Computer Science - Computer Hardware - Discussion

IBM created preset combinations of IRQs and I/O addresses for serial and parallel devices. These preset combinations are called ports. The word "port" simply means a portal or two-way access. The preset combinations are called COM ports for serial devices and LPT ports for parallel devices.

The purpose of a port is to make installation easier. Modems and printers, therefore, do not require IRQ or I/O settings. When assigned to an active port (as long as no other device is using that port) , they will work. The following table lists standard ports.

Port I/O Address IRQ.

COM1 : 3F8 : 4.
COM2 : 2F8 : 3.
COM3 :3E8 : 4.
COM4 : 2E8 : 3.
LPT1 : 378 : 7.
LPT2 : 278 : 5.

At the hardware level, each serial port uses a block of eight I/O addresses, and is identified by the base address of the block.

For example, a serial port that resides at addresses (3F8 hex to 3FF hex) , is simply referred to as a serial port at 3F8.

I/o add : iRQ
Com1 : 3f8 : 4
Com2 : 2f8 : 3
Com3 : 3e8 : 4
Com4 : 2e8 : 3