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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:
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:
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.