Meaning of MODE "Illegal Device Name - COMx" Message (73725)

The information in this article applies to:
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a

This article was previously published under Q73725


If the I/O port address for a COM port is set to 0 (zero) in the table of COM port addresses located at address 0000:0400 to 0000:0407, then the MODE COMx command returns the following error message:

Illegal device name - COMx
I/O port addresses are initialized at start up by the BIOS and can be changed through software.

A value of 0 for a COM port address typically indicates a lack of BIOS support.


The I/O port addresses of COM1 through COM4 are stored in low-byte, high-byte format in memory locations 0000:0400 through 0000:0407. MS-DOS 4.01 recognizes the first non-zero word (series of two bytes) located in this range as the base address of the COM1 port, the next non-zero word as the base address of COM2, the next as COM3, and the next as COM4.


If a partial memory dump shows

0000:0400 F8 03 F8 02 00 00 00 00 .. .. .. .. .. .. .. ..

the address of COM1 is 3F8, and COM2 is 2F8. MODE COM3 or MODE COM4 commands will return the "Illegal device name - COMx" message.

If a memory dump shows

0000:0400 00 00 F8 02 00 00 00 00 .. .. .. .. .. .. .. ..

the base address of COM1 is 2F8; and MODE COM2, MODE COM3, and MODE COM4 commands will return a "Illegal device name - COMx" message.

Modification Type: Major Last Reviewed: 10/13/2003
Keywords: KB73725