QBasic Supports Only COM1 and COM2 (95103)


The information in this article applies to:

  • Microsoft MS-DOS operating system 5.0
  • Microsoft MS-DOS operating system 5.0a
  • Microsoft MS-DOS operating system 6.0
  • Microsoft MS-DOS operating system 6.2
  • Microsoft MS-DOS operating system 6.21
  • Microsoft MS-DOS operating system 6.22
This article was previously published under Q95103

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.
Modification Type:MajorLast Reviewed:5/12/2003
Keywords:KB95103
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