Microsoft KB Archive/73725

= Meaning of MODE &quot;Illegal Device Name - COMx&quot; Message =

Article ID: 73725

Article Last Modified on 10/13/2003

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APPLIES TO


 * Microsoft MS-DOS 4.01 Standard Edition
 * Microsoft MS-DOS 5.0 Standard Edition
 * Microsoft MS-DOS 5.0a

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This article was previously published under Q73725



SUMMARY
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.



MORE INFORMATION
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.

Example
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 &quot;Illegal device name - COMx&quot; 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 &quot;Illegal device name - COMx&quot; message.

Additional query words: 4.01 5.00 5.00a

Keywords: KB73725

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