DOCUMENT:Q95103 19-NOV-1999 [msdos] TITLE :QBasic Supports Only COM1 and COM2 PRODUCT :Microsoft Disk Operating System PROD/VER:MS-DOS:5.0,5.0a,6.0,6.2,6.21,6.22 OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 ------------------------------------------------------------------------------- SUMMARY ======= QBasic supports only serial communication ports COM1 and COM2. COM3 and COM4 are not supported. QBasic supports no more than two serial communications ports because QBasic is a subset of Microsoft QuickBasic 4.5, which also does not support COM3 and COM4. MORE INFORMATION ================ Microsoft does not support more than two serial COM ports with Microsoft QuickBasic versions 1.0 through 4.5 because of a combination of the following factors: 1. MS-DOS 3.3 is the first operating system to support COM3 and COM4. It uses ROM BIOS Interrupt 14h to access those COM ports. 2. The ROM BIOS of a computer system must support COM3 and COM4. 3. COM3 and COM4 have not been widely available on computer systems. 4. A larger code size would be required for support due to the possible conflict created by IRQ sharing; COM1 and COM3 use the same IRQ as do COM2 and COM4. Additional query words: 6.22 5.00 5.00a 6.00 6.20 6.21 5.0a 5.0 ====================================================================== Keywords : Technology : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a Version : MS-DOS:5.0,5.0a,6.0,6.2,6.21,6.22 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 1999.