settings. This is a Windows 98 screen obtained from the Settings control panel. See Win Technical details.

Personal Use Only

Before/After Your Search Term
I LECI/O area
i-modeI/O bound
i-nameI/O bus
i-name serviceI/O card
I-netI/O channel
i-Net+I/O device
i-nodeI/O devise
i-numberI/O intensive
I/OI/O interface
I/O adapterI/O port

Terms By Topic
Click any of the following categories for a list of fundamental terms.
Computer Words You Gotta KnowSystem design
Job categoriesUnix/Linux
Interesting stuffPersonal computers
InternetIndustrial Automation/Process Control
Communications & networkingAssociations/Standards organizations
HistoryDesktop publishing
ProgrammingHealthcare IT
System design