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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Resources | Logical Devices ]

Logical Name Assignment

Logical device names are assigned by the system BIOS during the power-on self test, when the system is booted up. The BIOS searches for devices by I/O address in a predefined order, and assigns them a logical name dynamically, in numerical order. The following are the normal default assignments for COM ports, in order:

Port

I/O Address

Default IRQ

COM1

3F8-3FFh

4

COM2

2F8-2FFh

3

COM3

3E8-3EFh

4

COM4

2E8-2EFh

3

For parallel ports it is slightly more complicated. Originally IBM defined different defaults for monochrome-based PCs and for color PCs. Of course, all new systems have been color for many years, but even some new systems still put LPT1 at 3BCh. Here is how the two different labeling schemes typically work:

Port

"Monochrome" Systems

"Color" Systems

Default IRQ

LPT1

3BC-3BFh

378-37Fh

7

LPT2

378-37Fh

278-27Fh

5

LPT3

278-27Fh

--

5

Most new systems have LPT1 at 378-37Fh. Note that the sequences are really the same, in a way; on a "monochrome" system if you don't put a device at 3BC-3BFh but instead put it at 378-37Fh, the BIOS will make that LPT1 since it didn't find an LPT1 at 3BCh.

Tip: If you want to run three parallel ports (for some reason) you should put LPT1 at 3BCh. By default most new systems put LPT1 at 378h and will not support three parallel ports.

Next: Problems With Logical Device Names


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